vapour

Vapour, the British spelling of vapor, refers to the gaseous state of a substance that is normally liquid or solid at room temperature. A vapour exists in equilibrium with its condensed phase when conditions permit condensation to occur. Water vapour is the most familiar example and is a major component of the atmosphere.

The term vapour is often used to distinguish a gas that is near its condensation point from a gas that is simply in the gaseous state without reference to a condensed phase. In many contexts, vapour describes the gaseous form of a substance that can become liquid or solid under suitable pressure and temperature, whereas the word gas may be used more generally.

Key properties include vapour pressure, the pressure exerted by a vapour in equilibrium with its liquid or solid form at a given temperature. Substances with high vapour pressure are more volatile and evaporate readily. Evaporation is the process of a liquid turning into vapour at the surface, while boiling is the formation of vapour bubbles within the liquid. Condensation reverses these processes.

Vapours are important in science and industry. Water vapour is central to meteorology and climate, contributing to humidity and the greenhouse effect. In industry, vapour processes such as distillation, steam generation, and vapour deposition are used for purification, heating, and the production of thin films. Some vapours are toxic or flammable, requiring proper handling and ventilation.