Vapour pressure of water

Vapour Pressure of Water

The vapour pressure of water is the pressure exerted by water vapour in equilibrium with liquid water in a closed system at a given temperature. This physical property is crucial in understanding various phenomena in meteorology, climatology, and the field of thermodynamics. Vapour pressure is dependent on temperature; as the temperature increases, the kinetic energy of the water molecules increases, leading to an increase in vapour pressure.

Definition

Vapour pressure is defined as the pressure exerted by a vapour in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. For water, this equilibrium involves the transition of molecules between the liquid phase and the gaseous phase (water vapour).

Measurement

The vapour pressure of water can be measured using various methods, including the static method, dynamic method, and effusion method. Each technique has its advantages and is chosen based on the specific requirements of the experiment or application.

Factors Affecting Vapour Pressure

Several factors influence the vapour pressure of water:

• Temperature: The most significant factor affecting vapour pressure is temperature. As temperature increases, vapour pressure increases.
• Surface Area: In practical scenarios, the surface area of the liquid does not affect the vapour pressure, assuming the system reaches equilibrium.
• Impurities: The presence of non-volatile solutes (e.g., salt) in water can lower its vapour pressure, a phenomenon known as Raoult's law.

Applications

Understanding the vapour pressure of water is essential in various fields:

• In Meteorology, it helps in predicting weather patterns and understanding phenomena like humidity and dew point.
• In Climatology, it is crucial for modeling the Earth's climate system and understanding the water cycle.
• In Engineering and Thermodynamics, vapour pressure is key to designing and operating equipment like boilers and condensers.

Vapour Pressure and Boiling Point

The boiling point of water is the temperature at which its vapour pressure equals the atmospheric pressure. At higher elevations, where atmospheric pressure is lower, water boils at a lower temperature.

Calculations and Models

Several empirical equations and models exist for calculating the vapour pressure of water, including the Clausius-Clapeyron equation and the Antoine equation. These models are based on the principles of thermodynamics and provide a means to estimate vapour pressure at various temperatures.

See Also

• Phase of matter
• Equilibrium chemistry
• Thermodynamics
• Humidity
• Boiling point