(a) In ethanol, molecules are associated through intermolecular H-bonding. On adding acetone it enters between the host molecules and breaks some of the H-bonds resulting in positive deviation from Raoult’s law. This reaction is endothermic (ΔHmix = positive) in nature and thus a decrease in temperature is observed.
(b) Potassium chloride solution freezes at a lower temperature than water because when a non volatile solute like KCl is added to water, it will result into lowering of vapour pressure of solution than that of pure solvent. Now the solution will freeze at a temperature lower than the freezing point of pure solvent and this is called depression in freezing point.
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