💧 Vapor Pressure Calculator

How to Use This Calculator

Enter the heat of vaporization, one known pressure-temperature point, and your target temperature. Optionally enter external pressure to get the boiling point there.

Clausius-Clapeyron

Vapor pressure rises exponentially with temperature because more molecules can escape the liquid. The boiling point is simply where this curve crosses the external pressure line.

Worked Example

Water at 25 C: ln(P2/1) = -(40700/8.314)(1/298.15 - 1/373.15) = -3.74. P2 = e^-3.74 = 0.0238 atm, about 18 mmHg. Matches the known value.

Cooking at Altitude

At 5,000 feet, water boils at 203 F instead of 212 F. Pasta takes longer because it cooks at a lower temperature, not because of the pressure itself. Pressure cookers solve this by raising the pressure above 1 atm, pushing water's boiling point to 250 F and cooking food faster than sea level. Baking also changes: lower air pressure means cakes rise more and dry out faster, so high-altitude recipes cut leavening and sugar.

Evaporative Cooling

Sweating cools you because the highest-energy water molecules escape as vapor, carrying heat away. This only works if the air can accept more vapor; in humid heat, sweat drips instead of evaporating and cooling stalls. The same physics drives swamp coolers and explains why a wet cloth on a water jug keeps it cool in dry climates.

Distillation Relies on This

Fractional distillation separates liquids by their vapor pressure differences: the more volatile component vaporizes preferentially at each stage. Oil refineries stack dozens of trays to separate crude into fractions. Azeotropes, like 95 percent ethanol and water, resist separation because the mixture's vapor pressure curve has no useful difference to exploit.

Perfume Design

Perfumers engineer scents around vapor pressure: top notes are high-vapor-pressure molecules that evaporate in minutes, heart notes last hours, and base notes linger for days. A perfume is a carefully balanced mixture where each component's volatility determines when you smell it. This is applied Clausius-Clapeyron in a bottle.