Question

give me a hypothesis about How adding salt affect the freezing point of water?

Answer 1

salt will interrupt the crystal structure of ice and therefore lowering the freezing point of water

Answer 2

It can be defined on the basis of colligative property.
The freezing point is the temperature at which its solid and liquid phase has the same vapor pressure. The freezing point of a pure liquid is fixed, if we add nonvolatile solute and in your case it is salt, then there occur a lowering of vapor pressure of pure liquid (here it is water) and it becomes equal to the vapor pressure of solvent in solid state at a lower temperature....
Hence the freezing point is lowered.