Vapor pressure

Vapor Pressure is a term used in thermodynamics to describe the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid).

Definition

Vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid). A substance with a high vapor pressure at normal temperatures is often referred to as volatile.

Measurement

Vapor pressure can be measured in the lab by various methods such as the isoteniscope method and the gas saturation method. It can also be calculated using various theoretical methods such as the Clausius–Clapeyron relation.

Factors affecting Vapor Pressure

Vapor pressure is affected by various factors such as temperature, the nature of the liquid, and the presence of other substances in the liquid.

Temperature

The vapor pressure of a liquid increases with temperature, as the kinetic energy of the molecules increases. This is described by the Clausius–Clapeyron relation.

Nature of the Liquid

The nature of the liquid also affects its vapor pressure. Liquids with higher intermolecular forces tend to have lower vapor pressures.

Presence of other substances

The presence of other substances in the liquid can also affect the vapor pressure. For example, the addition of a non-volatile solute to a liquid lowers the vapor pressure of the liquid.

Applications

Vapor pressure has many practical applications. It is used in the formulation of nail polish, in the design of fuel pumps and in the operation of steam engines. It is also used in the prediction of boiling point and in the understanding of colligative properties.

See also

• Boiling point
• Clausius–Clapeyron relation
• Colligative properties
• Volatile substance

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