Temperature at which the pressure exerted by the surroundings upon a liquid is equalled by the pressure exerted by the vapour of the liquid; under this condition, addition of heat results in the transformation of the liquid into its vapour without raising the temperature. At any temperature a liquid partly vaporizes into the space above it until the pressure exerted by the vapour reaches a characteristic value called the vapour pressure of the liquid at that temperature. As the temperature is increased, the vapour pressure increases; at the boiling point, bubbles of vapour form within the liquid and rise to the surface. The boiling point of a liquid varies according to the applied pressure; theNormal boiling point is the temperature at which the vapour pressure is equal to the standard sea-level atmospheric pressure (101.325 kPa (1 atm, 760 mmHg, 760 Torr, 14.7 psia, 29.92 inches of Hg). At sea level, water boils at 100°C (212°F). At higher altitudes the temperature of the boiling point is lower. For methods of determination, see: Normal Boiling Point. If the boiling temperature is not measured at normal atmospheric pressure, the temperature dependence of the vapour pressure can be described by the Clausius- Clapeyron equation: log(pSAT) = (?vH/R.T) + const. Where: p = the vapour pressure of the substance in pascals, ?vH = its heat of vaporization in J/mol, R = the universal molar gas constant = 8.314 J/mol.K. and T = thermodynamic temperature in K