Raoult’s Law is a principle in chemistry that explains how the vapor pressure of a solution changes when a non-volatile solute is added to a solvent.
Definition:
Raoult’s Law states that the vapor pressure of a solvent in a solution is directly proportional to the mole fraction of the solvent present in the solution.
Key Points:
- When a non-volatile solute (like sugar or salt) is dissolved in a solvent (like water), the vapor pressure of the solvent decreases.
- The decrease happens because solute particles take up space at the liquid surface, reducing the number of solvent molecules that can escape into the vapor phase.
- The relation is expressed as:
- P₁ = X₁ × P₁°
- Where:
- P₁ = vapor pressure of solvent in solution
- X₁ = mole fraction of solvent
- P₁° = vapor pressure of pure solvent
Examples:
- Adding salt to water lowers its vapor pressure; this is why saltwater boils at a higher temperature than pure water.
- In industries, Raoult’s Law is used in processes like distillation and to explain colligative properties (boiling point elevation, freezing point depression, osmotic pressure).
In short: Raoult’s Law explains how dissolving a solute in a solvent lowers the solvent’s vapor pressure, and the effect depends on the mole fraction of the solvent.