Boiling Point Calculator | Antoine Equation Tool

What is a boiling point calculator?

A boiling point calculator finds the temperature at which a liquid turns into a gas at a chosen pressure. It works for water and other pure liquids using a formula called the Antoine equation. Enter a substance and a pressure, and the calculator returns the temperature at which that liquid boils.

How pressure changes the boiling point

A liquid boils when its vapor pressure, the pressure of the gas escaping from its surface, equals the pressure of the air or gas above it. Lower the outside pressure, and the liquid needs less heat to reach that balance, so it boils at a lower temperature. Raise the pressure, and the liquid needs more heat, so it boils at a higher temperature.

This is why water boils at 100°C (212°F) at sea level, where standard atmospheric pressure is 1 atm (760 mmHg), but boils below 100°C on a mountain, where the air is thinner. It also explains why a pressure cooker cooks food faster. Trapped steam raises the pressure inside the pot, which raises the boiling point of the water, so the food cooks at a higher temperature than it would in an open pot.

Boiling point also depends on the substance itself. Molecules held together by strong forces, such as the hydrogen bonds between water molecules, need more energy to separate, so they boil at higher temperatures than molecules with weaker forces.

The Antoine equation formula

The calculator uses the Antoine equation, a formula that links a liquid's vapor pressure to its temperature:

log₁₀(P) = A − B / (T + C)

P is the vapor pressure in millimeters of mercury (mmHg). T is the temperature in degrees Celsius. A, B, and C are constants specific to each substance, found through laboratory measurement.

To find the boiling point at a chosen pressure, the equation is rearranged to solve for T:

T = B / (A − log₁₀(P)) − C

Each substance's constants are only accurate within a specific temperature range, usually the range they were measured over. Outside that range the equation's predictions become unreliable, so the calculator flags any result that falls outside the tested range for a substance.

How to calculate a boiling point

1. Pick a substance, such as water, ethanol, or toluene, or enter custom A, B, and C constants for another liquid.
2. Enter a pressure and choose its unit: atm, mmHg, kPa, psi, or bar.
3. Choose the unit for the answer: Celsius, Fahrenheit, or Kelvin.
4. Read the calculated boiling point. The calculator also plots the pressure-temperature curve and marks the result on it.

Worked examples

Water at high altitude

At 5,000 feet, atmospheric pressure is about 0.83 atm, or 630.8 mmHg. Using water's Antoine constants (A = 8.07131, B = 1730.63, C = 233.426):

log₁₀(630.8) = 2.800 T = 1730.63 / (8.07131 − 2.800) − 233.426 = 94.9°C (202.8°F)

That is about 5°C below water's sea-level boiling point of 100°C. A recipe that calls for boiling water for a set number of minutes at sea level needs a longer time at this altitude, because the water is cooler.

Ethanol distillation under reduced pressure

Distilling ethanol at 0.5 atm, or 380 mmHg, with its constants (A = 8.20417, B = 1642.89, C = 230.3), gives:

log₁₀(380) = 2.580 T = 1642.89 / (8.20417 − 2.580) − 230.3 = 61.8°C (143.2°F)

Ethanol normally boils at 78°C at 1 atm. Boiling it at a lower temperature under reduced pressure uses less energy and lowers the risk of scorching heat-sensitive mixtures during distillation.

Vacuum distillation of toluene

A laboratory vacuum pump can lower pressure to 50 mmHg, about 0.066 atm. With toluene's constants (A = 6.95464, B = 1344.8, C = 219.482):

log₁₀(50) = 1.699 T = 1344.8 / (6.95464 − 1.699) − 219.482 = 36.4°C (97.5°F)

Toluene normally boils at 111°C at 1 atm. Under vacuum it boils at just 36.4°C, letting chemists purify heat-sensitive compounds without the higher heat that would break them down.

Where boiling point calculations are used

Chemical and pharmaceutical plants use them to plan distillation, the process of separating liquids by their different boiling points. Cooks adjust recipes at altitude because water boils at a lower temperature there. Refrigeration engineers choose refrigerants partly by their boiling point at working pressures. Vacuum ovens and freeze-dryers use the same principle: lowering the pressure lowers the temperature needed to remove water from a material.

Antoine equation vs other methods

The Antoine equation is a simplified version of a more fundamental relationship called the Clausius-Clapeyron equation, which needs extra data such as a substance's heat of vaporization. The Wagner equation covers wider temperature ranges but needs more constants. NIST steam tables give very precise values for water but cover no other substance. For most everyday and engineering calculations on common liquids, the Antoine equation strikes a good balance of simplicity and accuracy, typically within 1–2% of measured values inside its tested range.

Frequently asked questions

How do I calculate the boiling point of a liquid?

Look up the substance's Antoine constants (A, B, and C) and use T = B / (A − log₁₀(P)) − C, with pressure P in mmHg. The calculator does this automatically for common substances, or for any custom set of constants entered by the user.

What is the boiling point of water at standard pressure?

Water boils at 100°C (212°F) at 1 atm, or 760 mmHg, the standard atmospheric pressure at sea level.

Why does water boil at a lower temperature at high altitude?

Air pressure drops as altitude increases. Lower pressure means water's vapor pressure reaches the surrounding pressure sooner, at a lower temperature. As a rough guide, the boiling point falls by about 1°C for every 285 meters (935 feet) of elevation gained.

Why do different liquids boil at different temperatures?

Boiling point depends on how strongly a liquid's molecules attract each other. Substances with strong intermolecular forces, such as the hydrogen bonds in water, need more heat energy to break those bonds and boil, so they have higher boiling points than substances with weaker forces, such as hexane.

Does the Antoine equation work for mixtures?

No. It applies to pure substances only. Mixtures need different tools, such as Raoult's law or activity-coefficient models, because each component changes the vapor pressure of the others.

How accurate is the Antoine equation?

Within a substance's tested temperature range, it typically matches measured boiling points within 1–2%. Outside that range, or near a substance's critical point, its accuracy drops, so a result outside the listed range should be treated with caution.