I have browsed internet to understand the concept of vapour pressure but something isn't linking for me. How do we relate vapour pressure to temperature for liquid only and any simplified definition of vapour pressure would be more than welcome! Can we relate vapour pressure to critical temperature in any way? Thanks in advance.....


Molecules of liquid, similarly as molecules of gas, have a particular statistical distribution of energy.

Molecules of liquid bond each other by intermolecular bonds, that deny them to escape from liquid to gas and become free.

Well, it is denied for most of them. At any temperature, there always exists a fraction of molecules with high enough energy to break bonds and escape.

This fraction increases with temperature of liquid, approximately exponentially.

If you put liquid to a closed container with vacuum, the escaping molecules start forming a gas with growing pressure. That is the vapour pressure.

When a molecule of such a gas hits the liquid, it is trapped by the bonds and stays in the liquid. The rate of this process is proportional to the vapour pressure.

At the particular vapour pressure, dependent on temperature, the rate of escaping liquid molecules ( evaporation rate ) equals to rate of trapping of molecules ( condensation rate ) and the liquid evaporation ( and condensation ) rate is zero.

This vapour pressure is called the saturated vapour pressure ( for given liquid at given temperature ).

This saturated vapour pressure does not depend on presence of other gases, like air, where it is called the saturated vapour partial pressure.(with partial often omitted)

The partial pressure is the contribution of a particular gas compound to the total gas pressure, proportional to its molecular fraction within the gas.

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