Vapour Pressure

The vapor pressure (P°) is the pressure of the vapor of a compound in equilibrium with its pure condensed phase (solid or liquid). Vapour pressures depend strongly on the temperature and vary widely with different compounds due to differences in molecule – molecule interactions. The normal boiling point of a liquid is defined as the temperature at which the vapor pressure of the liquid is 1 atmosphere (P° = 1 atm). The vapour pressure of a substance is an intrinsic physical property that plays a crucial role in determining it’s distribution to and from gaseous environmental phases (the atmosphere, marsh bubble gas). The vapour pressure is also crucial for the prediction of equilibrium distribution coefficients to and from natural waters, such as Henry’s Law constant (KH).