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The pressure p of a fluid on a surface is defined as the normal force exerted by the fluid per unit area of the surface. If force is measured in Newtons and area in square meters, the unit of pressure is 1 N/m2 or 1 Pascal (1 Pa). Other important units are 1 bar = 105 Pa, 1 atm = 1.0133 × 105 Pa, 1 Torr = 1.3332 × 102 Pa, and 1 psi (pound per square inch) = 6.8948 × 103 Pa.

The total pressure exerted on a boundary wall is called the absolute pressure, whereas the pressure exerted by the atmosphere is called atmospheric pressure. Gauge pressure designates the difference between absolute and atmospheric pressure in a particular system and is normally measured by an instrument, which has atmospheric pressure as a reference. Since a vertical column of a fluid with density ρ under the influence of gravity exerts a pressure at its base in direct proportion to its height h,

(1)

pressure is also expressed as the equivalent height of a fluid column (e.g., mm water, mm mercury). The above definition of pressure is only valid for an area element sufficiently large so that the fluid may be treated as a continuum, i.e., as long as the average distance a fluid molecule travels between collisions is small compared with a side of the area element.

The partial pressure pi of a component in a mixture of gases is the pressure this component would exert if it solely occupied the entire volume of the mixture at the given temperature. For ideal gases, it is defined by the Ideal Gas Law (See Gas Law)

(2)

where Ni is the number of moles of component i, is the universal gas constant, T is the absolute temperature, and V is the volume.

The critical pressure is the pressure at the critical state of a substance. At pressures higher than the critical pressure, no distinction can be made between liquid and vapor phases.

The pressure at which a liquid vaporizes or a vapor condenses at a given temperature is called the saturation pressure.