Boiling Point Calculator

How to use this boiling point calculator

What this calculator does. It estimates the temperature at which a pure liquid's equilibrium vapor pressure reaches a chosen external pressure. The model uses one known pressure – temperature point and a molar enthalpy of vaporization to project a second boiling point. It is useful for thermodynamics exercises, laboratory planning, vacuum-process estimates, and checking how reduced or elevated pressure shifts boiling behavior. It does not replace a measured vapor-pressure curve, account for changing enthalpy near a critical point, or model mixtures, dissolved salts, decomposition, or superheating.

When to use it. Use it to estimate water boiling under partial vacuum, compare a solvent's expected boiling temperature in a pressure-controlled apparatus, verify a textbook Clausius – Clapeyron calculation, or explore how strongly pressure affects a liquid with a given enthalpy of vaporization.

How to calculate. The calculator opens with a complete water demonstration and a ready-to-download workbook. Follow these steps:

1. Replace Reference pressure with a positive pressure measured in hPa. Use a plain decimal such as 1013.25; commas, scientific notation, units typed into the field, zero, and negative values are rejected.
2. Enter the matching Reference boiling point in degrees Celsius. Values must be above absolute zero. For water near normal atmospheric pressure, 100 °C is a practical rounded reference.
3. Enter the positive Target pressure in hPa. Lowering this value generally lowers the estimated boiling point; raising it generally raises the result.
4. Enter Enthalpy of vaporization in kJ/mol. For water near its normal boiling point, 40.65 kJ/mol is a useful example. A larger value makes the projected temperature less sensitive to a given pressure ratio.
5. Read Estimated boiling point in °C, then use the Kelvin, Fahrenheit, Temperature shift, Pressure ratio, and Model status outputs for cross-checking. Select Download Excel to export the current validated inputs and results. Reset clears the demonstration values and may disable export until all required fields are complete again.

Output guide. Estimated boiling point is the projected target temperature and is an approximation based on constant ΔH. Kelvin is the absolute-temperature form used by the equation. Fahrenheit is an exact unit conversion of the displayed result. Temperature shift is T₂ – T₁, so a negative value means the target pressure produces a lower boiling point. Pressure ratio is P₂/P₁; one means no pressure change, below one means reduced pressure, and above one means increased pressure. Model status reports whether the current fields form a finite valid calculation.

Worked example. The startup values are P₁ = 1013.25 hPa, T₁ = 100 °C, P₂ = 800 hPa, and ΔH = 40.65 kJ/mol. Converting 100 °C gives 373.15 K. Substituting the pressure ratio 800/1013.25 and R = 8.314462618 J/(mol·K) gives T₂ = 366.54 K, which equals 93.39 °C or 200.10 °F. The temperature shift is – 6.61 °C.

Learn more. The underlying relationship is described in the Chemistry LibreTexts explanation of the Clausius – Clapeyron equation. For reference-quality thermophysical data, consult the NIST Chemistry WebBook. The IUPAC Gold Book definition of boiling point clarifies that boiling occurs when vapor pressure equals the applied pressure.