Water activity
= Water activity
{title2=$\mu_w=\mu_w^\circ+RT\ln a_w$}
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.