CBSE Class 12 Chemistry Notes: Solutions – Vapour Pressure

Vapour Pressure of a Liquid

• Vapour pressure is the pressure exerted by vapours when they are in equilibrium with the liquid phase at a given temperature. It depends on the nature of the liquid and temperature.
• Vapour pressure of pure liquid is always greater than the vapour pressure of its solution.
• Vapour pressure of a liquid helps us to have an idea of forces of attraction between the molecules of a liquid. More the force of attraction, lower is the vapour pressure and vice versa.
• Vapour pressure of a liquid increases with increase in temperature due to increase in kinetic energy of molecules.
• The variation of vapour pressure with temperature can be given by Clausius­Clapeyron equation as follows:

Vapour Pressure of a Solution

• When a miscible solute is added to a pure solvent, it results in the formation of solution. As some molecules of solute replace the molecules of the solvent from the surface, the escaping tendency of solvent molecules decreases. This causes a lowering of the vapour pressure.
• The vapour pressure of a solution is less than that of pure solvent.
• If the vapour pressure of a solvent is p° and that of solution is p_s then, lowering of vapour pressure = p° – p_s.
• The vapour pressure of a solution decreases as the surface area occupied by the solvent molecules decreases and density increases.

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