Boiling Point Calculator
Find the boiling point of water, ethanol, and other common liquids at any target pressure, using the Clausius-Clapeyron equation.
♨️ What is a Boiling Point Calculator?
A boiling point calculator like this one finds the temperature at which a liquid boils under any chosen pressure, not just the single normal boiling point printed in a reference table. It works from the Clausius-Clapeyron equation, using a substance's known normal boiling point (at 1 atm) and its heat of vaporization to predict boiling point at any other target pressure, whether that pressure is higher or lower than standard atmospheric pressure.
This calculation matters in genuinely practical situations: vacuum distillation in a laboratory lowers pressure specifically to reduce boiling point so that heat-sensitive organic compounds can be purified without decomposing, industrial vacuum evaporators concentrate liquid products at lower temperatures to save energy and protect delicate ingredients, pressure cookers raise pressure to raise boiling point and cook food faster, and chemical engineers routinely need boiling point at non-standard pressures when designing distillation columns and reactors.
A common misconception is that boiling point is a single fixed number for a given substance. It is not, boiling point is only fixed once a pressure is specified. Water boils at 100 degrees Celsius specifically at 1 atm (sea level pressure), it boils below that at lower pressure and above that at higher pressure, which is why this calculator treats pressure as the primary variable, not an afterthought.
This tool is useful because working the Clausius-Clapeyron equation by hand, especially converting between pressure units and correctly using natural logarithms, is easy to get wrong. Entering a substance preset or your own reference values and a target pressure gives an instant, correctly worked answer, along with the full pressure-temperature curve for context.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Water Under Vacuum (50 kPa)
Water, T₁=373.15 K, ΔHvap=40.7 kJ/mol, target pressure = 50 kPa.
Example 2 — Ethanol at Half an Atmosphere
Ethanol, T₁=351.5 K, ΔHvap=38.56 kJ/mol, target pressure = 0.5 atm.
Example 3 — Sanity Check at Standard Pressure
Water, T₁=373.15 K, ΔHvap=40.7 kJ/mol, target pressure = 760 mmHg.
❓ Frequently Asked Questions
🔗 Related Calculators
How do you calculate boiling point at a different pressure?
Use the Clausius-Clapeyron equation: 1/T2 = 1/T1 minus (R divided by the heat of vaporization) times the natural log of P2 divided by P1, where T1 and P1 are the normal boiling point and 1 atm, and P2 is the new target pressure. Solving for T2 gives the boiling point at that new pressure.
Why does boiling point drop at lower pressure?
A liquid boils when its vapor pressure equals the surrounding atmospheric pressure. At lower surrounding pressure, the liquid needs to generate less vapor pressure to reach that threshold, which happens at a lower temperature, so reducing pressure always lowers the boiling point.
What is the Clausius-Clapeyron equation used for?
It relates a pure substance's vapor pressure to its temperature along the liquid-vapor phase boundary. Given one known temperature-pressure pair, usually the normal boiling point at 1 atm, it lets you calculate the boiling point at any other pressure, or the vapor pressure at any other temperature.
What is the difference between normal boiling point and boiling point?
Normal boiling point specifically means the boiling point measured at exactly 1 atmosphere (101.325 kPa) of pressure, the value listed on most reference tables. Boiling point on its own is ambiguous unless a pressure is specified, since the same liquid boils at a different temperature under different pressure.
Why do I need the heat of vaporization to find boiling point at a new pressure?
The heat of vaporization sets how strongly vapor pressure responds to a temperature change for that particular substance. A substance with a large heat of vaporization needs a bigger temperature swing to produce the same relative change in vapor pressure than a substance with a small one, so it directly controls the steepness of the pressure-temperature curve.
Does this calculator work for substances other than water?
Yes. Choose any of the built-in presets, water, ethanol, methanol, benzene, or acetone, or select Custom and enter your own normal boiling point and heat of vaporization for any pure liquid you have reference data for.
Why is my result for very high or very low pressures unreliable?
The Clausius-Clapeyron equation used here assumes a constant heat of vaporization, which is a good approximation near the normal boiling point but becomes less accurate far from it, especially approaching the critical point where the liquid and vapor phases become indistinguishable and heat of vaporization actually falls toward zero.
Can this calculator find vapor pressure at a given temperature instead?
This particular tool is set up to solve for temperature given a target pressure. The same rearranged Clausius-Clapeyron formula can also solve for pressure given a target temperature, the underlying relationship is identical, just solved for a different unknown.
What units does this calculator accept for pressure?
You can enter the target pressure in kilopascals (kPa), standard atmospheres (atm), millimetres of mercury (mmHg, equivalent to torr), or pounds per square inch (psi). The calculator converts everything internally to kPa before applying the formula.
Why does a pressure cooker raise the boiling point of water?
A pressure cooker seals in steam, raising the internal pressure well above the normal 1 atm. Because higher pressure raises boiling point, water inside a sealed pressure cooker can reach well above 100 degrees Celsius before it boils, which is exactly why food cooks faster inside one.