(a) When ethanol is mixed with acetone, it shows positive deviation from Raoult's law and acetone molecules get in between the host molecules and break some hydrogen bonds, which requires higher energy than energy released in the formation of new hydrogen bonds. This results fall in temperature.
(b) According to Raoult's law, when a non-volatile solid is added to the solvent, its vapour pressure decreases resulting in decrease in freezing point. Also, the freezing point of water is higher than when water contains KCl.