The document discusses colligative properties of nonelectrolyte solutions, specifically vapor pressure lowering. It defines colligative properties as those that depend on the number of solute particles but not their identity. The four colligative properties are listed as vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. Vapour pressure lowering occurs when a nonvolatile solute is dissolved, reducing the number of solvent molecules available at the surface for vaporization compared to pure solvent. This leads to a lower vapor pressure. The extent of lowering depends on the mole fraction of solute and is thus a colligative property. Raoult's law and calculations using mole fractions and molar masses are also