The process by which molecules escape from a liquid into the gas phase is called

• A. evaporation
• B. sublimation
• C. boiling
• D. condensation

Answer: (A)

Escape of molecules from a liquid into the gas phase happens at the surface when some molecules have enough energy to break away from the liquid’s intermolecular forces. This process is evaporation. It can occur below the liquid’s boiling point and only at the surface, so the liquid doesn’t have to reach a uniform boiling temperature. Increasing temperature, increasing surface area, or reducing surrounding pressure speeds evaporation by giving more molecules the energy to escape or by letting vapor leave more easily. In contrast, boiling is when vapor forms throughout the liquid once the temperature reaches the boiling point and the vapor pressure matches the surrounding pressure. Sublimation is a direct solid-to-gas transition, and condensation is gas-to-liquid. So the described process is evaporation because it involves surface escape of liquid molecules into the gas phase, typically below the boiling point.