If an electric current is passed through water, made slightly acid to increase conductivity (ease of passage), the water will be decomposed or separated into its elements, oxygen and hydrogen, which can be collected in separate tubes. Separating compounds into their elements is called analysis.

In the case of water, for every volume of oxygen there will be found to be just twice the volume of hydrogen. Every molecule of water contains two atoms of hydrogen and one atom of oxygen. The chemical formula expressing this is H1O. In any formula the number of atoms is shown by writing the number below the symbol of the element and at the right.