The pressure exerted by a vapour in equilibrium with its solid or liquid phase is called vapour pressure.

If a substance is placed a in an evacuated closed container some of it would vapourise. The pressure in the space above the liquid would increase from zero and eventually stabilize at a constant value depending on the temperature.

Vapour pressures increase with temperature. At a higher temperature, more molecules have enough energy to escape from the liquid or solid. At a lower temperature, fewer molecules have sufficient energy to escape.

The types of molecules that make up a solid or liquid also determine its vapour pressure. If the intermolecular forces between molecules are relatively strong, the vapour pressure will be relatively low. For relatively weak intermolecular forces, the vapour pressure will be relatively high.

The surface area of the solid or liquid in contact with the gas has no effect on the vapour pressure.

Units and Measures

The vapour pressure is the equilibrium pressure of vapour above the liquid or solid resulting from evapouration and it is measured using barometer.

Following are the units for measuring vapour pressure:

  • SI Units: Atmospheric pressure (atm).
  • Imperial Units: Torr

Normal atmospheric pressure is defined as 1 atmosphere. 1 atm = 14.6956 psi = 760 torr

Example Values

Vapour pressure at 25°C

  • Diethyl ether - 0.7 atm
  • Bromine - 0.3 atm
  • Ethyl alcohol - 0.08 atm
  • Water - 0.03 atm

Useful References

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