Some liquids conduct electricity. Such liquids are called electrolytes. In order for a liquid to conduct electricity it must contain charged particles called ions, which are free to move in the liquid (electricity is basically just the flow of charge). Pure water is a poor electrolyte because it has very few ions, but can be made a much better electrolyte with the addition of a small amount of acid eg sulphuric acid. The acid introduces ions. Adding sulphuric acid to water producesand ions.When current is passed through water treated in this way, bubbles appear at the anode (the + terminal) and the cathode (the negative terminal). These bubbles can be collected and analysed. The bubbles at the positive terminal are found to be oxygen are those at the negative terminal are found to be hydrogen.
The molecular formula for water is H-2 O, so that in any amount of water there are twice as many hydrogen atoms as oxygen atoms. Because hydrogen and oxygen molecules both contain two atoms – they formandmolecules – and equal numbers of atoms of any gas occupy the same volume at the same temperature, the volume of hydrogen liberated will be twice the volume of ocygen liberated.