Calculate boiling points at different pressures and altitudes using the Clausius-Clapeyron equation. Supports multiple unit conversions with step-by-step solutions.
Problem: Water has a normal boiling point of 100°C at 1 atm. What is the boiling point of water at 2,000 meters altitude? (ΔH_vap = 40.65 kJ/mol)
Altitude to Pressure: At 2,000 m, atmospheric pressure ≈ 0.795 atm (using barometric formula approximating ~1.2% drop per 100 m).
Using Clausius-Clapeyron: ln(0.795/1) = −(40650/8.314)(1/T₂ − 1/373.15)
Result: T₂ ≈ 93.5°C
At 2,000 m altitude, water boils at about 93-94°C — that's why cooking times increase at higher elevations.
Problem: Ethanol's normal boiling point is 78.37°C at 1 atm, with ΔH_vap = 38.56 kJ/mol. What is its boiling point at 2 atm (typical pressure cooker)?
Given: P₁ = 1 atm, T₁ = 78.37°C (351.52 K), P₂ = 2 atm, ΔH_vap = 38.56 kJ/mol
Using Clausius-Clapeyron (solving for T₂):
Result: T₂ ≈ 96.3°C
In a pressure cooker at 2 atm, ethanol boils at about 96°C instead of 78°C.
Problem: Water boils at 100°C (373.15 K) at 1 atm. What is the vapor pressure of water at 80°C? (ΔH_vap = 40.65 kJ/mol)
Given: P₁ = 1 atm, T₁ = 100°C (373.15 K), T₂ = 80°C (353.15 K), ΔH_vap = 40.65 kJ/mol
Using Clausius-Clapeyron: ln(P₂/1) = −(40650/8.314)(1/353.15 − 1/373.15)
Result: P₂ ≈ 0.473 atm
At 80°C, water's vapor pressure is about 0.47 atm — it would boil at this temperature if the external pressure were 0.47 atm.
The boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure. The Clausius-Clapeyron equation relates vapor pressure to temperature and is the fundamental tool for predicting how boiling points change with pressure.
| Symbol | Meaning | Typical Units |
|---|---|---|
| P₁ | Reference pressure (usually 1 atm) | atm, kPa, mmHg |
| P₂ | Pressure at new conditions | atm, kPa, mmHg |
| T₁ | Reference boiling point (at P₁) | K (Kelvin) |
| T₂ | Boiling point at new pressure P₂ | K (Kelvin) |
| ΔH_vap | Enthalpy of vaporization | kJ/mol |
| R | Universal gas constant | 8.314 J/(mol·K) |
| h | Altitude | m, ft |
| Substance | Formula | Normal BP (°C) | ΔH_vap (kJ/mol) |
|---|---|---|---|
| Water | H₂O | 100.0 | 40.65 |
| Ethanol | C₂H₅OH | 78.37 | 38.56 |
| Acetone | C₃H₆O | 56.08 | 29.10 |
| Benzene | C₆H₆ | 80.10 | 30.72 |
| Methanol | CH₃OH | 64.70 | 35.21 |
| Ammonia | NH₃ | −33.34 | 23.35 |
| Mercury | Hg | 356.7 | 59.11 |
| Sulfur Hexafluoride | SF₆ | −63.8 | 9.60 |
The temperature at which the vapor pressure of a liquid equals the external pressure. At this point, bubbles of vapor form throughout the liquid.
At higher altitudes, atmospheric pressure is lower, so liquids boil at lower temperatures. Water boils at ~93°C at 2,000 m and ~71°C at the summit of Everest (8,849 m).
The energy required to convert one mole of liquid to gas. Higher ΔH_vap means stronger intermolecular forces and a steeper boiling point-pressure relationship.
Increasing external pressure raises the boiling point (as in a pressure cooker). Decreasing pressure lowers the boiling point (as at high altitude or in a vacuum).
The boiling point of a substance is the temperature at which its vapor pressure equals the external (atmospheric) pressure surrounding the liquid. At this temperature, the liquid transforms into vapor throughout its volume — not just at the surface (evaporation), but forming bubbles within the liquid itself.
The normal boiling point is defined as the boiling point at standard atmospheric pressure (1 atm = 101.325 kPa = 760 mmHg). For water, the normal boiling point is 100°C (212°F). However, boiling point varies significantly with pressure — this is why water boils at lower temperatures at high altitudes and why pressure cookers work by raising the boiling point.
The Clausius-Clapeyron equation is a fundamental thermodynamic relationship that describes how vapor pressure changes with temperature. It is derived from the condition of equilibrium between a liquid and its vapor phase. The equation assumes that the enthalpy of vaporization (ΔH_vap) is constant over the temperature range of interest and that the vapor behaves as an ideal gas.
This calculator uses the Clausius-Clapeyron equation to compute boiling points at different pressures, predict boiling points at various altitudes, and determine the vapor pressure at any given temperature. For altitude calculations, it additionally uses the barometric formula to estimate atmospheric pressure as a function of elevation.
Understanding boiling point variation is critical in numerous fields. In cooking, recipes must be adjusted at high altitudes because water boils at lower temperatures, requiring longer cooking times. In chemical engineering, distillation column design depends on accurate boiling point data at different pressures. In meteorology, understanding how pressure affects phase transitions helps predict weather patterns. In medicine, autoclaves use elevated pressure to raise water's boiling point for sterilization.
At 2,500 m elevation, water boils at ~91°C. This lower temperature means food takes longer to cook — pasta may need 50% more time, and meats require adjusted cooking methods.
Pressure cookers operate at 1.5-2.0 atm, raising water's boiling point to 110-120°C. This higher temperature speeds up cooking by 30-70% compared to conventional methods.
Fractional distillation separates mixtures by exploiting boiling point differences. Reduced-pressure distillation lowers boiling points, allowing separation of heat-sensitive compounds without decomposition.
Many industrial chemical reactions are conducted at specific pressures to control boiling points and maintain desired reaction conditions, optimizing yield and safety.
⚠️ Important Note: This Boiling Point Calculator is for educational and professional reference purposes. The Clausius-Clapeyron equation assumes constant enthalpy of vaporization and ideal gas behavior. For precise engineering applications, especially over wide temperature ranges or near the critical point, consult experimental data or more advanced thermodynamic models. The altitude-to-pressure conversion uses an approximate barometric formula and does not account for local weather variations, humidity, or temperature gradients. Always verify critical values with authoritative sources.