Freezing Point Depression

Freezing point depression means that a solution will freeze at a lower temperature than a pure liquid.

For instance, ocean water freezes at a lower temperature than pure water.

The reasons behind this have to do with the structure of ice and the arrangement of water molecules and solute particles in a solution.

Looking at the molecules of water in an ice crystal we see that they are arranged in a very specific 3 dimensional arrangement.

When a solute is dissolved in water, the molecules are arranged very differently - specifically they are arranged around the solute particles.

This means that in a solution, the waters are not arranged "correctly" to form ice. As a result, the waters must be pulled away from the solute particles, so that they can join the forming ice crystal.

To make that happen, the water-water attraction needs to be strong enough to pull molecules away from the solute particles. To make that attraction stronger, we need to slow the particles down (South Street Effect). That means we need to make it colder.

This same effect helps us understand why we put salt on icy sidewalks and streets.

When a crystal of salt falls onto ice, the attraction between the ions and the molecules of water can pull molecules out of the ice crystal, melting it.

The math:

The more solute that is added to a solution, the lower the freezing point goes. In other words, the change in the freezing point is proportional to the concentration of the solution. Specifically, the change in the freezing point can be calculated with the formula:

where &Delta: T is the change in the freezing point, k_f is the freezing point depression constant (which depends on the solvent), m is the molality, and i is the vant Hoff factor.