Oxygen has a great tendency to combine with various other elements, particularly metals. Thus, iron rust is due to the combination of oxygen with iron; the blackening and tarnishing of cooking pots is due to the slow oxidation of copper, etc.
In breathing, we take in oxygen from the air, which combines with the gases in the lungs, forming carbon dioxide. Curiously enough, plants thrive upon this gas, which is so poisonous to human beings, and in turn give off oxygen. Hence the value of plants and flowers in the room, or in any densely inhabited area.
Nitrogen is a very important element, entering into many chemical combinations. It forms the basis of explosives, used in war. Until relatively recently, this element had to be obtained from substances dug out of the ground, but during the late war, methods were devised for obtaining it from the air. “Nitrogen fixation” became possible. If it had not been for this discovery, Germany would have had to give up the war in 1916, at the latest.
Nitrogen combines with hydrogen, to form ammonia; with oxygen and water, to form nitric acid; with nitric acid and potash to form gunpowder, etc.
Certain oxides combine with water, to form what are known as bases. Bases can combine with acids, giving rise to salts.
Carbon is an essential element for all living matter; it combines with oxygen, to yield carbonic acid; with hydrogen, giving rise to a great number of compounds, such as benzene, turpentine, etc. Marsh gas, illuminating gas, acetylene, etc., are compounds of carbon.
Chlorine is a very important element, combining with sodium to form common salt. As we have seen, the saltness of the sea is due to this substance. Owing to its great affinity for hydrogen, chlorine decomposes water, setting free oxygen. The result of this is that a mixture of chlorine and water has strong bleaching qualities.
Chlorine also combines with hydrogen to form hydrochloric acid. On the other hand, it shows little sympathy for oxygen, forming but few stable compounds. Chloroform, so long useful in surgical anaesthesia, is a compound of chlorine, carbon and hydrogen.
Sulphur can assume a variety of appearances (allotrophic variation) as we have seen. Sulphuric acid, etc., are its compounds. The latter substance is used for bleaching violets, but the flowers become violet again when put into an ammonia solution.
Sodium is a metal, which burns when thrown into water. It is the other constituent of common salt, and enters into a great variety of combinations. Carbonate of soda is one of these.