Electrolysis+of+Water

Electrical energy can be used to break down water, H2O into its elements. Bubbles of oxygen gas and hydrogen gas form at separate electrodes. As electric current flows between the electrodes, atoms of one kind lose electrons at one electrode. At the other electrode, atoms of a different kind gain electrons. The chemical bonds between the oxygen and hydrogen atoms are broken by the electrical energy.

Use the law of conservation of matter to compare the numbers of hydrogen and oxygen atoms before and after this chemical reaction. Why is the volume of hydrogen formed twice that of oxygen?