It's winter. The sidewalks are icy. Salting the walkways makes the ice melt. Antifreeze keeps the coolant in the car radiator from freezing. How does this work? Antifreeze is very different from salt, but the chemical properties of these materials are not responsible for the effect they have on freezing. The very presence of particles in the water other than water molecules interferes with the orderly organization of water molecules into ice crystals, lowering the freezing point. The more non-water particles there are, the more the freezing point is lowered. The effect of solute on a solvent due to the number of particles is called a colligative property. This PowerPoint presentation explains what colligative properties are and explains the causes of freezing point depression and boiling point elevation. The differences in the effect of electrolytes and nonelectrolytes is discussed. Sample quantitative problems are provided and explained. Worksheet on this topic are available free of charge on my website at www.evanschemistrycorner.com.