The composition of pure water is same throughout. It always contains one part of hydrogen and eight parts of oxygen by weight.
The constituents of water cannot be separated by physical means. However, by electrochemical means water can be decomposed into hydrogen and oxygen.
Chemical reaction takes place with the liberation of heat and light energy when one part of hydrogen combines with the eight parts of oxygen by weight.
The properties of water are entirely different from the properties of its constituents. For example, hydrogen is a combustible gas, oxygen in supporter of combustion, but their compound water extinguishes fire.
