4/18/2012 - How the Atomic Makeup of Water was Discovered

The question of how did someone could find out what is in water is a simple one, it basically consisted of combining hydrogen and oxygen together in a combustion to yield out water.  The scientist who carried out these experiments first was none other than Henry Cavendish, the British man to have the Cavendish Laboratory in Cambridge to be named after, of which many famous physicists go through, including Paul Dirac, James Maxwell, and Ernest Rutherford.  Now at that time, he had not named these gases as Hydrogen and Oxygen, but rather dephlogisiticated air and phlogiston respectively.  Phlogiston being the fire element that is released during combustion.  Through many experiments he determined that the greatest efficiency of water was obtained through the use of two parts hydrogen gas with one part of oxygen gas.  This was a very skeptical idea at the time that water was produced out of two invisible gases, but he endured many more of his experiments to prove his hypothesis, and win respect of the skeptics.

2H₂ + O₂ → 2H₂O + Heat

Source: Jaffe, Bernard. Crucibles: The Story of Chemistry. London, Hutchinson's scientific and technical publications, 1931.