A molecule of water is composed of two atoms of hydrogen (H) and one atom of oxygen (O). Therefore the formula for water is H2O. Water is produced when hydrogen burns in the presence of oxygen. It is formed when plant and animal matter decay and when food is combined with oxygen in living organisms.

Water decomposes, or breaks down, into hydrogen and oxygen in certain high-temperature chemical reactions involving metals, or when it is electrolyzed by passage of an electric current through it.

Some water molecules contain forms of hydrogen that are heavier than ordinary hydrogen. These heavier forms of hydrogen—deuterium and tritium—form molecules of water called deuterium oxide and tritium oxide, respectively.