What is saturation vapor pressure?

Saturation vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid at a given temperature. When the surrounding pressure equals it, the liquid boils. For water at 100°C the saturation vapor pressure is 760 mmHg, which is why water boils at 100°C at sea level.

How is the pressure calculated?

The tool uses the Antoine equation, log10(P) = A − B/(C + T), where P is pressure in mmHg, T is temperature in degrees Celsius, and A, B, C are substance-specific constants fitted to experimental data. Each solvent has its own constants and a stated valid temperature range.

Why does vapor pressure rise so steeply with temperature?

More molecules gain enough energy to escape the liquid as temperature climbs, and the relationship is exponential rather than linear. The Antoine equation captures this curvature, which is why vapor pressure roughly doubles for every 10 to 15 degree rise near room temperature.

What does an out-of-range result mean?

Antoine constants are fitted over a specific temperature window. Outside that window the equation still returns a number, but it is an extrapolation and may be inaccurate. The tool greys out and flags any temperature beyond the constants' validity range.

How does vapor pressure relate to boiling point?

A liquid boils when its saturation vapor pressure equals the external pressure. At altitude, where atmospheric pressure is lower, water boils below 100°C. To find a boiling point, find the temperature at which the tabulated vapor pressure matches the ambient pressure.

Vapor Pressure Reference Table

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Vapor pressure governs evaporation, boiling, and distillation. This reference computes the saturation vapor pressure of water and common solvents at any temperature from the Antoine equation and shows the result in mmHg, kPa, and atm, alongside a quick reference table.

How it works

The saturation vapor pressure comes from the Antoine equation, an empirical fit to experimental data:

log10(P) = A − B / (C + T)

Here P is the vapor pressure in mmHg, T is the temperature in degrees Celsius, and A, B, and C are constants specific to each substance, determined by regression against measured data. Because the temperature sits in the denominator and the whole expression is a base-10 logarithm, pressure rises exponentially with temperature. The tool converts the mmHg result to kPa (× 0.13332) and atm (÷ 760).

Reading the result

A liquid boils when its vapor pressure reaches the surrounding pressure. At sea level that is 760 mmHg (1 atm), so the temperature at which a solvent’s vapor pressure hits 760 mmHg is its normal boiling point — about 100°C for water and 78°C for ethanol. Each set of Antoine constants is valid only over a stated temperature window; outside it the equation extrapolates and the tool flags the value as approximate.

Example and notes

At 25°C the tool gives water a vapor pressure near 23.8 mmHg (about 3.2 kPa), matching standard tables. Heat it to 100°C and the value reaches 760 mmHg, the boiling point at sea level. The greyed rows in the reference table mark temperatures outside each solvent’s Antoine validity range — treat those as rough extrapolations rather than precise figures, and consult NIST data for high-accuracy work.