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Water condenses into visible droplets (aerosol) after evaporating from a cup of hot tea

In physics a vapor (American English spelling) or vapour (British) is a substance in the gas phase at a temperature lower than its critical point,<ref name=Petrucci>R. H. Petrucci, W. S. Harwood, and F. G. Herring, General Chemistry, Prentice-Hall, 8th ed. 2002, p. 483–86.</ref> which means that the vapor can be condensed to a liquid by increasing its pressure without reducing the temperature. Vapor is a different substance than aerosol.<ref name=Cheng2014/> Aerosol is a suspension of tiny particles of liquid, solid or both within a gas.<ref name=Cheng2014>{{#invoke:Citation/CS1|citation |CitationClass=journal }}</ref>

For example, water has a critical temperature of 374 °C (647 K), which is the highest temperature at which liquid water can exist. In the atmosphere at ordinary temperatures, therefore, gaseous water (known as water vapor) will condense to liquid if its partial pressure is increased sufficiently.

A vapor may co-exist with a liquid (or solid). When this is true, the two phases will be in equilibrium, and the gas partial pressure will equal the equilibrium vapor pressure of the liquid (or solid).<ref name=Petrucci/>


Vapor sections
Intro   Properties   Vapor pressure   Examples   Measuring vapor  See also  References  

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