Vapor is the gaseous state into which solids and liquids pass when heated. In a technical sense, both oxygen and steam are vapors. But it is possible to make a distinction between gases and vapors. Gases, such as oxygen, remain gases when compressed at ordinary temperatures. Vapors, such as steam, resume their liquid or solid state under high pressure at ordinary temperatures. The process of converting a substance from liquid form into a vapor is called vaporization. Evaporation and boiling of liquids are forms of vaporization. In evaporation, the change to a vaporous condition takes place slowly. In boiling, it occurs very rapidly. The formation of vapor directly from a solid state is called sublimation. Vaporization in connection with atmospheric conditions bears an important relation to climate. Water vapor is always present in the air. When condensed under varying conditions, water vapor forms clouds, dew, rain, and snow. See also Boiling point ; Evaporation ; Gas .
