We already know that oxygen (O) is part of the air, but it is also a part of water, sand, soil, rock, and many other things. It may be hard to understand how a gas, like oxygen, can be a part of a liquid, like water, or of a solid like wood, but this is true. Oxygen is found in all plant and animal substance. In fact it is the most abundant element in the world, and is itself one-half of the solid material of the earth.
We shall see oxygen prepared in the laboratory, and shall discover that it is a colorless, odorless, and tasteless gas. It is heavier than air, will dissolve slightly in water, and most curious of all, though it will not burn, it nevertheless makes other things burn very rapidly. Iron, copper, and many other substances which do not seem to burn at all in the air will do so in oxygen, while sulphur and wood, which do burn in air, burn very fast in oxygen.
Test. It is the only substance which will cause a glowing splinter to burst into flame. This fact is utilized in testing whether a gas is oxygen or not, and is therefore called a test for oxygen. When anything unites with oxygen, the process is called oxidation, and the compound formed by the substance and the oxygen is called an oxide. Oxygen may unite with substances rapidly, as when a stick burns, or slowly, as when iron rusts. An oxide is always the product, and there is always a more important product, namely, heat energy.
Both plants and animals use oxygen. Heat energy is necessary for all life. All plants and animals therefore depend on oxygen which they take into their bodies by breathing, as we have seen in CKapter II. As the living tissues become oxidized they produce heat and energy, leaving a residue of oxides and other material to be thrown off as waste. The food assimilated as tissue contains the vital energy which oxidation releases.
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