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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; the Normal 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:
p = the vapour pressure of the substance in pascals
ΔvH = its heat of vaporization in J?mol-l
R = the universal molar gas constant = 8.314 J?mol-l?K-l
T = thermodynamic temperature in K