1 molecule of water is made up of 2 Hydrogen and 1 oxygen atoms because if electrolysis of water is done by adding a little dilute sulphuric acid (as pure water does not dissociate into ions)to it, there are three types of ions produced: 1. read more
Animesh gave indeed a nice proof of getting 2 volumes of a gaz on the cathode and a volume of the anode of an electrolysis system. It is very easy today to take a sample of each gaz and inject to a gas chromatograph and prove it is indeed Hydrogen and Oxygen. read more
Three main phases of development are critically re-examined, covering the historical period from the 1760s to the 1860s: the Chemical Revolution (through which water first became recognized as a compound, not an element), early electrochemistry (by which water’s compound nature was confirmed), and early atomic chemistry (in which water started out as HO and became H 2 O). read more