Question

what is vapour pressure of a liquid . on what factors does it depends?

Answer 1

The pressure exerted by the vapours of any liquid is called its vapour pressure.

Suppose a beaker is partially filled with any liquid. Then the molecules of that liquid at its surface will start evaporating on their own even without heating. This happens because the molecules at surface behave differently than those in the bulk. Now these molecules get converted into vapours which exert some pressure on the walls of the beaker, which is the vapour pressure of that liquid.

Vapour pressure of any liquid depends upon the following factors:
(i) Temperature: As the temperature of the liquid is raised, more molecules get evaporated and thus exert more pressure. So vapour pressure will increase.
(ii) Intermolecular forces: If the intermolecular attractions between the molecules of liquid are stronger, then its vapour pressure would be low as it would be difficult to convert that liquid into vapours.
(iii) Addition of solute: Addition of solute raises the boiling point of any liquid, i.e. its will boil at a higher temperature. Thus it will lower the vapour pressure of that liquid.