Pressure exerted by vapor in equilibrium with liquid.

• A. Partial pressure
• B. Atmospheric pressure
• C. Vapor pressure
• D. Osmotic pressure

Answer: (C)

Vapor pressure is the pressure exerted by the vapor when it is in equilibrium with its liquid. At a given temperature, molecules continually evaporate from the liquid surface and condense back from the vapor. When the rates of evaporation and condensation balance, the vapor above the liquid reaches a constant pressure—the vapor pressure. This value depends on temperature and the liquid’s intermolecular forces: weaker forces make it easier for molecules to escape, giving a higher vapor pressure; stronger forces make it harder, giving a lower vapor pressure. Boiling happens when the external pressure equals this vapor pressure, so the liquid can form vapor freely. Atmospheric pressure is the total pressure from the surrounding air, not the pressure of the vapor itself. Osmotic pressure is tied to solvent flow across membranes driven by solute concentration, not to vapor in equilibrium with a liquid. Partial pressure refers to the pressure contribution of a component in a gas mixture; for a liquid in equilibrium with its vapor, the relevant quantity is the vapor pressure of that liquid.