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Gas pressure results from collisions between moving gas particles and container walls. Each collision exerts a small force, but the cumulative effect of numerous collisions across a surface creates measurable pressure. According to basic postulates of kinetic molecular theory particle size energy and collision, pressure is directly proportional to the number of gas particles in a given volume, so fewer particles mean fewer collisions and lower pressure.

A barometer is a mercury-filled glass tube inverted into a dish of mercury. Atmospheric pressure pushes on the mercury surface outside the tube, causing the mercury column inside to rise to a height proportional to that pressure. At sea level, standard atmospheric pressure of 1 atm supports a mercury column 760 millimeters high.

A closed-end manometer has one sealed end in a vacuum and measures absolute gas pressure directly from the height difference in mercury. An open-end manometer has one end exposed to the atmosphere and measures pressure relative to atmospheric pressure. The gas pressure is calculated by adding or subtracting the height difference from atmospheric pressure depending on whether gas pressure exceeds or falls below it.

Mercury is preferred because it is 13.5 times denser than water. This density difference means atmospheric pressure supports a mercury column only about 0.76 meters tall, whereas a water column would need to be 10.3 meters tall. The shorter mercury column makes barometers practical and convenient for measuring pressure.

Pressure is defined as force exerted per unit area. It is directly proportional to force and inversely proportional to area. Pressure increases by applying more force or reducing the area over which force is distributed, and decreases by reducing force or increasing the contact area.

The SI unit of pressure is the pascal (Pa), equal to 1 newton per square meter. Larger pressures are often expressed in kilopascals (kPa) or bars, where 1 bar equals 100,000 Pa. Pressure can also be measured in atmospheres (atm), with 1 atm equal to 101,325 Pa or 760 millimeters of mercury.

Pressure is directly proportional to gas density, the number of particles per unit volume. Higher density means more gas particles occupy the same space, resulting in more frequent collisions with container walls and greater pressure. Conversely, lower density produces fewer collisions and reduced pressure.