Gay-Lussac's Law of Combining Volumes
The principle that gases react in volume ratios that are simple whole numbers, when measured at the same temperature and pressure.
Formula / equation
V_A : V_B = a : b
Unit
L
In depth
Combined with Avogadro's hypothesis that equal volumes contain equal numbers of molecules, the law proved that elemental gases such as hydrogen and oxygen must be diatomic. For gaseous reactions it allows stoichiometry to be done directly in volumes, with no need to convert to moles.
Examples in the real world
Two volumes of hydrogen combine with one of oxygen to give two volumes of steam, a clean 2:1:2 ratio.