Colligative Properties of Solutions

Key Concepts

• Colligative properties of solutions depend on the concentration of solute particles but NOT on their identity.
• Colligative properties depend on the lowering of the escaping tendency of solvent particles by the addition of solute particles.
• Colligative properties include:

1. vapor pressure lowering
2. boiling-point elevation
3. freezing-point depression
4. osmotic pressure

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Vapor Pressure Lowering

• The escaping tendency of a solvent is measured by its vapor pressure.
• Vapor pressure measures the concentration of solvent molecules in the gas phase.
• Adding a nonvolatile solute lowers the vapor pressure of the solvent since a smaller proportion of the molecules at the surface of the solution are solvent molecules, fewer solvent molecules can escape from the solution compared to the pure solvent.
• The quantitative relationship between vapor pressure lowering and concentration in an ideal solution is stated in Raoult's Law.

Boiling-Point Elevation

• A liquid boils at the temperature at which its vapor pressure equals atmospheric pressure.
• The presence of a nonvolatile solute lowers the vapor pressure of a solution so it is necessary to heat the solution to a higher temperature in order for it to boil.
• The amount by which the boiling point is raised is known as the boiling point elevation.
• The boiling-point elevation is proportional to the concentration of solute particles expressed as moles of solute per kilogram of solvent.

Freezing-Point Depression

• The presence of a nonvolatile solute lowers the freezing point of a solvent.
• In order to freeze the solvent, it must be cooled to a lower temperature in order to compensate for its lower escaping tendency.
• The amount by which the freezing point is lowered is known as the freezing point depression.
• The freezing-point depression is proportional to the concentration of solute particles expressed as moles of solute per kilogram of solvent.

Osmotic Pressure

• When two liquids, such as a solvent and a solution, are separated by a semipermeable membrane that allows only solvent molecules to pass through, then there is a net transfer of solvent molecules from the solvent to the solution. This process is called osmosis.
• Osmosis can be stopped by applying pressure to compensate for the difference in escaping tendencies. The pressure required to stop osmosis is called osmotic pressure.
• In dilute solutions, osmotic pressure is directly proportional to the molarity of the solution and its temperature in Kelvin.