•    All liquids vapourise or evaporate due to the molecules escaping from the free surface.

•    When the liquid is contained in a closed vessel, the vapour molecules get accumulated in the space between the free surface and the top of the vessel.

•    This accumulated vapor of the liquid exerts a partial pressur on the liquid surface is known as vapor pressur.

•    If the pressure on liquid is equal to or less than vapor pressure, it starts boiling or vaporize.

•    Higher the vapor pressure more volatile is the liquid, For example vapor pressure of petrol at 20⁰C is 30.4 Kn/m² while that of water is 2.345 Kn/m², hence petrol vaporizes faster than water.

•    As the temperature is increased, the vapor pressure is also increased.

•    Mercury has very low vapor pressure and high density. Therefore, it is used as a numeric liquid.

Vapur pressure = Force/Area

S.I.unit: N/m² or ML^-1T^-2

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