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Philip Wood
#7
Feb4-13, 12:39 PM
PF Gold
P: 956
Boiling is a very special case of evaporation. It involves bubbles of vapour forming below the surface of the water. The bubbles usually form on minute concavities on the inside of the vessel below the liquid level, or on lime-scale or other solids below the water level. [The water is evaporating into minute air pockets in these concavities.] The bubbles are able to grow, and then break away and rise to the surface, when the saturated vapour pressure inside them is equal to the external pressure on them. This pressure is that of the atmosphere (about 101000 Pa) plus the extra pressures (usually negligible) of the liquid column above them and of surface tension at the bubble surface. So at what temperature is the svp of water equal to 101000 Pa? 100C !

Ordinary evaporation from the exposed surface of a liquid is the unco-ordinated escape of individual molecules. They do not have to fight against atmospheric pressure, except inasmuch as they will collide with the molecules of air once they have escaped. So ordinary evaporation does not require the svp of the liquid to equal atmospheric pressure, as it does not involve bubbles of vapour having to grow and escape through the liquid against atmospheric pressure. Therefore ordinary evaporation can - and does - occur at much lower temperatures than 100C. Even clothing put out to dry on a washing line in the open air in temperatures below 0C will dry, though very slowly!