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How It Works
The Antoine equation estimates vapor pressure as a function of temperature using three experimentally determined constants (A, B, C). For water, two sets of constants cover the range from 1 °C to 374 °C. Enter a temperature to find the vapor pressure, or enter a pressure to find the boiling temperature at that pressure.
A liquid boils when its vapor pressure equals the surrounding atmospheric pressure. At standard pressure (760 mmHg), water boils at 100 °C. At higher altitudes with lower air pressure, water boils at a lower temperature.
Example Problem
What is the vapor pressure of water at 25 °C (room temperature)?
- Use the low-range constants: A = 8.07131, B = 1730.63, C = 233.426
- log₁₀(P) = 8.07131 − 1730.63 / (233.426 + 25) = 1.376
- P = 101.376 ≈ 23.8 mmHg
This is far below atmospheric pressure (760 mmHg), confirming that water is a liquid at room temperature under normal conditions.
Frequently Asked Questions
What is water vapor pressure?
Water vapor pressure is the pressure exerted by water molecules in the gas phase above a liquid water surface. It increases with temperature — at 20 °C it is about 17.5 mmHg, and at 100 °C it reaches 760 mmHg (the standard atmospheric pressure), which is why water boils at that temperature.
Why does water boil at a lower temperature at high altitude?
At high altitude, atmospheric pressure is lower. Since boiling occurs when vapor pressure equals atmospheric pressure, water reaches that threshold at a lower temperature. In Denver (5,280 ft), water boils at about 95 °C instead of 100 °C.
What is the Antoine equation used for?
The Antoine equation relates vapor pressure to temperature for pure substances. It is widely used in chemical engineering for distillation design, in meteorology for humidity calculations, and in environmental science for modeling evaporation rates.
How accurate is the Antoine equation for water?
With the standard published constants, the Antoine equation gives vapor pressure estimates within about 1% of measured values across the 1–374 °C range. For higher precision, the Wagner or Buck equations may be used.
What is the vapor pressure of water at body temperature?
At 37 °C (98.6 °F), the vapor pressure of water is approximately 47 mmHg. This value is important in respiratory physiology because inhaled air becomes fully saturated with water vapor in the lungs.
Related Calculators
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- Relative Humidity Calculator — compute relative humidity from temperature and dew point.
- Ideal Gas Law Calculator — relate pressure, volume, temperature, and moles for gases.
- Temperature Conversion Calculator — convert between Celsius, Fahrenheit, and Kelvin for vapor pressure equations.
- Pressure Unit Converter — convert vapor pressure between mmHg, Pa, atm, and mbar.