Q1(i). Pressure controlling boiling

Answer in brief

Boiling occurs when vapour pressure can balance the pressure actually imposed on the liquid and its vapour bubbles; the overlying or superincumbent pressure is the relevant pressure.

Key points

• A liquid boils when its equilibrium vapour pressure at that temperature equals the effective confining pressure (neglecting small hydrostatic and surface-curvature corrections).
• In an open vessel, the overlying pressure is atmospheric pressure, so ordinary external atmospheric pressure does determine the boiling point.
• In a closed or pressure-controlled vessel, the pressure directly over the liquid can differ from the ambient pressure outside the vessel.
• Thus the quoted distinction must not be read as saying that external pressure never affects boiling.