Vapor

Vapor is the gaseous state of a substance when that substance is at a temperature and pressure at which it normally exists as a liquid or solid. A common example is water vapor, which is water converted to gas. Vapor differs from ordinary gases such as oxygen in that it is near the conditions at which it can become liquid; consequently, an increase in pressure or decrease in temperature can return it to liquid form.

The process by which a liquid becomes vapor is called evaporation if it occurs at the surface of the liquid, or boiling if it occurs throughout the liquid when the vapor pressure equals atmospheric pressure. The reverse process is condensation, in which vapor returns to liquid form when cooled below the surface.

Vapor pressure

When a liquid is enclosed in a sealed container, some of it converts to gas until equilibrium is reached: the number of molecules leaving equals the number returning. The pressure exerted by vapor in this state is called saturated vapor pressure. It increases with temperature. Liquids with high vapor pressure—such as alcohol and ether—evaporate rapidly and are known as volatile.

Water vapor and air humidity

The air of the atmosphere always contains some water vapor. Relative humidity measures how much water vapor the air contains compared to the maximum it can hold at that temperature. When air cools to the dew point, vapor condenses into liquid water droplets, producing mist, dew, and clouds.

Industrial use

Water vapor under pressure (steam) is used to transfer energy: steam engines and power plant turbines convert heat energy into rotational motion. Similarly, the chemical separation process called distillation relies on substances in a mixture evaporating at different temperatures.