The pressure exerted by a vapor escaping from a liquid. It quantifies the tendency of molecules to enter the gaseous phase. The vapor pressure of water increases as temperature increases and reaches one atmosphere pressure (760 mm Hg or 14. 7 psia) at the boiling point (100掳C or 212掳F). The activity of an aqueous solution is the ratio of vapor pressures: a_w = p/p_o, where p = vapor pressure of a solution and p_o is vapor pressure of pure water. Since this is a ratio of vapor pressures, activity is not a strong function of temperature.