Calculate boiling points using the Clausius-Clapeyron equation. Determine how boiling point changes with pressure for any substance.
The boiling point of a substance is the temperature at which its vapor pressure equals the surrounding atmospheric pressure. This fundamental thermodynamic property depends on the strength of intermolecular forces, molecular weight, and most importantly, the external pressure. Understanding how boiling points vary with conditions is essential in chemistry, chemical engineering, and everyday applications from cooking to industrial distillation.
The Clausius-Clapeyron equation provides the theoretical relationship between vapor pressure and temperature, allowing prediction of boiling points at any pressure given the enthalpy of vaporization and a known reference point. This equation is derived from the fundamental thermodynamic relationship between the Gibbs free energy of the liquid and gas phases at equilibrium.
This calculator implements the Clausius-Clapeyron equation for common substances and allows custom input of thermodynamic properties. It is particularly useful for chemical engineers designing distillation columns, food scientists working at different altitudes, and researchers who need to predict phase behavior under non-standard conditions. The built-in database includes water, common organic solvents, and cryogenic liquids.
This calculator eliminates the need for manual Clausius-Clapeyron calculations and provides instant boiling point predictions for any pressure. Essential for lab planning, process design, altitude cooking adjustments, and vacuum distillation setup. This boiling point calculator helps you compare outcomes quickly and reduce avoidable mistakes when making day-to-day care decisions. Use the estimate as a planning baseline and confirm final decisions with a qualified professional when risk is high.
Clausius-Clapeyron equation: ln(P₂/P₁) = (ΔH_vap/R) × (1/T₁ - 1/T₂), where P = pressure, T = temperature (K), ΔH_vap = enthalpy of vaporization (J/mol), R = 8.314 J/(mol·K). Antoine equation: log₁₀(P) = A - B/(C + T), where A, B, C are substance-specific constants.
Result: Boiling point = 81.3°C
Using the Clausius-Clapeyron equation with water’s ΔH_vap = 40,700 J/mol and normal boiling point of 100°C at 1 atm, at 0.5 atm the boiling point drops to approximately 81.3°C. This is why water boils at lower temperatures at high altitudes.
The Clausius-Clapeyron equation is derived from the Clausius theorem and describes the slope of the phase boundary between liquid and vapor in a pressure-temperature diagram. In its integrated form, it assumes that the enthalpy of vaporization is constant over the temperature range of interest and that the vapor behaves as an ideal gas. These assumptions limit its accuracy to moderate pressure ranges near the normal boiling point, but it remains one of the most useful equations in physical chemistry for quick estimates.
For more precise calculations over wider temperature ranges, the Antoine equation uses three empirically fitted constants (A, B, C) specific to each substance. The NIST Chemistry WebBook and Perry’s Chemical Engineers’ Handbook are standard references for these constants. For even higher accuracy, the Wagner equation and extended Antoine models incorporate additional terms to describe the curvature of the vapor pressure curve near the critical point.
Distillation is the most common industrial separation technique, and boiling point data is fundamental to column design. Vacuum distillation allows temperature-sensitive compounds to be purified at lower temperatures, preventing thermal decomposition. In the pharmaceutical and food industries, understanding boiling point behavior under reduced pressure is essential for solvent removal, concentration, and drying operations. Environmental engineers use boiling point data to assess the volatility and atmospheric fate of pollutants.
Boiling occurs when vapor pressure equals external pressure. Lower external pressure means the liquid reaches this equilibrium at a lower temperature, so the boiling point decreases.
The normal boiling point is the boiling temperature at exactly 1 atmosphere (101.325 kPa) of pressure. It is the standard reference condition for tabulating boiling points.
It is most accurate near the normal boiling point and for moderate pressure ranges. For wide pressure ranges, the Antoine equation or more complex models provide better accuracy.
ΔH_vap is the energy required to convert one mole of liquid to gas at constant temperature and pressure. Stronger intermolecular forces lead to higher ΔH_vap values.
This calculator is designed for pure substances. For mixtures, you need Raoult’s law or more complex vapor-liquid equilibrium models that account for interactions between components.
Atmospheric pressure decreases with altitude. At the top of Mount Everest (~0.34 atm), water boils at about 70°C, which is why cooking takes longer at high elevations.