Clausius-Clapeyron Vapor Pressure Calculator
Vaporization and Pressure
When liquid molecules escape into the gas phase, they exert a pressure known as vapor pressure. This pressure increases with temperature because more molecules have enough energy to break free from the surface. Understanding vapor pressure is essential in meteorology, chemical engineering, and many laboratory procedures. It dictates when a substance will boil and influences how rapidly evaporation proceeds under various conditions.
Understanding Phase Changes
The transition between liquid and vapor involves significant energy as intermolecular bonds are broken. The heat required to convert one mole of liquid into vapor at constant temperature is the enthalpy of vaporization, often symbolized ΔHvap. Because a system in equilibrium experiences equal rates of evaporation and condensation, the vapor pressure at a specific temperature reflects the balance of these competing processes.
Clausius-Clapeyron Equation Basics
The Clausius-Clapeyron equation links vapor pressure and temperature for a pure substance through the relationship
Here P₁ and T₁ are the reference pressure and temperature, while P₂ is the pressure at the new temperature T₂. R is the universal gas constant. The equation assumes the enthalpy of vaporization remains constant across the temperature range.
Worked Vapor Pressure Scenarios
The table below compares three scenarios using a reference pressure of 101.3 kPa at 373 K—the boiling point of water at sea level. Varying the target temperature shows how quickly vapor pressure drops away from boiling conditions.
| T₂ (K) | ΔT (K) | Calculated P₂ (kPa) | Interpretation |
|---|---|---|---|
| 363 | -10 | 72.0 | Water boils vigorously at lower pressure. |
| 353 | -20 | 50.7 | At ~50 kPa, cooking times lengthen significantly. |
| 323 | -50 | 18.8 | Pressure cookers reach this regime during rapid cooling. |
Limitations and Best Practices
The Clausius-Clapeyron relationship assumes ΔHvap is constant and the vapor behaves ideally. Near the critical point or for very wide temperature ranges the estimate can deviate from experimental data. Still, within the moderate ranges used in laboratories and distillation design, the equation offers quick, reliable insight. Combining the calculator with measured data helps refine ΔHvap for a specific substance.
Continue exploring phase behavior with the Heat of Vaporization Calculator, Vapor Pressure Deficit Calculator, and the Water Evaporation Rate Calculator to connect thermodynamics with practical lab and weather scenarios.