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(US), vapor
1. particles of moisture or other substance suspended in air and visible as clouds, smoke, etc.
2. a gaseous substance at a temperature below its critical temperature
3. a substance that is in a gaseous state at a temperature below its boiling point



the gaseous state of matter that arises when the gas phase of a substance is in equilibrium with the liquid or solid phase of the same material. As a rule, the term “vapor” is used when a phase equilibrium is achieved at temperatures and pressures that are characteristic of natural occurring conditions. For example, one speaks of the vapors of alcohol, benzene, iodine, and naphthalene. However, the term “vapor” is also used to refer to the gaseous state of water above the critical values of temperature and pressure, even though such values are not obtained under spontaneous, natural conditions. Conversely, CO2 is referred to as a gas below the critical temperature of 31.04°C, even though by strict usage gaseous CO2 is a vapor, since it is found in the gaseous state under spontaneous, natural conditions (seeCRITICAL STATE). The terms “gas” and “vapor” are interchangeable in the terminology of the thermodynamics of phase transformations.

The vapor of a chemically pure substance is saturated at the saturation temperature and pressure and unsaturated at temperatures above the saturation temperature when the value of pressure is fixed. Unsaturated vapor, which in industrial terminology is called superheated steam or, simply, gas, is less dense than saturated vapor. A vapor is supersaturated when the pressure is raised above the saturation pressure while the temperature is kept constant.



A gas at a temperature below the critical temperature, so that it can be liquefied by compression, without lowering the temperature.