Water activity (aw) is a measure of the availability of water in a substance for microbial growth, chemical reactions, and biochemical processes. It is defined as the ratio of the partial vapor pressure of water in a material to the partial vapor pressure of pure water at the same temperature. Water activity values range from 0 to 1, with pure water having an aw of 1.0.
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Water activity measures the chemical potential of water relative to a chosen pure-water standard state. Under ideal-vapour conditions above a flat solution, it is the equilibrium vapour pressure divided by that above pure liquid water at the same temperature. Dissolved solute usually lowers activity. Raoult's law gives the ideal dilute-solution limit; nonideal interactions require an activity model.