Colligative Properties: Vapor Pressure Lowering

Colligative Properties

• Colligative properties are properties of a solution that depend on the concentration of the solute, not its identity.
• Examples of colligative properties include vapor pressure lowering, boiling-point elevation, and freezing-point depression.

Vapor Pressure Lowering

• Nonvolatile solutes reduce the ability of surface solvent molecules to escape the liquid, resulting in a lower vapor pressure.
• The amount of vapor pressure lowering depends on the number of solute particles.
• Solute particles take up space in a solution, decreasing the number of solvent particles on the surface, and subsequently reducing the number of solvent particles that can evaporate from the surface.

Boiling-Point Elevation

• The boiling point of a liquid is the temperature at which the vapor pressure of the liquid phase equals the atmospheric pressure.
• Adding a nonvolatile solute to a liquid solvent decreases the vapor pressure of the solvent, requiring additional kinetic energy to raise the vapor pressure of the liquid phase to atmospheric pressure and initiate boiling.

Freezing Point Depression

• Adding a solute causes the freezing point of the solvent to become lower.
• The amount the freezing point is lowered is directly related to the amount of solute present.
• The freezing-point depression is the difference between the freezing points of the pure solvent and a solution in that solvent.

Osmosis

• Osmosis is the diffusion of water from an area of high water concentration to an area of low water concentration.
• Osmosis is the minimum pressure needed to prevent the flow of solvent molecules through a semipermeable membrane.
• Solvent molecules flow across the membrane to equalize concentration, resulting in an increase in the level of the solution.
• The pressure required to stop the osmosis is called the osmotic pressure.

Reverse Osmosis

• Reverse osmosis occurs when a pressure greater than the osmotic pressure is applied.
• This process is used to purify water, such as in the treatment of brackish water or sea water.
• The semipermeable membrane used in reverse osmosis is typically made of cellulose acetate or polymer.