vapours

Vapour is the gaseous state of a substance that is typically a liquid or solid at room temperature. Vapours form when a substance escapes into the air as a gas through vaporisation, which can occur by evaporation from a surface or by boiling in a liquid. In everyday language, steam and mist are often described as vapour, but steam specifically refers to water vapour produced by heating water, while mist describes tiny liquid droplets suspended in air.

Vapour is distinct from gas in that vapour typically refers to the gaseous phase of a substance that is normally not gaseous at room temperature. If a substance is already a gas at room temperature (such as nitrogen or oxygen), its gaseous form is not usually called a vapour. The behaviour of vapours is governed by vapour pressure—the pressure exerted by the vapour in equilibrium with its condensed phase. Vapour pressure increases with temperature; substances with high vapour pressure evaporate easily, whereas those with low vapour pressure are less prone to vaporisation.

Water vapour is a common example, forming part of the atmospheric mixture and contributing to humidity and weather phenomena. Steam refers specifically to water vapour produced by boiling water. Other examples include ethanol vapour, acetone vapour, and iodine vapour, which occur when their liquids or solids are heated or otherwise vaporised.

Applications and considerations include distillation, evaporation in industrial processes, fragrance and aroma dispersion, and the presence of volatile organic compounds in air. Vapours can be hazardous or flammable, and proper ventilation and safety measures are important in contexts involving inhalation risk or flammable vapours. The term vapour is the British English spelling; the American form is vapor.