What is vapor pressure? Vapor pressure definition
Vapor pressure is the pressure exerted by a vapor that is in thermodynamic equilibrium with its condensed phases (solid or liquid) in a closed system at a given temperature. The equilibrium — in other words, steady state — between evaporation and condensation occurs when:
The rate of evaporation of the liquid = Rate of condensation of the gas
Vapor pressure is one of the fluid characteristics: it's a measure of the tendency of a material to change into the gaseous/vapor state. The vapor pressure of a liquid can be measured in many ways, e.g., by a manometer connected to the flask with the measured liquid.
Factors influencing vapor pressure
There are two factors that influence the vapor pressure:
Temperature
The higher the temperature is, the more molecules have enough energy to escape from the liquid or solid, which leads to higher vapor pressure values.
- Temperature of a liquid increases (T↑) → kinetic energy of its molecules increases (Ek↑) → number of molecules transitioning into a vapor increases → vapor pressure increases (P↑)
- At lower temperatures, fewer molecules have sufficient energy.
Substance nature (types of molecules)
The vapor pressure will be relatively low for substances with stronger intermolecular forces. On the contrary, the vapor pressure is relatively high for relatively weak forces.
Note: The surface area of a liquid/solid substance in contact with the gas does not affect the vapor pressure. It doesn't matter whether the liquid is placed in a wide flask or a thin graduated cylinder — the vapor pressure remains the same.
Vapor pressure formulas
There are many different formulas to calculate the vapor pressure of water. Our calculator supports five methods:
1. Antoine Equation
The most widely used formula, based on the Clausius–Clapeyron equation:
log₁₀(P) = A − B / (C + T)Where P is in mmHg and T in °C. For water in the range 1–100 °C: A = 8.07131, B = 1730.63, C = 233.426 (NIST constants). For 60–150 °C: A = 8.14019, B = 1810.94, C = 244.485.
2. Buck Equation (1981)
Highly accurate for meteorological applications:
P = 0.61121 · exp((18.678 − T/234.5) · T / (257.14 + T)) [kPa, T in °C, over liquid water]
P = 0.61115 · exp((23.036 − T/333.7) · T / (279.82 + T)) [kPa, T in °C, over ice]3. Magnus Formula
A simplified approximation widely used in meteorology:
P = 0.61078 · exp(17.27 · T / (T + 237.3)) [kPa, T in °C, over liquid water]
P = 0.61078 · exp(21.87 · T / (T + 265.5)) [kPa, T in °C, over ice]4. Tetens Formula
Similar to Magnus, using base-10 exponentiation:
P = 0.61078 · 10^(7.5 · T / (237.3 + T)) [kPa, T in °C, over liquid water]
P = 0.61078 · 10^(9.5 · T / (265.5 + T)) [kPa, T in °C, over ice]5. Simple Formula
A straightforward exponential approximation expressed in terms of absolute temperature:
P = e^(20.386 − 5132/T_K) [result in mmHg, T_K in Kelvin]Vapor pressure of water
Water has one of the most studied vapor pressure curves. Key reference points:
- 0 °C (32 °F) — 611.7 Pa (0.006117 atm) — melting point of ice
- 20 °C (68 °F) — 2338 Pa (≈ 17.5 mmHg) — typical room temperature
- 37 °C (98.6 °F) — 6280 Pa — human body temperature
- 100 °C (212 °F) — 101325 Pa (1 atm) — normal boiling point
- 374 °C (705 °F) — 22.06 MPa — critical point of water
How to use the vapor pressure of water calculator
- Select unit system — Metric (°C) or Imperial (°F).
- Enter the temperature — valid range: −50 °C (−58 °F) to 374 °C (705 °F).
- Choose a formula — Antoine is recommended for most applications; Buck is most accurate for meteorological use.
- Click "Calculate" — results appear instantly in ten pressure units: Pa, hPa, kPa, mbar, bar, atm, mmHg, Torr, psi, and inHg.
FAQs
Does vapor pressure depend on the amount of liquid?
No. Vapor pressure is an intensive property — it depends only on the temperature and the nature of the substance, not on the quantity of liquid or the size of the container.
What happens at the boiling point?
A liquid boils when its vapor pressure equals the surrounding atmospheric pressure. At sea level (≈ 101325 Pa), water boils at exactly 100 °C (212 °F).
Why does water boil at a lower temperature at high altitude?
At high altitude, atmospheric pressure is lower. Therefore, water's vapor pressure reaches that lower pressure at a lower temperature, causing it to boil sooner.
Which formula is most accurate?
For the range 0–100 °C over liquid water, the Antoine and Buck equations are both highly accurate (error < 0.1%). The Buck equation is generally recommended for meteorological and engineering applications because it performs well over both ice and liquid phases.
What is the difference between vapor pressure and partial pressure?
Vapor pressure refers to the pressure exerted by a pure substance in equilibrium with its liquid/solid phase. Partial pressure is the contribution of one component to the total pressure in a gas mixture (e.g., the partial pressure of water vapor in humid air).