What is the formula of water?

The formula of water is H2O: each molecule contains two hydrogen atoms covalently bonded to one oxygen atom.

The subscript 2 applies only to hydrogen, while oxygen has no subscript because one oxygen atom is present. This notation describes the molecule’s composition, not whether the substance is liquid, solid, or gas. Ice, liquid water, and water vapor all consist of H2O molecules, although their molecular arrangements and motions differ.

Chemists established water’s elemental composition during the late eighteenth and early nineteenth centuries. Henry Cavendish showed in 1781 that water was composed of hydrogen and oxygen, and William Nicholson and Anthony Carlisle performed the first electrolysis of water in 1800. In 1805, Alexander von Humboldt and Joseph Louis Gay-Lussac established the two-to-one hydrogen-to-oxygen ratio.

A common mix-up is H2O2, hydrogen peroxide, which contains an additional oxygen atom and has very different properties. O2 is molecular oxygen, while CO2 is carbon dioxide. Water’s molar mass is approximately 18.015 grams per mole.