Colligative properties are physical properties of a solution that depend solely on the number of solute particles present, regardless of their identity or nature. These properties are primarily applicable to solutions, which are homogeneous mixtures of a solvent and one or more solutes.
The four main colligative properties are:
1. Lowering of Vapor Pressure: When a non-volatile solute is added to a solvent, the vapor pressure of the solvent decreases. This is because the solute particles occupy space on the liquid surface, reducing the number of solvent molecules that can escape into the vapor phase. Raoult's law describes this relationship mathematically.
2. Elevation in Boiling point: The boiling point of a solvent increases when a non-volatile solute is dissolved in it. This occurs because the presence of solute particles disrupts the formation of vapor bubbles, requiring a higher temperature for the liquid to reach its boiling point. The extent of boiling point elevation is proportional to the concentration of solute particles in the solution.
3. Depression in Freezing point: The freezing point of a solvent decreases when a non-volatile solute is added to it. This happens because the solute particles disrupt the formation of the solvent's regular crystal lattice, making it more difficult for the solvent to solidify. The extent of freezing point depression is directly proportional to the concentration of solute particles in the solution.
4. Osmotic pressure: Osmosis is the movement of solvent molecules from an area of lower solute concentration to an area of higher solute concentration through a semi-permeable membrane. Osmotic pressure is the pressure required to prevent osmosis from occurring. It depends on the concentration of solute particles in the solution and follows the van 't Hoff equation.
These colligative properties are useful for determining the molecular weight of a solute, as they provide a means of indirectly measuring the number of solute particles present in a solution. By comparing the observed changes in these properties with the expected changes based on the ideal behavior of solutions, one can calculate the molar mass or molecular weight of the solute. |
