Water molecules. Computer artwork showing the molecular (top) and atomic (bottom) structure of water (H2O). Atoms are colour-coded: hydrogen (blue) and oxygen (white). Water is one of the most abundant chemicals on Earth, covering almost 75 per cent of its surface. It is also present as vapour in the atmosphere, as ice at temperatures below zero degrees Celsius, and as steam at temperatures above 100 degrees Celsius. Atoms are colour-coded: oxygen (red) and hydrogen (black).