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.