Volatility in the context of chemistry, physics and thermodynamics is a measure of the tendency of a substance to vaporize. It has also been defined as a measure of how readily a substance vaporizes. At a given temperature, substances with higher vapor pressures will vaporize more readily than substances with a lower vapor pressure.

Although usually applying to liquids, volatility can apply to solid materials such as dry ice and ammonium chloride, which can change directly from solid to vapor without becoming liquid. The direct transition of a solid to a vapor is called sublimation.

Vapor pressure is the pressure of a vapor which is in equilibrium with its non-vapor phases (i.e., liquid or solid). Most often the term is used to describe a liquid's tendency to evaporate. It is a measure of the tendency of molecules and atoms to escape from a liquid or a solid. A liquid's atmospheric pressure boiling point corresponds to the temperature at which its vapor pressure is equal to the surrounding atmospheric pressure and it is often called the normal boiling point.

The higher the vapor pressure of a liquid at a given temperature, the higher the volatility and the lower the normal boiling point of the liquid.

See also Reid vapor pressure.

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