Vapor pressure

Pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases

Vapor pressure (or vapour pressure) is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. It is an indication of a liquid's evaporation rate. The vapor pressure of a substance is a critical property in determining its volatility.

Diagram showing the relationship between vapor pressure and temperature.

Vapor pressure in a cloud chamber.

Graph of water vapor pressure.

Definition

Vapor pressure is defined as the pressure exerted by a vapor in equilibrium with its liquid or solid phase. It is a measure of the tendency of particles to escape from the liquid or solid phase into the gaseous phase. The vapor pressure of a liquid varies with its temperature, increasing as the temperature rises.

Factors affecting vapor pressure

Several factors influence vapor pressure:

• Temperature: As temperature increases, the kinetic energy of the molecules increases, leading to a higher vapor pressure.
• Nature of the liquid: Different liquids have different vapor pressures at the same temperature due to variations in intermolecular forces.
• Presence of solutes: The addition of a non-volatile solute to a liquid decreases its vapor pressure, a phenomenon known as Raoult's law.

Measurement

Vapor pressure can be measured using various methods, including the manometer method, the dynamic method, and the static method. Each method has its own advantages and limitations, depending on the substance being measured and the conditions of the experiment.

Applications

Vapor pressure is a crucial concept in many fields:

• In meteorology, it is used to describe the humidity of the air.
• In chemistry, it is important for understanding distillation and evaporation processes.
• In engineering, it is used in the design of equipment such as pressure vessels and heat exchangers.

Related pages

• Boiling point
• Partial pressure
• Saturation vapor pressure