Freezing Point

The number of particles dissolved in a solution significantly affects both boiling and freezing points. (For more information, click on the Boiling & Freezing Points button.)

Every gram of solute particles added to water lowers the freezing point by a precise amount. Specifically, adding 1 gram molecular weight of nonionizing solute (like sugar) to 1 liter of water lowers the freezing point by -1.86 degrees Celsius (C).

1 gram molecular weight nonionizing solute per liter = -1.86 degrees C decrease in freezing point

This formula remains constant as solute is added. The more solute is added and dissolved, the more concentrated the solution becomes and the lower the freezing point.

Compared to a nonionizing solute, an ionizing solute (like salt) has a far greater impact on the freezing temperature of water. The amount that the freezing temperature lowers depends on the number of particles into which the ionizing solute dissolves. For example, using salt (which divides into two particles) lowers the freezing temperature of water twice as much as a nonionizing solute (like sugar) -- by -3.72 degrees C per gram molecular weight of the solute per liter of water. An ionizing solute that divides into three particles would lower the freezing temperature of water three times lower than a nonionizing solute, and so on.

1 gram molecular weight ionizing solute per liter = -1.86 degrees C decrease in freezing point per particle

That's because each salt compound dissolves into two particles, which doubles the decrease in freezing temperature. This continues until freezing point reaches approximately -18 degrees C, the temperature at which salt is no longer soluble.

For more information, click on Making Sherbet.

Click on another colligative property in the left-hand column to continue. When you're finished, click on the Continue button.