Boiling Point Calculator

When a liquid begins to boil, its vapor pressure is equal to the pressure acting on the liquid. By changing the external pressure, the boiling point changes accordingly.

This calculator allows you to calculate the boiling point of water and ten other common substances at any given pressure. It is also possible to do the calculation in reverse: if you want a particular substance to boil at a certain temperature, this tool will tell you what pressure is required for that to happen.

In addition, two calculations can be performed that would normally only be possible by reference to experimental notes: the calculation of the heat of vaporization from a set of measured values and the conversion of the boiling point actually observed at a given altitude into altitude.

What is a boiling point?

The molecules in a liquid are constantly escaping into the space above the surface of the liquid. The pressure exerted by these escaping vapours is called the vapour pressure. When a liquid is heated more molecules gain enough energy to escape, so the vapour pressure increases.

Once the vapor pressure reaches the ambient pressure, boiling begins. At this point bubbles form not only at the surface but anywhere inside the liquid as well, which is what distinguishes boiling from evaporation.

The principle behind each of the results on this page is the same: when the ambient pressure is lowered, the liquid boils at a lower temperature because less heat is required to reach its boiling point. When the pressure is increased, the liquid must be heated to a higher temperature.

The Clausius–Clapeyron relation

The relationship between pressure and boiling point is given by the Clausius–Clapeyron relation. In its integrated form, it can be used to relate two points on a vapor-pressure curve of the same liquid.

By rearranging the equation, you get the desired boiling temperature.

Conversely, by another adjustment, one can obtain the pressure required for a specific target temperature.

There are two common sources of error here: first, the absolute temperature must be used, so 273.15 is added to the Celsius temperature; second, the units for enthalpy of vaporization are per mole, not per gram.

Example: water at 0.8 atm.

Let's take water at 0.80 atm as an example. This is about what a barometer would read at an altitude of 1900 meters. The normal boiling point for water is 373.15 K and the heat of vaporization is 40,660 J/mol.

First we'll look at the logarithmic term. The pressure ratio is .80, and the natural log of .80 is -.2231. Multiply this by the ratio of R over dHvap, which is 8.314 divided by 40660, or multiply it by 2.045 times ten to the negative fourth power.

So 1/T2 = 0.0027255 where T2 = 366.90 K which is 93.75°C. The boiling point of water would normally be about six and a half degrees lower, just because the amount of air acting on the water has been reduced by one fifth.

If you do the same numbers in reverse order, you get a different result. If you want to find out at what conditions water boils at 90 °C, this relationship gives approximately 0.697 atm or 70.6 kPa.

How to use each mode.

Calculating boiling point at a given pressure.

Choose a substance, enter the pressure and read off the temperature. This is the most common case in everyday life. Examples are vacuum lines with 50 mmHg, closed containers with 3 bar or weather systems with 98 kPa.

Pressure according to target temperature.

This mode is useful when temperature is a constraint and not a variable parameter. For example, if you are distilling a substance that decomposes at temperatures above 60 °C, then this will allow you to determine exactly how far the pump needs to reduce pressure.

Two reference points.

If two sets of pressure and temperature data are entered, the tool will calculate dHvap without using standard values. This is a standard exercise in academic labs and the most reliable method when dealing with substances not on a predetermined list.

The values obtained are then used to calculate the boiling point for any target pressure entered, thus giving two different results from just two measurements.

Boiling points by altitude

When an altitude is entered the calculator first estimates the air pressure at that location based on the International Standard Atmosphere model and then calculates the boiling point of water at that pressure. In Denver it will be about 94.5 degrees Celsius, while on top of Mount Everest it will be about 69 degrees Celsius.

Vaporization enthalpy and Trouton's rule

For all predefined substances a vaporization enthalpy (dHvap) is given which was measured at the standard boiling point. The value of 40.66 kJ/mol for water is unusually high for a molecule of this size, because each molecule is fixed by hydrogen bonds and these bridges must be broken before it can separate from the liquid state.

The last column shows the vaporization entropy, i.e., the value of dHvap divided by the boiling point in Kelvin temperature scale. In 1884, Thornton noted that for most liquids this quantity is approximately 88 J/(mol K). The table shows how well this rule holds for benzene, acetone and diethyl ether.

The values for water and alcohols are well above this value. This shows that the hydrogen bonds are being expressed in the data. The values for methane are below this value because it is already disordered in liquid state, so there is little entropy gained during the transition to gas phase.

If you select a particular substance and set the heat of vaporization to zero, then the calculator will apply the Trouton's rule based on the standard boiling point and continue with the calculation. This is only an approximation but for non-associated liquids the error is usually within a few percentage points.

Real life applications:

A rotary evaporator is a typical example. By lowering the pressure to about 50 mbar, a solvent that normally only evaporates at 80 °C already evaporates at room temperature, thus protecting temperature-sensitive products from damage caused by high temperatures.

A pressure cooker is the opposite. In a sealed environment at about 2 atm, water inside will reach about 120°C rather than 100°C. This extra 20 degrees significantly reduces cooking time.

Even chefs at high altitudes are often unaware of these physical laws. Since water boils at about 94.5 °C in a Denver kitchen, it takes significantly longer to cook pasta or beans and cake recipes must be adjusted.

The boiling point of water is a classic reference value used in the calibration of outdoor thermometers. The actual value varies according to the pressure at that location and therefore needs to be corrected. This calculation tool presented here offers exactly this correction.

When the model becomes inaccurate:

The Clausius-Clapeyron relation in this form is based on two assumptions: that the change in enthalpy of vaporization (ΔHvap) is independent of temperature and that the vapor behaves as an ideal gas with a volume significantly larger than that of the liquid.

In limited areas near the reference point both assumptions are generally valid. When the pressure range is extended by an order of magnitude, dHvap actually decreases as temperature approaches the critical point, leading to inaccuracies.

For larger ranges and higher accuracy it is better to use the Antoine equation with substance-specific constants. This is not based on the assumption of constant enthalpy but is determined from experimental data. The two-reference-point mode is a practical compromise where this relationship is calibrated using your own measurements.

These results can be used for educational purposes, experiment planning and everyday use but are only estimates. Vapor pressure data from different sources may vary slightly, and in real systems the boiling point may differ due to dissolved substances, impurities or non-ideal behavior. For critical applications, you should verify the results with actual measurements.