Dalton's Law of Partial Pressure

Partial pressure is the pressure that an individual gas would exert if it occupied the total volume available in a container. In a mixture of ideal gases, each component exerts its own partial pressure independently, determined by the number of molecules and their collisions with container walls. The partial pressure reflects the thermodynamic activity of that gas's molecules.

Dalton's law states that the total pressure in a container of mixed ideal gases equals the sum of the partial pressures of its components. When gases reach thermal equilibrium, their molecules have the same average kinetic energy and each gas obeys the ideal gas equation separately. This additive property is consistent with Pascal's principle and assumes no chemical reactions between components.

In a gas mixture at thermal equilibrium, molecules move randomly and collide with each other and container walls. Each gas component maintains the same temperature and average kinetic energy as the others, allowing each to obey the ideal gas law separately. This independence means each gas exerts pressure on the walls as if it alone occupied the entire volume.

Vapor pressure is the partial pressure of a vapor when it is in equilibrium with the liquid phase of the same substance at a given temperature. The partial pressure of water vapor in air cannot exceed the vapor pressure of water at that temperature. When partial pressure reaches vapor pressure, condensation occurs, forming dew.

Relative humidity is the ratio of the partial pressure of water vapor to the vapor pressure of water at a specific temperature. It indicates how much moisture the air contains compared to its maximum capacity. This ratio determines whether condensation will occur and helps predict dew point conditions.

The dew point is the temperature at which condensation first appears on a cooled surface. It can be measured by slowly cooling a metal ball and recording the temperature when water droplets form on its surface. This temperature indicates the point where the partial pressure of water vapor equals the vapor pressure of water.

In a mixture of ideal gases at thermal equilibrium, the number of molecules of each gas is proportional to its partial pressure. Gases with more molecules exert higher partial pressures because more molecular collisions occur with container walls. This relationship allows calculation of partial pressures using the ideal gas law when molecular counts are known.