Colligative Properties Versus Pure Solvent
A pure solvent is when you only have the solvent. For example, when you only have water. When discussing colligative properties, these are the properties of a solution that are different from that of the pure solvent. Adding a solute (it does not matter what the solute is- salt, sugar, etc...) to a solvent will change the following properties of the solution:
- Boiling Point Elevation
- Freezing Point Depression
- Vapor Pressure Lowering
- Vapor Pressure Lowering: Vapor pressure is the force exerted by a gas that lies above the solution in a sealed system. In a pure liquid, there is a point when molecules on the surface of the liquid evaporate. At the same time, molecules that are in the gaseous phase will condense into a liquid. When the rates of these two processes are equal, the system is in equilibrium. The pressure on the surface of the solvent at that equilibrium point is called vapor pressure, and the vapor pressure is lowered when a solute is added.
- Freezing Point Depression: The more solute particles in a solution, the lower the freezing point. This is because the orderly pattern of molecules in a solid is disrupted by the solute molecules. For example, putting salt in water lowers the freezing point of water to below 0 degrees C. Next, you can calculate the exact values. Simply, multiply the molality of the solution by the freezing point constant for the solvent.
- Boiling Point Elevation: The more solute particles in solution means a higher boiling point. Some solute molecules sit at the surface of the solution and prevent the solvent molecules from escaping into the air, requiring more energy for the liquid to boil. For example, engine coolant has a solute added to water to raise the boiling point of the solution, so it does not boil over. Next, you can calculate the exact value. Simply, multiple the molality of the solution be the boiling point constant for the solvent.
Try the following simulation. Which colligative property is occurring?