These water-binding effects reduce the vapour pressure of the food substrate according to Raoult’s Law. Comparing this vapour pressure with that of pure water (at the same temperature) results in a ratio called water activity (aw). Pure water has an aw of 1, one molal solution of sugar - 0.98, and one molal solution of sodium chloride - 0.9669. A saturated solution of sodium chloride has a water activity of 0.755. This same NaCl solution in a closed container will develop an equilibrium relative humidity (ERH) in a head space of 75.5%. A relationship therefore exists between ERH and aw since both are based on vapour pressure.