How does a solute affect the freezing point?
The key to understanding this effect is that the solute is present in the liquid solution, but not in the pure solid solvent.
Example: think of pure ice cubes floating in salt water. The temperature would have to be lowered somewhat below
At the freezing point of any solution the rates of melting and freezing are exactly balanced, allowing both phases to coexist in a thermodynamically stable state. The introduction of a solute reduces the activity of the liquid phase solvent, thereby reducing the rate of freezing.
You can think of this reduction in activity as solute molecules "getting in the way" of solvent molecules from attaining the correct alignment for freezing at the surface. On the other hand, the rate of melting is unaffected by the presence of solute because this is simply the rate of liberation from the frozen surface into the liquid phase.
In the presence of solute, it is necessary to lower the temperature to slow down the rate of melting so that it once again matches the rate of freezing. This is why the freezing point is lowered, or depressed, by the presence of solute in the liquid phase.