Why does adding salt lower the freezing point of water?Watch
A proper explanation requires the use of chemical potentials... Are you actually an undergraduate student? I ask because so many A-level students put undergrad because they think their problem is just hard.
I would have said that the hydrolysed ions prevent the hydrogen bonding lattice from forming.
Comes about from considering this system. Obv a substance boils when the chemical potential is the same as the chemical potential of the vapour phase. Adding a solute lowers the chemical potential (see the equation for the chemical potential of a solution) of the liquid phase and hence a higher temperature is needed to boil the solvent.
Equally, the lower chemical potential of the solution means the solution needs to be cooled more than pure solvent as the boundry between the solid and liquid phase happens at a lower temp.
Afraid I don't have my lecture notes atm so I can't dig up the actual derivation but it's explained fairly well here (with a nice graph!)