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.