Difference Between Vapor Pressure and Partial Pressure

Vapor Pressure vs Partial Pressure
 

Partial pressure and vapor pressure are two important properties of gaseous systems. This article will compare and contrast the definitions, applications and differences between vapor pressure and partial pressure.

Vapor Pressure

To understand vapor pressure, a clear understanding in the concept of pressure, is required. Pressure is defined as the force per unit area applied in a direction perpendicular to the object. The pressure of a static fluid is equal to the weight of the fluid column above the point the pressure is measured. Therefore, the pressure of a static (non-flowing) fluid is dependent only of the density of the fluid, the gravitational acceleration, the atmospheric pressure and the height of the liquid above the point the pressure is measured. The pressure can also be defined as the force exerted by the collisions of particles. In this sense, the pressure can be calculated using the molecular kinetic theory of gasses and the gas equation. Vapor pressure is the pressure exerted by the vapor in a system, which is in equilibrium, or the pressure exerted by a vapor. A system is in equilibrium, when the gas state and the liquid or condensed state of the vapor are in contact with each other, in a closed system. A liquid is vaporized by the heat. Therefore, the temperature of the system is a measurement of the vaporization of the liquid. The temperature is also a measurement of the amount of vapor molecules a system can accommodate without forcing condensation. There are two types of vapor pressures. They are namely saturated vapor pressure and unsaturated vapor pressure. When a closed system has both the vapor and the corresponding liquid in equilibrium, the system accommodates the highest amount of vapor possible. Therefore, the system is said to be saturated. When a system has only the vapor present, it is said to be an unsaturated system, and any liquid added, until the saturation point, will evaporate. It must be noted that the saturated vapor pressure of a system only depends on the temperature of the system and the substance itself.

Partial Pressure

The partial pressure of a system is the ratio of the pressure exerted by the considered gas to the total pressure of the system. Partial pressure of a gas is merely a number. Partial pressure can only vary in the range of zero to one. The partial pressure of a pure gas is 1, while the partial pressure from a missing substance is zero. It can be proven that the partial pressure of a gas is also equal to the molecular ratio of gas for a perfect gas. The molecular ratio is the number of molecules of the gas considered divided by the total number of gas molecules. Partial pressure multiplied by the total pressure of the system yields the pressure from the considered gas.