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What is Vapour Pressure?

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October 14, 2008 1:49PM

Vapour pressure is the pressure of the steam of a substance at a certain temperature.

This does not automatically mean that the substance is completely evaporated. When the vapour pressure reaches the surrounding (atmospheric) pressure, the substance starts to boil or sublimate.

Also at very low temperatures, even when the substance is in solid state, a certain amount of substance already exists as vapour. Though, for the most substances, the proportion of vapour is very small compared to the solid portion.

Finally, a substance does not have to melt before it evaporates. This is called sublimation and occurs, e.g., when solid carbon dioxide (dry ice) is used: It will evaporate directly from the solid state (under normal ambient physical conditions), because, with rising temperature, the vapour pressure is already exceeding the atmospheric pressure before the melting point is reached.

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