 |
As the months go by, winter freezes the few pools that remain. No snow falls. Living creatures die by the tens of thousands. But the winter is less cold than usual, because there is now so much water vapor in the air that it acts like a great blanket holding in the earth's heat.
With spring no showers come. The dead trees send forth no buds. No birds herald the coming of warm weather. The continents of the world have become vast, uninhabitable deserts. People have all moved to the shores of the ocean, where their chemists are extracting salt from the water in order to give them something to drink. By using this saltless water they can irrigate the land near the oceans and grow some food to live on. Each continent is encircled by a strip of irrigated land and densely populated cities close to the water's edge.
It is many years before the oceans disappear. But in time they too are transformed into water vapor, and no more life as we know it is possible in the world. The earth has become a great rocky and sandy ball, whirling through space, lifeless and utterly dry.
That which prevents this from really happening is very simple: In the world as it is, water vapor condenses and changes to drops of water whenever it gets cool enough.
HOW WATER VAPOR GETS INTO THE AIR. The water vapor gets into the air by evaporation. When we say that water evaporates, we mean that it changes into water vapor. As you already know, it is heat that makes water evaporate; that is why you hang wet clothes in the sun or by the fire to dry: you want to change the water in them to water vapor. The sun does not suck up the water from the ocean, as some people say; but it warms the water and turns part of it to vapor.
What happens down among the molecules when water evaporates is this: The heat makes the molecules dance around faster and faster; then the ones with the swiftest motion near the top shoot off into the air. The molecules that have shot off into the air make up the water vapor.
The water vapor is entirely invisible. No matter how much of it there is, you cannot see it. The weather is just as clear when there is a great deal of water vapor in the air as when there is very little, as long as none of the vapor condenses.
HOW CLOUDS ARE FORMED. But when water vapor condenses, it forms into extremely small drops of real water. Each of these drops is so small that it is usually impossible to see one; they are so tiny that you could lay about 3000 of them side by side in one inch! Yet, small as they are, when there are many of them they become distinctly visible. We see them floating around us sometimes and call them fog or mist. And when there are millions of them floating in the air high above us, we call them a cloud.
The reason clouds form so high in the air is this: When air or any gas expands, it cools. Do you remember Experiment 31, where you let the gas from a tank expand into a wet test tube and it became so cold that the water on the test tube froze? Well, it is much the same way with rising air. When air rises, there is less air above it to keep it compressed; so it expands and cools. Then the water vapor in it condenses into droplets of water, and these form a cloud.
Each droplet forms a gathering place for more condensing water vapor, and therefore grows. When the droplets of water in a cloud are very close together, some may be jostled against one another by the wind. And when they touch each other, they stick together, forming a larger drop. When a drop grows large enough it begins to fall through the cloud, gathering up the small droplets as it goes. By the time it gets out of the cloud it has grown to a full-sized raindrop, and falls to earth. The complete story of rain, then, is this:
HOW RAIN IS CAUSED. The surface of the oceans and lakes is warmed by the sun. The water evaporates, turning to invisible water vapor. This water vapor mingles with the air. After a while the air is caught in a rising current and swept up high, carrying the water vapor with it. As the air rises, there is less air above it to press down on it; so it expands. When air expands it cools, and the water vapor which is mingled with it likewise cools. When the water vapor gets cool enough it condenses, changing to myriads of extremely small drops of water. These make a cloud.
A wind comes along; that is, the air in which the cloud is floating moves. The wind carries the cloud along with it. More rising air, full of evaporated water from the ocean, joins the cloud and cools, and the water forms into more tiny droplets. The droplets get so close together that they shut out the sun's light from the earth, and people say that the sky is darkening.
Meanwhile some of the droplets begin to touch each other and to stick together. Little by little the drops grow bigger by joining together. Pretty soon they get so big and heavy that they can no longer float high in the air, and they fall to the ground as rain.
Part of the rain soaks into the ground. Some of it gradually seeps down through the ground to an underground stream. This has its outlet in a spring or well, or in an open lake or the ocean. But the rain does not all soak in. After the storm, some of the water again evaporates from the top of the ground and mixes with the warm air, and it goes through the same round. Other raindrops join on the ground to form rivulets that trickle along until they meet and join other rivulets; and all go on together as a brook. The brook joins others until the brooks form a river; and the river flows into a lake or into the ocean.
Then again the sun warms the surface of the ocean or lake; the water evaporates and mixes with the air, which rises, expands, and cools; the droplets form and make clouds; the droplets join, forming big drops, and they fall once more as rain. The rain soaks into the ground or runs off in rivulets, and sooner or later it is once more evaporated. And so the cycle is repeated again and again.
And all this is accounted for by the simple fact that when water evaporates its vapor mingles with the air; and when this vapor is sufficiently cooled it condenses and forms droplets of water.
THE BAROMETER. In predicting the weather a great deal of use is made of an instrument called the barometer. The barometer shows how hard the air around it is pressing. If the air is pressing hard, the mercury in the barometer rises. If the air is not pressing hard the mercury sinks. Just before a storm, the air usually does not press so hard on things as at other times; so usually, just before a storm, the mercury in the barometer is lower than in clear weather. You will understand the barometer better after you make one. Here are the directions for making a barometer:
EXPERIMENT 87. To be done by the class with the aid of the teacher. Use a piece of glass tubing not less than 32 inches long, sealed at one end. Fill this tube to the brim with mercury (quicksilver), by pouring the mercury into it through a paper funnel. Have the sealed end of the tube in a cup, to catch any mercury that spills.[7] When the tube is full, pour mercury into the cup until there is at least half an inch of it at the bottom. Now put your forefinger very tightly over the open end of the tube, take hold of the sealed end with your other hand, and turn the tube over. Lower the open end, with your finger over it, into the cup. When the mercury in the cup completely covers your finger and the end of the tube, remove your finger carefully so that no air can get up into the tube of mercury. Let the open end of the tube rest gently on the bottom of the cup, and hold the tube upright with your hand or by clamping it to a ring stand. Hold a yardstick or meter stick beside the tube, remembering to keep the tube straight up and down. Measure accurately the height of the mercury column from the surface of the mercury in the cup. Then go to the regular barometer hanging on the wall, and read it.
[Footnote 7: If mercury spills on the floor or table during this experiment, gather it all into a piece of paper by brushing even the tiny droplets together with a soft brush; squeeze it through a towel into a cup to clean it. It is expensive; so try not to lose any of it.]
The reason your barometer may not read exactly the same as the expensive laboratory instrument is that a little air and water vapor stick to the inside of the tube and rise into the "vacuum" above the mercury; also, the tube may not be quite straight up and down. Otherwise the readings would be the same.
Of course you understand what holds the mercury up in the tube. If you could put the cup of mercury into a vacuum, the mercury in the tube would sink down into the cup. But the pressure of the air on the surface of the mercury in the cup keeps the mercury from flowing out of the tube and so leaving a vacuum in there. If the air pushes down hard on the mercury in the cup, the mercury will stand high in the tube. This is called high pressure. If the air does not press hard on the mercury in the cup, the mercury stands low in the tube. This is called low pressure.
HOW WEATHER IS FORECAST. Weather forecasters make a great deal of use of the barometer, for storms are usually accompanied by low pressure, and clear weather nearly always goes with high pressure.
The reason storms are usually accompanied by low pressure is this: A storm is almost always due to the rising of air, for the rising air expands and cools, and if there is much water vapor in it, this condenses when it cools and forms clouds and rain. Now air rises only when there is comparatively little pressure from above. Therefore, before and during a storm there is not so much pressure on the mercury of the barometer and the barometer is low.
Clear weather, on the other hand, is often the result of air being compressed, for compressing air warms it. When air is being warmed, the water vapor in it will not condense; so the air remains clear. But when the air is being compressed, it presses hard on the mercury of the barometer; the pressure is high, and the mercury in the barometer rises high. Therefore when the mercury in the barometer is rising, the weather is usually clear.
These two statements are true only in a very general way, however. If weather forecasters had only their own barometers to go by, they would not be of much value; for one thing, they could not tell us that a storm was coming much before it reached us. But there are weather stations all over the civilized world, and they keep in touch with each other by telegraph. It is known that storms travel from west to east in our part of the world. If one weather man reports a storm at his station, and tells how his barometer stands, the weather men to the east of him know that the storm is coming their way. From several such reports the weather men to the east can tell how fast the storm is traveling and exactly which way it is going. Then they can tell when it will reach their station and can make the correct prediction.
Weather men do not have to wait for an actual storm to be reported. If the reports from the west show that the air is rising as it swirls along—that is, if the barometer readings in the west are low—they know that this low-pressure air is approaching them. And they know that low pressure usually means air that is rising and cooling and therefore likely to drop its moisture. In the same way, if the barometers to the west show high pressure, the eastern weather men know that the air that is blowing toward them is being compressed and warmed, and is therefore not at all likely to drop its moisture; so they predict fair weather.
The weather man is not ever certain of his forecasts, however. Sometimes the air will begin to rise just before it gets to him. Then there may be a shower of rain when he has predicted fair weather. Or sometimes the air that has been rising to the west, and which has made him predict bad weather, may stop rising; the storm may be over before it reaches his station. Then his prediction of bad weather is wrong. Or sometimes the storm unexpectedly changes its path. There are many ways in which a weather prophecy may go wrong; and then we blame the weather man. We are likely to remember the times that his prophecy is mistaken and to forget the many, many times when it is right.
HOW SNOW IS FORMED. The difference between the ways in which snow and rain are formed is very slight. In both cases water evaporates and its vapor mingles with the warm air. The warm air rises and expands. It cools as it expands, and when it gets cool enough the water vapor begins to condense. But if the air as it expands becomes very cold, so cold that the droplets of water freeze as they form and gather together to make delicate crystals of ice, snow is formed. The ice crystals found in snow are always six-sided or six-pointed, because, probably, the water or ice molecules pull from six directions and therefore gather each other together along the six lines of this pull. At any rate, the tiny crystals of frozen water are formed and come floating down to the ground; and we call them snowflakes. After the snow melts it goes through the same cycle as the rain, most of it finally getting back to the ocean through rivers, and there, in time, being evaporated once more.
Hail is rain that happens to be caught in a powerful current of rising air as it forms, and is carried up so high that it freezes in the cold, expanding air into little balls of ice, or hail stones, which fall to the ground before they have time to melt.
WHY ONE SIDE OF A MOUNTAIN RANGE USUALLY HAS RAINFALL. When air that is moving along reaches a mountain range, it either would have to stop, or rise and go over the mountain. The pressure of the air behind it, moving in the same direction, keeps it from stopping, and so it has to go up the slopes and over the range. But as it goes up, there is less air above it to push down on it; so it expands. This makes it cool, and the water vapor in it begins to condense and form snow or rain. Therefore the side of mountain ranges against which the wind usually blows, almost always has plenty of rainfall.
It is different on the farther side of the mountain range. For here the air is sinking. As it sinks it is being compressed. And as it is compressed it is heated. If you hold your finger over the mouth of a bicycle pump and compress the air in the pump by pushing down on the handle, you will find that the pump is decidedly warmed. When the air, sinking down on the farther side of the mountain range, is heated, the water vapor in it is not at all likely to condense. Therefore rain seldom falls on the side of the mountains which is turned away from the prevailing winds.
HOW DEW AND FROST ARE FORMED. The heat of the earth radiates out into the air and on out into space. At night, when the earth loses its heat this way and does not receive heat from the sun, it becomes cooler. When the air, carrying its water vapor, touches the cool leaves and flowers, the water vapor is condensed by the coolness and forms drops of dew upon them. Or, if the night is colder, the droplets freeze as they form, and in the morning we see the grass and shrubs all covered with frost.
THE CAUSE OF FOGS. When warm air is cooled while it is down around us, the water vapor in it condenses into myriads of droplets that float in the air and make it foggy. The air may be cooled by blowing in from the warm lake or ocean in the early morning, for at night the land cools more rapidly than the water does. This accounts for the early morning fogs in many cities that are on the coasts.
Likewise when the wind has been blowing over a warm ocean current, the surface of the warm water evaporates and fills the air with water vapor. Then when this air passes over a cold current, the cold current cools the air so much that the moisture in it condenses and forms fog. That is why there are fog banks, dangerous to navigation, in parts of the ocean, particularly off Labrador.
WHY YOU CAN SEE YOUR BREATH ON COLD DAYS. You really make a little fog when you breathe on a cold morning. The air in your lungs is warm. The moisture in the lungs evaporates into this warm air, and you breathe it out. If the outside air is cold, your breath is cooled; so some of the water vapor in it condenses into very small droplets, and you see your breath.
Here are two experiments in condensing water vapor by cooling the air with which it is mixed. Both work best if the weather is warm or the air damp.
EXPERIMENT 88. Put the bell jar on the plate of the air pump and begin to pump the air out of it. Watch the air in the jar. If the day is warm or damp, a slight mist will form.
As part of the air is pumped out, the rest expands and cools, as warm air does when it rises and is no longer pressed on so hard by the air above it. And as in the case of the rising warm air, the water vapor condenses when it cools, and forms the mist that you see. This mist, like all clouds and fog, consists of thousands of extremely small droplets.
EXPERIMENT 89. Hold a saucer of ice just below your mouth. Open your mouth wide and breathe gently over the ice. Can you see your breath?
Now put the ice into half a glass of water and cover the glass. Be sure the outside of the glass is thoroughly dry. Set it aside and look at it again in a few minutes.
What caused the mist when you breathed across the ice?
Where did the water on the outside of the glass of ice water come from? What made it condense?
APPLICATION 66. Explain why clouds are formed high in the atmosphere; why we have dew at night instead of in the daytime; why clothes dry more quickly in a breeze than in still air; why clothes dry more quickly on a sunny day than on a foggy one.
INFERENCE EXERCISE
Explain the following:
411. A gas-filled electric lamp gets hotter than a vacuum lamp.
412. You can remove a stamp from an envelope by soaking it in water.
413. We see our breath on cold days and not on warm days.
414. The electric arc is exceedingly hot.
415. Rock candy is made by hanging a string in a strong syrup left open to the air.
416. Dishes in which candy has been made should be put to soak.
417. Moisture gathers on eyeglasses when the wearer comes from a cold room into a warm one.
418. Sprinkling the street on a hot day makes the air cool.
419. You cannot see things in a dark room.
420. Where air is rising there is likely to be rain.
SECTION 45. Softening due to oil or water.
Why does fog deaden a tennis racket?
How does cold cream keep your face from becoming chapped?
Let us now imagine that animal and plant substances have suddenly lost their ability to be softened by oil or water.
All living things soon feel very uncomfortable. Your face and hands sting and crack; the skin all over your body becomes harsh and dry; your mouth feels parched. The shoes you are wearing feel as if they had been dried over a radiator after being very wet, only they are still harder and more uncomfortable.
A man driving a horse feels the lines stiffening in his hands; and the harness soon becomes so dry and brittle that it cracks and perhaps breaks if the horse stops suddenly.
The leaves on the trees begin to rattle and break into pieces as the wind blows against them. Although they keep their greenness, they act like the driest leaves of autumn.
I doubt whether you or any one can stay alive long enough to notice such effects. For the muscles of your body, including those that make you breathe and make your heart beat, probably become so harsh and stiff that they entirely fail to work, and you drop dead among thousands of other stiff, harsh-skinned animals and people.
So it is well that in the real world oil and water soften practically all plant and animal tissues. Of course, in living plants and animals the oil and water come largely from within themselves. Your skin is kept moist and slightly oily all the time by little glands within it, some of which, called sweat glands, secrete perspiration and others of which secrete oil. But sometimes the oil is washed off the surface of your hands, as when you wash an article in gasoline or strong soap. Then you feel that your skin is dry and harsh.
And when you want to soften it again you rub into it oily substances, like cold cream or vaseline.
In the same way if harness or shoes get wet and then are dried out, they can be made properly flexible by oiling. You could wet them, of course, and this would soften them as long as they stayed wet. But water evaporates rather quickly; so when you want a thing to stay soft, you usually apply some kind of oil or grease.
Just as diffusion and the forming of solutions are increased by heat, this softening by oil and water works better if the oil or water is warm. That is why you soak your hands in warm water before manicuring your nails.
APPLICATION 67. Explain why women dampen clothes before ironing them; why crackers are put up in waterproof cartons; why an oil shoe polish is better than one containing water.
INFERENCE EXERCISE
Explain the following:
421. You can shorten your finger nails by filing them.
422. You can do it more quickly after washing them than before.
423. After a flashlight picture is taken, the smoke soon reaches all parts of the room.
424. A jeweler wears a convex lens on his eye when he works with small objects.
425. Shoemakers soak the leather before half-soling shoes.
426. Lightning often sets fire to houses or trees that it strikes.
427. The directions on many bottles of medicine and of preparations for household use say, "Shake well before using."
428. If you set a cold tumbler inside of one that has just been washed in hot water, the outer one will crack in a few minutes.
429. A dry cloth hung out at night becomes wet, while a wet cloth hung out on a clear day dries.
430. Putting cold cream or tallow around the roots of your finger nails will help to prevent hangnails.
CHAPTER TEN
CHEMICAL CHANGE AND ENERGY
SECTION 46. What things are made of: Elements.
What is water made of?
What is iron made of?
Is everything made out of dust?
One of the most natural questions in the world is, "What is this made of?" If we are talking about a piece of bread, the answer is, of course, "flour, water, milk, shortening, sugar, salt, and yeast." But what is each of these made of? Flour is made of wheat, and the wheat is made of materials that the plant gets from the earth, water, and air. Then what are the earth, water, and air made of? A chemist is a person who can answer these questions and who can tell what almost everything is made of. And a strange thing that chemists have found out is this: Everything in the world is made out of one or more of about eighty-five simple substances called elements.
WHAT AN ELEMENT IS. An element is a substance that is not made of anything else but itself. Gold is one of the eighty-five elements; there are no other substances known to man that you can put together to make gold. It is made of gold and that is all. There is a theory that maybe all the elements are made of electrons in different arrangements, or of electrons and one other thing; but we do not know that, it is only a theory. Carbon is another element; pure charcoal is carbon. The part of the air that we use when we breathe or when we burn things is called oxygen. Oxygen is an element; it is not made of anything but itself. There is another gas which is often used to fill balloons that are to go very high; it is the lightest in the world and is called hydrogen. Hydrogen is an element.
For a long time people thought that water was an element. Water certainly looks and seems as if it were made only of itself. Yet during the thousands of years that people believed water was an element, they were daily putting two elements together and making water out of them. When you put a kettle, or anything cold, over a fire, tiny drops of water always form on it. These are not drops of water that were dissolved in the air, and that condense on the sides of the cold kettle; if they were, they would gather on the kettle better in the open air than over the hot fire. Really there is some of that very light gas, hydrogen, in the wood or coal or gas that you use, and this hydrogen joins the oxygen in the air to make water whenever we burn ordinary fuel.
But the best way to prove that water is made of two gases is to take the water apart and get the gases from it. Here are the directions for doing this:
EXPERIMENT 90. A regular bought electrolysis apparatus may be used, or you can make a simple one as follows:
Use a tumbler and two test tubes. If the test tubes are rather small (3/8'' X 3'') they will fill more quickly. Dissolve a little lye (about 1/8 teaspoonful) in half a pint of water to make the water conduct electricity easily, or you may use sulfuric acid in place of lye. Pour half of this solution into the tumbler. Pour as much more as possible into the test tubes, filling both tubes brim full. Cover the mouth of each test tube with a small square of dry paper or cardboard, and turn it upside down, lowering it into the tumbler.
The "electrodes" are two 3/4'' pieces of platinum wire (#30), which are soldered to two pieces of insulated copper wire, each about 2 feet long.[8] The other ends of the copper wire are bare. Fasten the bare end of one copper wire to one nail of the nail plug if you have direct current (d. c.) in the laboratory, and fasten the bare end of the other wire to the other nail; then turn on the electricity. If you do not have direct current in the laboratory, attach the copper wires to the two poles of a battery instead.
[Footnote 8: If the copper wire is drawn through a piece of 1/4-inch soft glass tubing so that only the platinum wire projects from the end of the tube, and the tube is then sealed around the platinum by holding it in a Bunsen burner a few minutes, your electrodes will be more permanent and more satisfactory. The pieces of glass tubing should be about 6 inches long (see Fig. 160).]
Bend the platinum electrodes up so that they will stick up into the test tubes from below. Bubbles should immediately begin to gather on the platinum wire and to rise in the test tubes. As the test tubes fill with gas, the water is forced out; so you can tell how much gas has collected at any time by seeing how much water is left in each tube.
One tube should fill with gas twice as fast as the other. The gas in this tube is hydrogen; there is twice as much hydrogen as there is oxygen in water. The tube that fills more slowly contains oxygen.
When the faster-filling tube is full of hydrogen—that is, when all of the water has been forced out of it—take the electrode out and let it hang loose in the glass. Put a piece of cardboard about 1 inch square over the mouth of the test tube; take the test tube out of the water and turn it right side up, keeping it covered with the cardboard. Light a match, remove the cardboard cover, and hold the match over the open test tube. Does the hydrogen in it burn?
When the tube containing the oxygen is full, take it out, covered, just as you did the hydrogen test tube. But in this case make the end of a stick of charcoal glow, remove the cardboard from the tube, and then plunge the glowing charcoal into the test tube full of oxygen.
Only oxygen will make charcoal burst into flame like this.
When people found that they could take water apart in this way and turn it into hydrogen and oxygen, and when they found that whenever they combined hydrogen with oxygen they got water, they knew, of course, that water was not an element. Maybe some day they will find that some of the eighty-five or so substances that we now consider elements can really be divided into two or more elements; but so far the elements we know show no signs of being made of anything except themselves.
The last section of this book will explain something about the way the chemist goes to work to find out what elements are hidden in compounds.
THE QUICK WAY CHEMISTS WRITE ABOUT ELEMENTS. Since everything in the world is made of a combination or a mixture of elements, chemists have found it very convenient to make abbreviations for the names of the elements so that they can quickly write what a thing is made of. They indicate hydrogen by the letter H. O always means oxygen to the chemist; C means carbon; and Cl means chlorine, the poison gas so much used in the World War. The abbreviation stands for the Latin name of the element instead of for the English name, but they are often almost alike. The Latin name for the metal sodium, however, is natrum, and chemists always write Na when they mean sodium; this is fortunate, because S already stands for the element sulfur. Fe means iron (Latin, ferrum). But I stands for the element iodine. (The iodine you use when you get scratched is the element iodine dissolved in alcohol.) It is not necessary for you to remember the chemical symbols unless you mean to become a chemist or unless you read a good deal about chemistry. But almost every one knows at least that H means hydrogen, O means oxygen, and C means carbon.
When a chemist wants to show that water is made of hydrogen two parts and oxygen one part, he writes it very quickly like this: H2O (pronounced "H two O"). "H2O" means to a chemist just as much as "w-a-t-e-r" means to you; and it means even more, because it tells that water is made of two parts hydrogen and one part oxygen. If a chemist wanted to write, "You can take water apart and it will give you two parts of hydrogen and also one part of oxygen," this is what he would put down:
H_2O -> 2H+O.
If he wanted to show that you could combine two parts of hydrogen and one part of oxygen to form water, he would write it quickly like this:
2H+O -> H_2O.
These are called chemical equations. You do not need to remember them; they are put here merely so that you will know what they look like. Some of them are much longer and more complicated, like this:
HC_2H_3O_2+NaHCO_3 -> H_2O+CO_2+NaC_2H_3O_2.
This is the chemist's way of saying, "Vinegar is made of one part of hydrogen gas that will come off easily and that gives it its sour taste, two parts of carbon, three parts of hydrogen that does not come off so easily, and two parts of oxygen. When you put this with baking soda, which is made of one part of the metal sodium, one part of hydrogen, one part of carbon, and three parts of oxygen, you get water and carbon dioxid gas and a kind of salt called sodium acetate." Or, more briefly, "If you put baking soda with vinegar, you get water, a gas called carbon dioxid, and a salt." You can see how much shorter the chemist's way of writing it is.
SOME ELEMENTS YOU ALREADY KNOW. Here is a list of some elements that you are already pretty well acquainted with. The abbreviation is put after the name for each. This list is only for reference and need not be learned.
Aluminum (Al) Carbon (C) Charcoal, diamonds, graphite (the lead in a pencil is graphite), hard coal, and soot are all made of carbon. Chlorine (Cl) A poison gas that was used in the war. Copper (Cu) Gold (Au) Hydrogen (H) The lightest gas in the world; you got it from water in the last experiment and will get it from an acid in the next. Iodine (I) It is a solid; what you use is iodine dissolved in alcohol. Iron (Fe) Lead (Pb) Mercury (Hg) This is another name for quicksilver. Nickel (Ni) Nitrogen (N) About four fifths of the air is pure nitrogen. Oxygen (O) This is the part of the air we use in breathing. You got some out of water, and you will have it to deal with in another experiment. Phosphorus (P) Phosphorus makes matches glow in the dark, and it makes them strike easily. Platinum (Pt) Radium (Ra) Silver (Ag) Sodium (Na) You are not acquainted with sodium by itself, but when it is combined with the poison gas, chlorine, it makes ordinary table salt. Sulfur (S) Tin (Sn) Zinc (Zn)
For the rest of the elements you can refer to any textbook on chemistry.
HOW ELEMENTS HIDE IN COMPOUNDS. One strange thing about an element is that it can hide so completely, by combining with another element, that you would never know it was present unless you took the combination apart. Take the black element carbon, for instance. Sugar is made entirely of carbon and water. You can tell this by making sugar very hot. When it is hot enough, it turns black; the water part is driven off and the carbon is left behind. Yet to look at dry, white sugar, or to taste its sweetness, one would never suspect that it was made of pure black, tasteless carbon and colorless, tasteless water. Mixing carbon and water would never give you sugar. But combining them in the right proportions into a chemical compound does produce sugar.
Not only is carbon concealed in sugar, but it is present in all plant and animal matter. That is why burning almost any kind of food makes it black. You drive off most of the other elements and separate the food into its parts by getting it too hot; the water evaporates and so does the nitrogen; what is left is mainly black carbon.
MAKING HYDROGEN COME OUT OF HIDING. The light gas, hydrogen, conceals itself as perfectly as carbon does by combining with other elements. It is hiding in everything that is sour and in many things that are not sour. And you can get it out of sour things with metals. In some cases it is harder to separate than in others; and some metals separate it better than others do. But one sour compound that you can easily get the hydrogen out of is hydrochloric acid (HCl), which is hydrogen combined with the poison gas, chlorine. One of the best metals to get the hydrogen out with is zinc. Here are the directions for doing it and incidentally for making a toy balloon:
EXPERIMENT 91. Do this experiment on the side of the laboratory farthest from any flames or fire. Do not let any flame come near the flask in which you are making hydrogen.
In the bottom of a flask put two or three wads of zinc shavings, each about the size of your thumb. Fit a one-hole rubber stopper to the flask. Take the stopper out and put a piece of glass tubing about 5 inches long through the hole of the stopper, letting half an inch or so stick down into the flask when the stopper is in place (Fig. 162). With a rubber band fasten the mouth of a rubber balloon over the end of the glass tube that will be uppermost. Fill the balloon by blowing through the glass tube to see if all connections are tight, and to see how far it may be expanded without danger of breaking. You can tell when the balloon has about all it will hold, by pressing gently with your fingers. If the rubber feels tight, do not blow any more. Let the air out of the balloon again.
Now get some hydrochloric acid (HCl) diluted with three parts of water. Find the bottle marked "HCl, dilute 1-3," in which the acid is already diluted. Before you open the bottle, get some solution of soda, and keep it near you; if in this experiment or any other you spatter acid on your hands or face or clothes, wash it off immediately with soda solution. Remember this. Ammonia will do as well as the soda solution to wash off the acid, but be careful not to get it into your eyes.
Pour the hydrochloric acid (HCl) on the zinc shavings in the bottom of the flask, until the acid stands about an inch deep. Then quickly put the rubber stopper with its attachments into the flask, so that the gas that bubbles up will blow up the balloon.
If the bubbles do not form rapidly, ask the teacher to pour a little strong hydrochloric acid into the flask; but this will probably not be necessary. Let the balloon keep filling until it is as large as you blew it. But if the bubbles stop coming before it gets as large as that, close the neck of the balloon by pinching it tightly, and take the stopper out. Let some one add more zinc shavings and more acid to the flask; put the stopper back in, and stop pinching the neck of the balloon. In this and all other experiments when you use strong acids, pour the used acids into the crockery jar that is provided for such wastes. Do not pour them into the sink, as acids ruin sink drainpipes.
When the balloon is full, close the neck by slipping the rubber band up from the part of the neck that is over the glass tube on to the upper part of the neck. Pull the balloon off the glass tube and pinch the neck firmly shut. Take the stopper out and rinse the flask several times with running water. Any zinc that is left should be rinsed thoroughly, dried, and set aside so that it may be used again. Now tie one end of a long thread firmly around the mouth of the balloon and let the balloon go. Does it rise? If it does not, the reason is that you did not get it full enough. In that case make more hydrogen and fill it fuller, as explained above.
Here is another experiment with hydrogen:
EXPERIMENT 92. Put a wad of zinc shavings, about the size of the end of your little finger, into the bottom of a test tube. Cover it with hydrochloric acid (HCl) diluted one to three, as in the preceding experiment. After the bubbles have been rising for a couple of minutes, take the test tube to the side of the laboratory where the burners are, and hold a lighted match at its mouth. Will hydrogen burn?
Remember that the hydrogen which the zinc is driving out of the acid is exactly the same as the hydrogen you drove out of water with an electric current. There is a metal called sodium (Na) and another called potassium (K) which are as soft as stiff putty and as shiny as silver; if you put a tiny piece of sodium (Na) or potassium (K) on water, it will drive the hydrogen out of the water just as zinc drove it out of the acid. The action is so swift and violent and releases so much heat that the hydrogen which is set free catches fire. This makes it look as if the metal were burning as it sputters around on top of the water. There is so much sputtering that the experiment is dangerous; people have been blinded by the hot alkaline water spattering into their eyes. So you cannot try this until sometime when you take a regular course in chemistry.
GETTING OXYGEN, A GAS, FROM TWO SOLIDS. Oxygen (O) can hide just as successfully as hydrogen. Practically all elements can do the same by combining with others. Here is an experiment in which you can get the gas, oxygen, out of a couple of solids. If you went to the moon or some other place where there is no air, you could carry oxygen very conveniently locked up in these solid substances. Oxygen, you remember, is the part of the air that keeps us alive when we breathe it.
EXPERIMENT 93. In a test tube mix about one half teaspoonful each of white potassium chlorate crystals and black grains of manganese dioxid. Put a piece of glass tubing through a cork so that the tubing will stick down a little way into the test tube. Do not put the glass tubing through the cork while the cork is in the test tube: insert the glass tubing first, then put the cork into the test tube. Put one end of a 2-foot piece of rubber tubing over the glass tube and put the other end into a pan of water.
Fill a flask or bottle to the brim with water, letting it overflow a little; hold a piece of cardboard firmly over the mouth of the bottle; turn the bottle upside down quickly, putting the mouth of it under water in the pan; take the cardboard away. The water should all stay in the bottle.
Now shove the rubber tube into the neck of the bottle until it sticks up an inch or two. During this experiment, be careful not to let the neck of the bottle or flask pinch the rubber tubing; small pieces of wood or glass tubing laid beside the rubber tubing where it goes under the run of the neck will prevent this.
Hold, the test tube, tightly corked, over the flame of a burner, keeping the tube at a slant and moving it slightly back and forth so that all the material in it will be thoroughly heated. If you stop heating the test tube even for a couple of seconds, take the cork out; if you do not remove the cork, the cooling gas in the test tube will shrink and allow the water from the pan to be forced through the rubber tube into the test tube, breaking it into pieces.
When enough gas has bubbled up into the bottle to force all the water out, and when bubbles begin to come up outside the bottle, uncork the test tube and lay it aside where it will not burn anything; then slide the cardboard under the mouth of the bottle and turn it right side up; leave the cardboard on the bottle.
Light a piece of charcoal, or let a splinter of wood burn a few minutes and then blow it out so that a glowing coal will be left on the end of it. Lift the cardboard off the bottle and plunge the glowing stick into it for a couple of seconds. Cover the bottle after taking out the stick, and repeat, using a lighted match or a burning piece of wood instead of the glowing stick. If you dip a piece of iron picture wire in sulfur and light it, and then plunge it into the bottle, you will see iron burn.
Both manganese dioxid and potassium chlorate have a great deal of oxygen bound up in them. When they join together, as they do when you heat them, they cannot hold so much oxygen, and it escapes as a gas. In the experiment, the escaping oxygen passed through the tube, filled the bottle, and forced the water out.
WHAT BURNING IS. When anything burns, it is simply joining oxygen. When a thing burns in air, it cannot join the oxygen of the air very fast, for every quart of oxygen in the air is diluted with a gallon of a gas called nitrogen. Nitrogen will not burn and it will not help anything else to burn. But when you have pure oxygen, as in the bottle, the particles of wood or charcoal or picture wire can join it easily; so there is a very bright blaze.
Although free oxygen helps things to burn so brilliantly, a match applied to the solids from which you got it would go out. And while hydrogen burns very easily, you cannot burn water although it is two-thirds hydrogen. Water is H_2O, you remember.
WHAT COMPOUNDS ARE. When elements are combined with other elements, the new substances that are formed are called _compounds_. Water (H_2O) is a compound, because it is made of hydrogen and oxygen combined.
When elements unite to form compounds, they lose their original qualities. The oxygen in water will not let things burn in it; the hydrogen in water will not burn. Salt (NaCl) is a compound. It is made of the soft metal sodium (Na), which when placed on water sputters and drives hydrogen out of the water, and the poison gas chlorine (Cl), combined with each other. And salt is neither dangerous to put in water like sodium, nor is it a greenish poison gas like chlorine.
MIXTURES. But sometimes elements can be mixed without their combining to form compounds, in such a way that they keep most of their original properties. Air is a mixture. It is made of oxygen (O) and nitrogen (N). If they were combined, instead of mixed, they might form laughing gas,—the gas dentists use in putting people to sleep when they pull teeth. So it is well for us that air is only a mixture of oxygen and nitrogen, and not a compound.
You found that things burned brilliantly in oxygen. Well, things burn in air too, because a fifth of the air is oxygen and the oxygen of the air has all its original properties left. Things do not burn as brightly in air as they do in pure oxygen for the same reason that a teaspoonful of sugar mixed with 4 teaspoonfuls of boiled rice does not taste as sweet as pure sugar. The sugar itself is as sweet, but it is not as concentrated. Likewise the oxygen in the air is as able to help things burn as pure oxygen is; but it is diluted with four times its own volume of nitrogen.
A solution is a mixture, too; for although substances disappear when they dissolve, they keep their own properties. Sugar is sweet whether it is dissolved or not. Salt dissolved in water makes brine; but the water will act in the way that it did before. It will still help to make iron rust; and salt will be salty, whether or not it is dissolved in water. That is why solutions are only mixtures and are not chemical compounds.
EVERYTHING IN THE WORLD IS MADE OF ATOMS. Everything in the world is either an element or a compound or a mixture. Most plant and animal matter is made of very complicated compounds, or mixtures of compounds. All pure metals are elements; but metals, when they are melted, can be dissolved in each other to form alloys, which really are mixtures. Most of the so-called gold and silver and nickel articles are really made of alloys; that is, the gold, silver, or nickel has some other elements dissolved in it to make it harder, or to impart some other quality. Bronze and brass are always alloys; steel is generally an alloy made chiefly of iron but with other elements such as tungsten, of which electric lamp filaments are made, dissolved in it to make it harder. An alloy is a special kind of solution not quite like an ordinary solution.
You remember that in the opening chapters we often spoke of molecules, the tiny particles of matter that are always moving rapidly back and forth. Well, if you were to examine a molecule of water with the microscope which we imagined could show us molecules, you would find that the molecule of water was made of three still smaller particles, called atoms. Two of these would be atoms of hydrogen and would probably be especially small; the third would be larger and would be an oxygen atom.
In the same way if you looked at a molecule of salt under this imaginary microscope, you would probably find it made of two atoms, one of sodium (Na) and one of chlorine (Cl), held fast together in some way which we do not entirely understand.
The smallest particle of an element is called an atom.
The smallest particle of a compound is called a molecule.
Molecules are usually made of two or more atoms joined together.
APPLICATION 68. In the following list tell which things are elements, which are compounds, and which are mixtures, remembering that both solutions and alloys are mixtures:
Air, water, salt, gold, hydrogen, milk, oxygen, radium, nitrogen, sulfur, baking soda, sodium, diamonds, sweetened coffee, phosphorus, hydrochloric acid, brass.
INFERENCE EXERCISE
Explain the following:
431. Although in most electric lamps there is a vacuum between the glowing filaments and the glass, the glass nevertheless becomes warm.
432. Clothes left out on the line overnight usually become damp.
433. You can separate water into hydrogen and oxygen, yet you cannot separate the hydrogen or the oxygen into any other substances.
434. Wet paper tears easily.
435. Windows are soiled on the outside much more quickly in rainy weather than in clear weather.
436. If you stir iron and sand together, the iron will rust as if alone.
437. Rust is made of iron and oxygen, yet you cannot separate the iron from the oxygen with a magnet.
438. A reading glass helps you to read fine print.
439. Stretching the string of a musical instrument more tightly makes the note higher.
440. Mayonnaise dressing, although it contains much oil, can readily be washed off a plate with cold water.
SECTION 47. Burning: Oxidation.
What makes smoke?
What makes fire burn?
Why does air keep us alive?
Why does an apple turn brown after you peel it?
If oxygen should suddenly lose its power of combining with other things to form compounds, every fire in the world would go out at once. You could go on breathing at first, but your breathing would be useless. You would shiver, begin to struggle, and death would come, all within a minute or two. Plants and trees would die, but they would remain standing until blown down by the wind. Even the fish in the water would all die in a few minutes,—more quickly than they usually do when we take them out of the water. In a very short time everything in the world would be dead.
Then suppose that this condition lasted for hundreds and hundreds of years, the oxygen remaining unable to combine with other elements. During all that time nothing would decay. The trees would stay as they fell. The corpses of people would dry and shrivel, but they would lie where they dropped as perfectly preserved as the best of mummies. The dead fish would float about in the oceans and lakes.
This is all because life is kept up by burning. And burning is simply the combining of different things with oxygen. If oxygen ceased to combine with the wood or gas or whatever fuel you use, that fuel could not burn; how could it when "burning" means combining with oxygen? The heat in your body and the energy with which you move come entirely from the burning (oxidation) of materials in your body; and that is why you have to breathe; you need to get more and more oxygen into your body all the time to combine with the carbon and hydrogen in the cells of which your body is made. Plants breathe, too. They do not need so much oxygen, since they do not keep warm and do not move around; but each plant cell needs oxygen to live; there is burning (oxidation) going on in every living cell. Fishes breathe oxygen through their gills, absorbing the oxygen that is dissolved in the water. They do not take the water apart to get some of the combined oxygen from it; there is always some free oxygen dissolved in any water that is open to the air. It is clear that fires would all go out and everything would die if burning (combining with oxygen) stopped.
The reason things would not decay is that decay usually is a slow kind of oxidation (burning). When it is not this, it is the action of bacteria. But bacteria themselves could not live if they had no oxygen; so they could not make things decay.
Not only would the dead plants and animals remain in good condition, but the clothes people were wearing when they dropped dead would stay unfaded and bright colored through all the storms and sunshine. And the iron poles and car tracks and window bars would remain unrusted. For bleaching and rusting are slow kinds of oxidation or burning (combining with oxygen).
Here are two experiments which show that you cannot make things burn unless you have oxygen to combine with them:
EXPERIMENT 94. Light a candle not more than 4 inches long and stand it on the plate of the air pump. Cover it with the bell jar and pump the air out. What happens to the flame?
EXPERIMENT 95. Fasten a piece of candle 3 or 4 inches long to the bottom of a pan. Pour water into the pan until it is about an inch deep. Light the candle. Turn an empty milk bottle upside down over the candle. Watch the flame. Leave the bottle over the candle until the bottle cools. Watch the water around the bottom of the bottle. Lift the bottle partly out of the water, keeping the mouth under water.
The bubbles that came out for a few seconds at the beginning of the experiment were caused by the air in the bottle being heated and expanded by the flame. Soon, however, the oxygen in the air was used so fast that it made up for this expansion, and the bubbles stopped going out. When practically all the oxygen was used, the flame went out.
The candle is made mostly of a combination of hydrogen and carbon. The hydrogen combines with part of the oxygen in the air that is in the bottle to form a little water. The carbon combines with the rest of the oxygen to make carbon dioxid, much of which dissolves in the water below. So there is practically empty space in the bottle where the oxygen was, and the air outside forces the water up into this space. The rest of the bottle is filled with the nitrogen that was in the air and that has remained unchanged.
About how much of the air was oxygen is indicated by the space that the water filled after the oxygen was combined with the candle.
CARBON AND HYDROGEN THE CHIEF ELEMENTS IN FUEL. Carbon and hydrogen make up the larger part of almost every substance that is used for fuel, including gas, gasoline, wood, and soft coal; alcohol, crude oil, kerosene, paper, peat, and the acetylene used in automobile and bicycle lamps. Hard coal, coke, and charcoal are, however, chiefly plain carbon. Since burning is simply the combining of things with oxygen, it is plain that when the carbon of fuel joins oxygen we shall get carbon dioxid (CO_2). When the hydrogen in the fuel joins oxygen, what must we get?
When things do not burn up completely, the carbon may be left behind as charcoal. That is what happens when food "burns" on the stove. But if anything burns up entirely, the carbon or charcoal burns too, passing off as the invisible gas, carbon dioxid, just as the hydrogen burns to form steam or water.
It is because almost every fuel forms water when it burns, that we find drops of water gathering on the outside of a cold kettle or cold flatiron if either is put directly over a flame. The hydrogen in the fuel combines with the oxygen of the air to form steam. As the steam strikes the cold kettle or iron, it condenses and forms drops of water.
NOTHING EVER DESTROYED. One important result of the discovery that burning is only a combining of oxygen with the fuel was that people began to see that nothing is ever destroyed. There is exactly as much carbon in the carbon dioxid that floats off from a fire as there was in the wood that was burned up; and there is exactly as much hydrogen in the water vapor that floats off from the fire as there was in the wood. Chemists have caught all the carbon dioxid and the water vapor and weighed them and added their weight to the weight of the ashes; and they have found them to weigh even more than the original piece of wood, because of the presence of the oxygen that combined with them in the burning.
If everything in the world were to burn up, using the oxygen that is already here, the world would not weigh one ounce more or less than it does now. All the elements that were here before would still be here; but they would be combined in different compounds. Instead of wood and coal and oxygen we should have water and carbon dioxid; instead of diamonds, we should have just carbon dioxid; and so on with everything that can burn.
WHY WATER PUTS OUT A FIRE. Water puts out a fire because it will not let enough free oxygen get to the wood, or whatever is burning, to combine with it. The oxygen that is locked up in a compound, like water, you remember, has lost its ability to combine with other things. Sand puts out a fire in the same way that water does. Most fire extinguishers make a foam of carbon dioxid (CO_2) which covers the burning material and keeps the free oxygen in the air from coming near enough to combine with it.
Water will not put out burning oil, however, as the oil floats up on top of the water and still combines with the oxygen in the air.
WHY ELECTRIC LAMPS ARE USUALLY VACUUMS. Electric lamps usually have vacuums inside because the filament gets so hot that it would burn up if there were any oxygen to combine with it. But in a globe containing no oxygen the filament may be made ever so hot and it cannot possibly burn.
High-power electric lamps are not made with vacuums but are "gas-filled." The gas that is oftenest put into lamps is nitrogen,—the same gas that is mixed with the oxygen in air. By taking all the oxygen out of a quantity of air, the lamp manufacturers can use in perfect safety the nitrogen that is left. It will not combine with the glowing filament. There is no oxygen to combine with the filament; so the lamp does not burn out.
WHAT FLAMES ARE. When you look at a flame, it seems as if fire were a real thing and not merely a process of combining something with oxygen. The flame is a real thing. It is made up of hot gases, rising from the hot fuel, and it is usually filled with tiny glowing particles of carbon. When you burn a piece of wood, the heat partly separates its elements, just as heating sugar separates the carbon from the water. Some of the hydrogen gas in the wood and some of the carbon too are separated from the wood by the heat. These are pushed up by the cooler air around and combine with the oxygen as they rise. The hydrogen combines more easily than the carbon; part of the carbon may remain behind as charcoal if your wood does not all burn up, and many of the smaller carbon particles only glow in the burning hydrogen, instead of burning. That is what makes the flame yellow. If you hold anything white over a yellow flame, it will soon be covered with black soot, which is carbon.
WHAT SMOKE IS. Smoke consists mostly of little specks of unburned carbon. That is why it is gray or black. When you have black smoke, you may always be sure that some of the carbon particles are not combining properly with oxygen.
Yellow flames are usually smoky; that is, they usually are full of unburned bits of carbon that float off above the flame. But by letting enough air in with the flame, it is possible to make all the little pieces of carbon burn (combine with the oxygen of the air) before they leave the heat of the burning hydrogen. That is why kerosene lamps do not smoke when the chimney is on. The chimney keeps all the hot gases together, and this causes a draft of fresh air to blow up the chimney to push the hot gases on up. The fresh air blowing up past the flame gives plenty of oxygen to combine with the carbon. The drum part of an oil heater acts in the same way; when the drum is open, the heater smokes badly; when it is closed up, enough air goes past the flame to burn up all the carbon. But if you turn either lamp or heater too high, it will smoke anyway; you cannot get enough air through to combine with all the carbon.
The hottest flames are the blue flames. That is because in a blue flame all the carbon is burning up along with the hydrogen of the fuel—both are combining with the oxygen of the air as rapidly as possible. A gas or gasoline stove is so arranged that air is fed into the burner with the gas. You will see this in the following experiment:
EXPERIMENT 96. Light the Bunsen burner in the laboratory. Open wide the little valve in the bottom. Now put your finger and thumb over the hole in the bottom of the burner. What happens to the flame? Turn the valve so that it will close the hole in the same way. Now hold a white saucer over the flame, being careful not to get it hot enough to break. What is the black stuff on the bottom of the saucer?
Examine the gas plate (small gas stove) in the laboratory. Light it, and see if you can find the place where the air is fed in with the gas. Close this place and see what happens. Open it wider and see what happens. If the air opening is too large, the flame "blows"; there is too much cold air coming in with the gas, and your flame is not as hot as it would be if it did not "blow." Also, the stove is liable to "back-fire" (catch fire at the air opening) when the air opening is too wide.
APPLICATION 69. An oil lamp tipped over and the burning oil spread over the floor. Near by were a pail of water, a pan of ashes, a rug, and a seltzer siphon. Which of these might have been used to advantage in putting out the fire?
APPLICATION 70. My finger was burned. I wanted the flesh around it to heal and new skin cells to live and grow rapidly around the burn.
"Put a rubber finger cot on the finger and keep all air out," one friend advised me. "Air causes decay and will therefore be bad for the burn."
"He's wrong; you should bandage it with clean cloth; you want air to reach the finger, I've heard," said another friend.
"Oh, no, you don't; air makes things burn, and the burn will therefore get worse," still another one said. What should I have done?
APPLICATION 71. Two students were discussing how coal was formed.
"The trees must have fallen into water and been completely covered by it, or they would have decayed," said one.
"Water makes things decay more quickly; there must have been a drought and the trees must have fallen on dry ground," said the second.
Which was right?
APPLICATION 72. A gas stove had a yellow flame. In front, by the handles, was a metal disk with holes so arranged that turning it to the left allowed air to mix with the gas on the way to the flame, and turning it to the right shut the air off (see Fig. 170).
One member of the family said, "Turn the disk to the left and let more air mix with the gas."
But another objected. "It has too much air already; that's why the flame is yellow. Turn it to the right and shut off the air from below."
"You're both wrong. Why do you want to change it?" said a third member of the family. "The yellow flame is the hottest, anyway. Can't you see that the yellow flame gives more light? And don't you know that light is just a kind of radiant heat? Of course the yellow flame is the hottest. Leave the stove alone."
Who was right?
INFERENCE EXERCISE
Explain the following:
441. Iron tracks are welded together with an electric arc.
442. The cool mirror in a bathroom becomes covered with moisture when you take a hot bath.
443. This prevents you from seeing yourself in the mirror.
444. Carbon dioxid has oxygen in it, yet a burning match dropped into a bottle of it will go out.
445. A ship that sinks to the bottom of the ocean does not decay.
446. When women put their hair in curlers, they usually moisten the hair slightly.
447. To dry a pan after washing it, a person often sets it on the hot stove for a few minutes.
448. When you put a kettle of cold water over a gas flame, drops of water appear on the lower part of the sides of the kettle.
449. Electric power plants are often situated where running water will turn the dynamo. Explain the necessity of turning the dynamo.
450. We make carbon dioxid by burning carbon, but you cannot put different things together to make carbon.
SECTION 48. Chemical change caused by heat.
Why do you have to strike a match to make it burn?
How does pulling the trigger make a gun go off?
What makes cooked foods taste different from raw ones?
Has it struck you as strange that we do not all burn up, since burning is a combining with oxygen, and we are walking around in oxygen all the time? The only reason we do not burn up is that it usually requires heat to start a chemical change. You already know this in a practical way. You know that you have to rub the head of a match and get it hot before it will begin to burn; that gunpowder does not go off unless you heat it by the sudden blow of the gun hammer which you release when you pull the trigger; that you have to concentrate the sun's rays with a magnifying glass to make it set a piece of paper on fire; and that to change raw food into food that tastes pleasant you have to heat it. If heat did not start chemical change, you could never cook food,—partly because the fire would not burn, and partly because the food would not change its taste even if heated by electricity or concentrated sunlight.
Here is an experiment to show that gas will not burn unless it gets hot enough:
EXPERIMENT 97. Hold a wire screen 2 or 3 inches above the mouth of a Bunsen burner. Turn on the gas and light a match, holding the lighted match above the screen. Why, do you suppose, does the gas below the screen not burn? Hold a lighted match to the gas below the screen. Does it burn now?
The reason the screen kept the gas below it from catching fire although the gas above it was burning was this: The heat from the flame above was conducted out to the sides by the wire screen as soon as it reached the screen; so very little heat could get through the screen to the gas below. Therefore the gas below the screen never got hot enough for the chemical change of oxidation, or burning, to take place. So the gas below it did not catch fire.
Another simple experiment with the Bunsen burner, that shows the same thing in a different way, is this:
EXPERIMENT 98. Light the Bunsen burner. Open the air valve at the bottom all the way. Hold the wood end of a match (not the head) in the center of the inner greenish cone of flame, about half an inch above the mouth of the burner. Does the part of the match in the center of the flame catch fire? Does the part on the edge? What do you suppose is the reason for this? Where are the cold gas and air rushing in? Can they get hot all at once, or will they have to travel out or up a way before they have time to get hot enough to combine?
APPLICATION 73. Explain why boiled milk has a different taste from fresh milk; why blowing on a match will put it out; why food gets black if it is left on the stove too long.
INFERENCE EXERCISE
Explain the following:
451. When you want bread dough to rise, you put it in a warm place.
452. Ink left long in an open inkwell becomes thick.
453. A ball bounces up when you throw it down.
454. When the warm ocean air blows over the cool land in the early morning, there is a heavy fog.
455. Striking a match makes it burn.
456. When you have something hard to cut, you put it in the part of the scissors nearest the handles.
457. A magnet held over iron filings makes them leap up.
458. Dishes in which flour thickening or dough has been mixed should be washed out with cold water.
459. A woolen sweater is liable to stretch out of shape after being washed.
460. When a telegraph operator presses a key in his set, a piece of iron is pulled down in the set of another operator.
SECTION 49. Chemical change caused by light.
How can a camera take a picture?
Why does cloth fade in the sun?
What makes freckles?
If light could not help chemical change, nothing would ever fade when hung in the sun; wall paper and curtains would be as bright colored after 20 years as on the day they were put up, if they were kept clean; you would never become freckled, tanned, or sunburned; all photographers and moving-picture operators would have to go out of business; but worst of all, every green plant would immediately stop growing and would soon die. Therefore, all cows and horses and other plant-eating animals would die; and then the flesh-eating animals would have nothing to eat and they would die; and then all people would die.
You will be able better to understand why all this would happen after you do the following experiments, the first of which will show that light helps the chemical change called bleaching or fading.
EXPERIMENT 99. Rinse two small pieces of light-colored cloth. (Lavender is a good color for this experiment.) Lay one piece in the bright sun to dry; dry the other in a dark cabinet or closet. The next day compare the two cloths. Which has kept its color the better? If the difference is not marked, repeat the experiment for 2 or 3 days in succession, putting the same cloth, wet, in the sun each time.
Bleaching is usually a very slow kind of burning. It is the dye that is oxidized (burned), not the cloth. Most dyes will combine with the oxygen in the air if they are exposed to the sunlight. The dampness quickens the action.
WHY SOME PEOPLE FRECKLE IN THE SUN. When the sunlight falls for a long time on the skin, it often causes the cells in the under part of the skin to produce some dark coloring matter, or pigment. This dark pigment shows through the outer layer of skin, and we call the little spots of it freckles. Some people are born with these pigment spots; but when the freckles come out from long exposure to the sunlight, they are an example right in our own skins of chemical change caused by the action of light. Tan also is due to pigment in the skin and is caused by light.
The next experiments with their explanations will show you how cameras can take pictures. If you are not interested in knowing how photographs are made, do the experiments and skip the explanations down to the middle of page 332.
EXPERIMENT 100. Dissolve a small crystal of silver nitrate (AgNO3) in about half an inch of pure water in the bottom of a test tube. Distilled water is best for this purpose. Now add one drop of hydrochloric acid (HCl). The white powder formed is a silver salt, called silver chlorid (AgCl); the rest of the liquid is now a diluted nitric acid (HNO3).
Pour the suspension of silver chlorid (AgCl) on a piece of blotting paper or on a paper towel, so that the water will be absorbed. Spread the remaining white paste of silver chlorid (AgCl) out over the blotter as well as you can. Cover part of it with a key (or anything that will shut off the light), and leave the other part exposed. If the sun is shining, put the blotter in the sunlight for 5 minutes. Otherwise, let as much daylight fall on it as possible for about 10 minutes. Now take the key off the part of the silver chlorid (AgCl) that it was covering and compare this with the part that was exposed to the light. What has the light done to the silver chlorid (AgCl) that it shone on?
What has happened is that the light has made the silver (Ag) separate from the chlorine (Cl) of the silver chlorid (AgCl). Chemists would write this:
AgCl -> Ag + Cl.
That is, silver chlorid (AgCl) has changed into silver (Ag) and chlorine (Cl). Chlorine, as you know, is a poisonous gas, and it floats off in the air, leaving the fine particles of silver behind. When silver is divided into very tiny particles, it absorbs light instead of reflecting it; so it looks dark gray or black.
HOW PHOTOGRAPHS ARE MADE. All photography depends on this action of light. The plates or films are coated with a silver salt,—usually a more sensitive salt than silver chlorid. This is exposed to the light that shines through the lens of the camera. As you have learned, the lens brings the light from the object to a focus and makes an image on the film or plate. The light parts of this image will change the silver salt to silver; the dark parts will not change it. So wherever there is a white place on the object you are photographing, there will be a dark patch of silver on the film or plate, and wherever there is a dark spot on the object, there will be no change on the film or plate.
As a matter of fact, the film or plate is exposed such a short time that there is not time for the change to be completed. So the photographer develops the negative; he washes it in some chemicals that finish the process which the light started.
If he exposed the whole plate to the light now, however, all the unchanged parts of the silver salt would also be changed by the light, and there would be no picture left. So before he lets any light shine on it, except red light which has no effect on the silver salt, he dissolves off all the white unchanged part of the silver salt, in another kind of chemical called the fixing bath. This is called "fixing" the negative.
The only trouble with the picture now is that wherever there should be a patch of white, there is a patch of dark silver particles; and wherever there should be a dark place, there is just the clear glass or celluloid, with all the silver salt dissolved off. This kind of picture is called a negative; everything is just the opposite shade from what it should be. A white man dressed in a black suit looks like a negro dressed in a white suit.
HOW A PHOTOGRAPHIC PRINT IS MADE. The negative not only has the lights and shadows reversed, but it is on celluloid or glass, and except for moving pictures and stereopticons, we usually want the picture on paper. So a print is made of the negative. The next experiment will show you how this is done.
EXPERIMENT 101. In a dark room or closet, take a sheet of blueprint paper from the package, afterwards closing the package carefully so that no light can get to the papers inside. Hold the piece of blueprint paper under your waist or coat, to keep it dark when you go into the light. Now lay it, greenish side downward, on a negative. Hold the two together, or place them in a printing frame, and turn them over so that the light will shine through the negative upon the greenish side of the blueprint paper. Be sure that the paper is held firmly against the negative and not moved around. Let the sun shine through the negative upon the paper for 1 or 2 minutes according to the brightness of the sun, or let the gray light of the sky, if it is cloudy, shine on it for 5 or 10 minutes. Now quickly put the blueprint paper (not the negative) into a basin of water, face down. Wash for a couple of minutes. Turn it over and examine it. If it has been exposed to the light too long, it will be dark; if it has been exposed too short a time, it will be too light; in either case, if the print is not clear, repeat with a fresh piece of blueprint paper, altering the time of exposure to the sunlight to improve the print.
You can make pretty outline pictures of leaves and pressed flowers, or of lace, by laying these on the blueprint paper in place of the negative and in other respects doing as directed above.
In making blueprints you are changing an iron salt instead of a silver salt, by the action of light. Regular photographic prints are usually made on paper treated with a silver salt rather than with iron salt, and sometimes a gold or platinum salt is used. But these other salts have to be washed off with chemicals since they do not come off in water, as the unchanged part of the iron salt comes off when you fix the blueprint paper in the water bath.
Since the light cannot get through the black part of a negative, the coating on the paper behind that part is not affected and it stays light colored; and since the light can get through the clear parts of the negative, the coating on the paper back of those parts is affected and becomes dark. Therefore, the print is "right side out,"—there is a light place on the print for every white place on the object photographed, and there is a dark place on the print for every black place on the object.
Moving-picture films are printed from one film to another, just as you printed from a negative to a piece of paper. The negative is taken on one film, then this is printed on another film. The second film is "right side out."
LIGHT AND THE MANUFACTURE OF FOOD IN PLANTS. Much the most important chemical effect of light, however, is not in making photographs, in bleaching things, or in "burning" your skin. It is in the putting together of carbon and water to make sugar in plants. Plants get water (H2O) from the earth and carbon dioxid (CO2) from the air. When the sun shines on chlorophyll, the green substance in plants, the chlorophyll puts them together and makes sugar. The plant changes this sugar into starch and other foods, and into the tissues of the plant itself. Nothing in the world can put carbon dioxid and water together and make food out of them except certain bacteria and the chlorophyll of plants. And light is absolutely necessary for this chemical action. Try this experiment:
EXPERIMENT 102. Pin together two pieces of cork on opposite sides of a leaf that is exposed to the sun. The next day take this leaf from the plant and heat it in a beaker of alcohol until the green coloring matter is removed from the leaf. Then place the leaf in a glass of water that contains iodine. The iodine will color the leaf dark where the cells contain starch. (See Experiment 115, page 373.) Is starch formed where the light does not reach the leaf?
No plant can make food except with the help of light. The part of the plant that can put carbon dioxid and water together is the green stuff or chlorophyll, and this can work only when light is shining on it. So all plants would die without light.
But if all plants should die, all animals would die also, for animals cannot make food out of carbon dioxid and water, as they do not have the chlorophyll that puts these things together. A lion does not live on leaves, it is true, but he lives on deer and other animals that do live on leaves and plants. If the plants died, all plant-eating animals would die. Then there would be nothing for the flesh-eating animals to eat except each other, and in time no animals would be left in the world. The same thing would happen to the fish. And man, of course, could no longer exist. The food supply of the world depends on the fact that light can start chemical change.
OXYGEN RELEASED IN THE MANUFACTURE OF PLANT FOOD. Besides in one way or another giving us all of our food, plants, helped by light, also give us most of the free oxygen that we breathe. We and all animals get the energy by which we live by combining oxygen with the hydrogen of our food (forming water) and by combining oxygen with the carbon in our food (forming carbon dioxid). This combining (burning or oxidizing) gives us our body heat and the energy to move. The free oxygen is carried to the different parts of our bodies by the red blood corpuscles that float in the liquid part of the blood. The liquid part of the blood also carries the food to the different parts of the body, and the food contains the carbon and hydrogen that is to be burned. Then in a muscle, for instance, the oxygen that has been carried by the corpuscles combines with the carbon to form carbon dioxid, and with the hydrogen to form water. The corpuscles carry part of the carbon dioxid back to the lungs, and the water is carried with other wastes and the rest of the carbon dioxid in the liquid part of the blood. In the lungs the carbon dioxid is exchanged for the free oxygen we have just inhaled, and we exhale the carbon dioxid. A good deal of water is also breathed out, as you can tell from the way the mist gathers on a window pane when you blow on it.
If there were only animals (including people) in the world, all the free oxygen in the air would in time be combined by the animals with hydrogen to make water and with carbon to make carbon dioxid (CO_2). As animals cannot breathe water and cannot get any good from carbon dioxid, they would all smother.
But the plants, as we have already said, use carbon dioxid (CO2) and water (H2O) to make food. They do not need so much oxygen, and so they set some of it free. The countless plants in the world set the oxygen free as rapidly as the countless animals combine it with hydrogen to make water and with carbon to make carbon dioxid. Since the water and carbon dioxid are the main things a plant needs to make its food, the animals really are as helpful to the plants as the plants are to the animals. For the animals furnish the materials to the plants for making their food in exchange for the ready-made food furnished by the plant. And both plants and animals would die if light stopped helping to bring about chemical change.
APPLICATION 74. Explain why the heart of a cabbage is white instead of green like the outside leaves; why a photographer works in a dark room with only a ruby light; why you get freckled in the sun.
INFERENCE EXERCISE
Explain the following:
461. If a pin is put through a lamp cord, a fuse is likely to blow out.
462. The wall paper back of a picture is often darker than that on the rest of the wall.
463. If you wet an eraser, it rubs through the paper.
464. Clothes are hot after being ironed.
465. If you drop candle grease on your clothes, you can remove the grease by placing a blotter over it and pressing the blotter with a warm iron.
466. Milliners cover hats that are on display in windows where the sun shines in on the hats.
467. You pull down on a rope when you try to climb it.
468. In taking a picture, you expose the sensitive film or plate to the light for a short time.
469. Good cameras have an adjustable front part so that the lens may be moved nearer to the plate or film, or farther from it, according to the distance of the object to be photographed.
470. A pencil has to be resharpened frequently when it is much used.
SECTION 50. Chemical change caused by electricity.
How are storage batteries charged?
How is silver plating done by electricity?
You have already done an experiment showing that electricity can start chemical change, for you changed water into hydrogen and oxygen by passing a current of electricity through the water.
The plating of metals is made possible by the fact that electricity helps chemical change. You can nickel plate a piece of copper in the following manner:
EXPERIMENT 103. Dissolve a few green crystals of "double nickel salts" in water, until the water is a clear green. The water should be about 2 or 3 inches deep in a glass or china bowl that is not less than 5 inches across.
Lay two bare copper wires across the bowl, about 3 inches apart, as shown in Figure 177. Connect the positive wire from a storage battery, or the wire from the carbon of a battery of three or four cells, to an end of one bare wire. Connect the negative wire from the storage or the negative wire from the zinc of the other battery to an end of the second bare wire.
Now fasten a fine bare wire 5 or 6 inches long around a small piece of copper, and another like it around a piece of nickel, as shown in Figure 176. Then put the piece of copper in the bottom of an evaporating dish, with the wire hanging out, as in Figure 177.
Pour over the piece of copper enough of the cleansing solution to cover it.[9] The cleansing solution contains strong acids. If you get any on your skin or clothes, wash it off immediately with ammonia or soda. As soon as the copper is bright and clean, take it out of the cleansing solution and suspend it by the negative wire in the green nickel solution. You can tell if you have it on the negative wire, for in that case bubbles will rise from it during the experiment. The copper should be entirely covered by the nickel solution, but should not touch the bottom or sides of the bowl. Pour the cleansing solution from the evaporating dish back into the bottle. Suspend the nickel, in the same way as the copper, from the positive wire crossing the bowl. When set up, the apparatus should appear as shown in Figure 178.
[Footnote 9: The formula for making the cleansing solution is as follows:
1 cup water.
1 cup concentrated sulfuric acid.
1 cup concentrated nitric acid.
1 teaspoonful concentrated hydrochloric acid.
The sulfuric and nitric acids must be measured in glass or china cups, and the hydrochloric acid must be measured in a silver-plated spoon or in glass—not in tin.]
Turn on the electricity. If the copper becomes black instead of silvery, clean it again in the cleansing solution, and move the two bare wires much farther apart,—practically the full width of the bowl. If the copper still turns black, it means that too much electricity is flowing. In that case use fewer batteries.
The electricity has started two chemical changes. It has made part of the piece of nickel combine with part of the solution of nickel salt to form more nickel salt, and it has made some of the nickel salt around the copper change into metallic nickel. Then the negative electricity in the copper has attracted the positive bits of nickel metal made from the nickel salt, and made them cling to the copper. If there is no dirt or grease on the copper, the particles of nickel get so close to it that they stick by adhesion, even after the electric attraction has ceased. This leaves the copper nickel-plated, but to make it shiny the nickel plating must be polished.
Silver plating and gold plating are done substantially in the way that you have done the nickel plating, only gold salt or silver salt is used instead of nickel salt.
Just as electricity helps chemical changes in plating, it helps changes in a storage battery. But in the storage battery the new compounds formed by "charging" the battery change back again and generate electricity when the poles of the battery are connected with each other by a good conductor.
APPLICATION 75. Explain how spoons can be silver plated; how water can be changed into hydrogen and oxygen.
INFERENCE EXERCISE
Explain the following:
471. Clothes dry best in the sun and wind.
472. Proofs of photographs that have not been thoroughly "fixed" fade if left out of their envelope.
473. Blowing a match puts it out, yet a good draft is necessary for a hot fire.
474. A cup does not naturally fall apart, yet after it is broken it falls apart even if you fit the pieces together again.
475. Crayon leaves marks on a blackboard.
476. A baked potato tastes very different from a raw one.
477. An air-filled automobile tire is harder at noon than in the early morning.
478. When a live trolley wire breaks and falls to the street, it becomes so hot that it burns.
479. Glass jars of fruit should be kept in a fairly dark place.
480. You wash dishes in hot water.
SECTION 51. Chemical change releases energy.
Why is fire hot?
What makes glowworms glow?
Why does cold quicklime boil when you pour cold water on it?
If no energy were released by chemical change, we should run down like clocks, and could never be wound up again. We could breathe, but to do so would do us no more good than it would if oxygen could not combine with things. Oxidation would go on in our bodies, but it would neither keep us warm nor help us to move. A few spasmodic jerks of our hearts, a few gasps with our lungs, and they would stop, as the muscles would have no energy to keep them going.
The sunlight might continue to warm the earth, as we are not sure that the sun gets any of its heat from chemical change. But fires, while they would burn for an instant, would be absolutely cold; no energy would be given out by the fuel combining with oxygen. But the fires could not burn long, because there would be nothing to keep the gases and fuel hot enough to make them combine with the oxygen.
Even during the instant that a fire lasted it would be invisible, for it would give off no light if no energy were released by the chemical change. Only electric lights and heaters would continue to work, and even some of these would fail. The electric motors in submarines and electric automobiles would instantly stop; battery flashlights would go out as quickly as the fire; no doorbells would ring. In short, all forms of electric batteries would stop sending currents of electricity out through their wires, and everything depending upon batteries would stop running.
A fire gives out heat and light; both are kinds of energy. And it is the electric energy caused by the chemical change in batteries that runs submarines, electric automobiles, flashlights, and doorbells. Since burning (oxidation) is simply a form of chemical change, it is not difficult to realize that chemical change releases energy.
WHY GLOWWORMS GLOW. When a glowworm glows at night, or when the head of a match glows as you rub it on your wet hand in the dark, we call the light phosphorescence. The name "phosphorus" means light-bearing, and anything like the element phosphorus, that glows without actively burning, is said to be phosphorescent. Match heads have phosphorus in them. Phosphorescence is almost always caused by chemical change. The energy released is a dim light, not heat or electricity. Sometimes millions of microscopic sea animals make the sea water in warm regions phosphorescent. They, like fireflies, glowworms, and will-o'-the-wisps, have in them some substance that is slowly changing chemically, and energy is released in the form of dim light as the change takes place. Most luminous paint is phosphorescent for the same reason,—there is a chemical change going on that releases energy in the form of light.
When you poured the hydrochloric acid on the zinc to make hydrogen, the flask became warm; the chemical change going on in the flask released heat energy.
APPLICATION 76. Explain why pouring cold water on cold quicklime makes the slaked lime that results boiling hot; why a cat's eyes shine in the dark; why a piece of carbon and a piece of zinc placed in a solution of sal ammoniac will make electricity run through the wire that connects them; why fire is hot.
INFERENCE EXERCISE
Explain the following:
481. A baking potato sometimes bursts in the oven.
482. Turpentine is used in mixing paint.
483. Sodium is a metal; chlorine is a poisonous gas; yet salt, which is made up of these two, is a harmless food. |
|
