♨️ Vapor Pressure & Boiling — Clausius-Clapeyron Equation

Watch molecules evaporate and condense until vapor pressure matches external pressure — the exact moment boiling begins. Tune temperature and pressure to see boiling points shift with altitude.

ChemistryInteractive
Left: liquid ⇌ vapor equilibrium; bubbles appear once vapor pressure ≥ external pressure · Right: P vs T curve — its crossing with the external pressure line is the boiling point

How it Works

In a sealed container, liquid molecules constantly evaporate from the surface into the space above while vapor molecules condense back into the liquid. At a given temperature, these two rates settle into balance: dynamic equilibrium. The pressure exerted by the vapor at that balance point is the equilibrium vapor pressure, and it grows with temperature following the Clausius-Clapeyron equation, ln(P) = −ΔHvap/R·(1/T) + C, calibrated here against each substance's real boiling point and enthalpy of vaporization.

Boiling is defined precisely as the temperature at which the liquid's vapor pressure equals the external pressure pushing down on it. Below that temperature, vapor can only escape at the exposed surface (evaporation). At and above it, vapor bubbles can nucleate and survive anywhere inside the liquid bulk, rising and escaping rapidly — this is why lowering external pressure (as at high altitude) lowers the temperature at which boiling starts.

Clausius-Clapeyron: ln(P) = −ΔHvap/R · (1/T) + C
Calibrated to boiling point: P(T) = exp[ −ΔHvap/R · (1/T − 1/T_b) ] (P in atm, T in K)
Boiling point at pressure P: T_b(P) = [ 1/T_b − (R/ΔHvap)·ln(P) ]⁻¹
Boiling condition: P_vapor(T) = P_external

Frequently Asked Questions

What is vapor pressure and why does it increase with temperature?

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid in a sealed container, at a given temperature. As temperature rises, more molecules gain enough kinetic energy to escape the liquid surface, so more of them enter the vapor phase and the equilibrium vapor pressure increases — rapidly at higher temperature because the Maxwell-Boltzmann energy distribution has an exponentially growing high-energy tail.

What is the Clausius-Clapeyron equation?

The Clausius-Clapeyron equation, ln(P) = −ΔHvap/R·(1/T) + C, describes how the equilibrium vapor pressure P of a liquid varies with absolute temperature T. Plotting ln(P) against 1/T gives a straight line with slope −ΔHvap/R, which is why the equation is used to measure a substance's enthalpy of vaporization from pressure-temperature data.

What does ΔHvap (enthalpy of vaporization) represent?

ΔHvap is the energy required to pull molecules out of the liquid phase against the intermolecular attractions holding them together, converting them into free-moving gas molecules. A larger ΔHvap means stronger intermolecular forces, so vapor pressure rises less steeply with temperature — the liquid is comparatively "harder to boil".

What is the precise physical definition of boiling?

Boiling is not simply "very hot" — it is the specific condition where a liquid's vapor pressure equals the pressure of its surroundings (the external/atmospheric pressure). Only at that temperature can vapor bubbles form and survive throughout the bulk liquid instead of collapsing under the surrounding pressure.

Why does boiling point change with altitude?

Atmospheric pressure drops with altitude, so a liquid needs a lower vapor pressure — and therefore a lower temperature — to match the external pressure and start boiling. Water boils at about 95°C near 2000 m elevation instead of 100°C at sea level, which is why food takes longer to cook in the mountains.

What is the difference between evaporation and boiling?

Evaporation happens at any temperature, only at the exposed liquid surface, and is relatively slow because only the fastest surface molecules escape. Boiling happens only at the specific boiling-point temperature for the current pressure, occurs throughout the bulk of the liquid via vapor bubbles, and is much faster.

Why do different liquids have different boiling points at the same pressure?

Each liquid has its own combination of intermolecular forces and enthalpy of vaporization, which sets a unique vapor-pressure curve P(T). Liquids with weak intermolecular attractions, like diethyl ether, reach a high vapor pressure at low temperature and boil early; liquids with strong hydrogen bonding, like water, need much more thermal energy before their vapor pressure catches up to atmospheric pressure.

How does a pressure cooker work?

A pressure cooker seals in steam, letting internal pressure rise above 1 atmosphere. Because boiling point rises with external pressure, water inside can reach well above 100°C before boiling, cooking food faster than in an open pot at atmospheric pressure.

What is vacuum distillation and why is it used?

Vacuum distillation lowers the external pressure above a liquid so it boils at a much lower temperature than it would at atmospheric pressure. It is used in oil refining to separate heavy fractions that would decompose at their normal boiling point, and in freeze-drying and food processing to remove water without heat damage.

About this simulation

Written by MySimulator Team · Reviewed by MySimulator Editorial Review

Last updated: 11 July 2026

This simulator shows liquid molecules evaporating from a surface and vapor molecules condensing back into it, settling into dynamic equilibrium at the vapor pressure predicted by the Clausius-Clapeyron equation for the selected substance. Raise the temperature or lower the external pressure until the calculated vapor pressure meets the pressure pushing down from outside, and bubbles start nucleating throughout the liquid bulk — the exact physical moment boiling begins, not just "getting hot."

🔬 What it shows

Two synchronized views: a sealed container where molecules cross the liquid-vapor interface in both directions and bubbles appear in the bulk once boiling starts, and a vapor-pressure curve P(T) with a horizontal line at your chosen external pressure — their crossing point is the boiling point.

🎮 How to use

Drag the temperature slider to heat or cool the liquid, and the external pressure slider to simulate altitude (lower pressure) or a sealed pressure cooker (higher pressure). The substance dropdown loads real approximate boiling points and enthalpies of vaporization for water, ethanol, and diethyl ether.

💡 Did you know?

Because boiling only requires vapor pressure to match external pressure, water boils at around 68°C on top of Mount Everest, while a pressure cooker can push water's boiling point above 120°C — both are the same Clausius-Clapeyron relationship, just with different external pressures.

Frequently asked questions

What does the temperature slider control?

The temperature slider sets the liquid's temperature in °C. The simulation converts it to Kelvin and plugs it into the Clausius-Clapeyron equation for the selected substance to compute the equilibrium vapor pressure shown in the stats box and on the right-hand curve.

What does the external pressure slider represent?

It represents the pressure pushing down on the liquid from outside — atmospheric pressure at sea level is 1 atm, while lower values simulate higher altitude and higher values simulate a sealed or pressurized container such as a pressure cooker.

How does the simulation decide when to show bubbles?

Bubbles only appear once the calculated vapor pressure at the current temperature meets or exceeds the external pressure slider value — the exact physical condition for boiling. Below that point you'll only see molecules crossing the surface (evaporation and condensation), never bubbles forming inside the liquid.

What do the three substance presets change?

Water, ethanol, and diethyl ether each use their real approximate boiling point at 1 atm and enthalpy of vaporization, so the vapor-pressure curve, the boiling point at any external pressure, and the point at which bubbling begins are all different for each substance.

What does the marker on the right-hand chart show?

The dot marks the current (temperature, vapor pressure) point on the substance's Clausius-Clapeyron curve. The dashed horizontal line is your chosen external pressure — where it crosses the curve is the boiling point at that pressure.

What does the evaporation/condensation indicator mean?

It compares how many simulated vapor molecules are currently present to the equilibrium number for the current temperature: "Net evaporation" means the vapor population is still growing toward equilibrium, "Net condensation" means it's shrinking back down, and "Equilibrium" means evaporation and condensation are effectively balanced.