Vapor Pressure & Boiling | mysimulator.uk

A dynamic equilibrium, not a static one

In a closed container, molecules at a liquid's surface are constantly evaporating into the space above it, while vapor molecules are simultaneously condensing back into the liquid. Equilibrium vapor pressure is simply the pressure at which those two opposing rates happen to be equal — a dynamic balance, not a static, unmoving state. Individual molecules keep switching between phases continuously even once equilibrium is reached.

Boiling: when vapor pressure meets the outside pressure

Boiling begins at the exact temperature where a liquid's equilibrium vapor pressure equals the ambient external pressure pressing down on it. At that point, vapor bubbles can form and grow throughout the liquid's whole volume, not just evaporate slowly from the exposed surface — that's the defining physical difference between simple evaporation and true boiling. The familiar 100°C boiling point of water is specifically the temperature at which its vapor pressure reaches 1 atmosphere.

The Clausius-Clapeyron equation

The Clausius-Clapeyron relation connects how a liquid's vapor pressure changes with temperature to its molar enthalpy of vaporization, treating the vapor as an ideal gas over the liquid-vapor coexistence line:

dP/dT = ΔH_vap / (T ΔV)     (Clapeyron equation)

Integrated (ideal-gas, ΔH_vap ~ constant over a small range):
  ln(P₂/P₁) = -(ΔH_vap / R) · (1/T₂ - 1/T₁)

Given a liquid's vapor pressure at one known temperature and its enthalpy of vaporization, this lets you predict its vapor pressure — and therefore its boiling point — at any other temperature, or under any other external pressure.

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Why altitude changes the boiling point

Atmospheric pressure falls steadily with altitude, so a liquid needs a lower vapor pressure — reached at a lower temperature — to match the surrounding air pressure and start boiling. At roughly 3,000 metres, water can boil at around 90°C instead of the sea-level 100°C, which is exactly why food takes noticeably longer to cook properly at high altitude.

P_ext < 1 atm   →   lower boiling point   (high altitude)
P_ext > 1 atm   →   higher boiling point  (pressure cooker)

Pressure cookers do the opposite

A sealed pressure cooker lets internal pressure climb well above 1 atmosphere as steam accumulates with nowhere to escape. Because boiling still requires vapor pressure to match the surrounding pressure, water inside must reach a higher temperature — commonly around 120°C — before it can boil. Cooking at that elevated temperature speeds up the underlying heat-transfer and chemical processes involved in cooking, cutting typical cooking times substantially.

Frequently asked questions

Is equilibrium vapor pressure a static condition?

No — it's dynamic, not static. Molecules are constantly evaporating from the liquid surface and condensing back from the vapor at the same time; equilibrium vapor pressure is simply the pressure at which those two rates happen to be equal, not a state where nothing is moving. Even at equilibrium, individual molecules keep switching phases continuously.

Why does water boil at a lower temperature at high altitude?

Boiling begins the instant a liquid's equilibrium vapor pressure equals the surrounding external pressure. Atmospheric pressure falls with altitude, so a lower vapor pressure — reached at a lower temperature — is now enough to match it. At roughly 3000 meters, water can boil around 90°C instead of 100°C, which is also why food takes noticeably longer to cook at altitude.

How does a pressure cooker cook food faster?

A sealed pressure cooker lets internal pressure build well above 1 atmosphere as steam accumulates. Because boiling requires vapor pressure to match the surrounding pressure, water inside must reach a higher temperature — commonly around 120°C — before it can boil. Cooking at that elevated temperature speeds up the heat-transfer and chemical reactions involved in cooking food, cutting cooking time substantially.

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Vapor Pressure and Boiling: The Clausius-Clapeyron Equation