Common Science Part 40

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.

[Ill.u.s.tration: FIG. 165. Filling a bottle with oxygen.]

[Ill.u.s.tration: FIG. 166. The iron really burns in the jar of oxygen.]

Both manganese dioxid and pota.s.sium 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 pa.s.sed 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 bra.s.s 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, bra.s.s.

INFERENCE EXERCISE

Explain the following:

431. Although in most electric lamps there is a vacuum between the glowing filaments and the gla.s.s, the gla.s.s 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 gla.s.s 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 s.p.a.ce in the bottle where the oxygen was, and the air outside forces the water up into this s.p.a.ce.

The rest of the bottle is filled with the nitrogen that was in the air and that has remained unchanged.