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.