Elements and their Classification
If we pass an electric current through the water using platinum or gold electrodes to introduce the current into the water, we find that two gases are give off at the electrodes and the water is used up. If we collect the gas given off at the negative electrode, we find that it is very light, that the soap bubbles filled with it rise like balloons and that it will burn in air with a faintly luminous very hot flame. This gas is hydrogen. The other gas, given off the positive electrode, will not burn in air, but burning substances thrust into it will continue to burn with greatly increased vigour and brilliancy. We know this gas to be oxygen, also one of the constituents of air.
From this analysis of water we may conclude that water is a compound of hydrogen and oxygen. The analysis might be made in many other ways. We might also synthesize water by allowing hydrogen and oxygen to combine, which they do very rapidly.
It was never possible to resolve lead, oxygen, and hydrogen into simpler substances. These substances and all others, which have never by ordinary chemical reactions been resolved into simpler substances, were given the name of elements. About one hundred elements are known at the present time. They are found to form almost endless combinations with each other.
The elements were classified in various ways, any of which might be useful for certain purposes. Some of them are solids, some liquids, and some gases at ordinary temperature and pressure. This is, however, not fundamental distinction, as each element may be made to exist in any of these states by suitable alterations in temperature and pressure. One of the most useful divisions was that into metals and non-metals. Several characteristics of metals and non-metals are familiar to all.
Metals are characterized by an appearance called metallic lustre, by malleability and ductility and by relatively high conductivity for heat and electricity. These physical differences are accompanied by fundamental differences in their chemical behaviour.
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