I was just learning about vapor pressure and got very confused. Vapor pressure was defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system, which was the pressure the vapor exerts on the closed container when it reaches the dynamic equilibrium. However, this is merely a calculation AFTER it reaches equilibrium, and I don't understand how vapor pressure can also be interpreted as the measure of the tendency of a material to change into the vapor state. Also, if it is the lack of enough vapor pressure, or "tendency of the liquid to change to vapor" compared to the atmospheric pressure that's keeping the liquid from vaporizing, how can evaporation occur with lower pressure than the atmospheric pressure(because if the vapor pressure is higher than atmospheric pressure it is already boiling), since evaporation is supposed to be the molecules on the surface of the liquid randomly gaining enough energy to overcome the atmospheric pressure, implying that it has greater pressure than the atmosphere. I feel like I am overthinking the topic, but I didn't just want to give up thinking about it, so please help if possible!


1 Answer 1


Vapour pressure is not merely calculation, it is easily measureable.

Evaporation is due molecules leaving liquid not by overcoming atmospheric pressure, but by overcoming cohesive intermolecular forces. The saturated vapour pressure is a measure of the rate of this massive leaving of molecules.

Boiling is just a special evaporation case. Above boiling point, vapour is always unsaturated and liquid evaporate at its surface regardless of gas composition. Additionally, random processes create miniature bubbles ( or these may be bubbles of formerly dissolved gasses ), that have the same or higher pressure then external pressure and expands themselves.

The result is more or less massive and turbulent evaporation with creation of vapour bubbles.

Liquids can be frequently overheated above their boiling point, if there are absent "bubble seeds". Once available, e.g. by strirring or repulsion of dissolved gas, creation of vapour bubbles quickly cools liquid down to its boiling point.


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