Question

what happened when an impurity is added to a pure solvent?

Answer 1

When an impurity is added to a pure solvent then elevation in boiling point and depression in freezing point takes place.

Impurity that is generally non volatile when mixed with volatile pure solvent it decreases the vapour content thus decreases the vapour pressure.Hence temperature has to be increased to increase the vapour pressure equal to atmospheric pressure.Hence the boiling point increases because boiling point is the temperature at which vapour pressure is equal to the atmospheric pressure.

Depression in freezing point occurs due to presence of impurities because when solvent particles freezes they leave behind solute particles and then solute - solute interactions becomes predominant.Thus entropy is reduced and hence freezing point gets depressed.