In the reaction between hydrogen and oxygen gives water vapour, the ratio of volumes is 2:1:2. This illustrates the law of

$2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$

2 volumes of hydrogen gas combines with 1 volume of oxygen to give 2 volumes of water vapour. Thus, the ratio of volumes is 2:1:2. As, the ratio bears a simple ratio, hence signifying the law of combining volumes.