Pressure is defined as 'the force exerted on a surface divided by the area of the surface'. The mathematical equation for pressure is

For a gas, the force that creates pressure is exerted by the gas molecules or atoms as they constantly collide with the walls of their container. Pressure results from the collisions of gas molecules with the walls of the balloon.

The effects of pressure are also observed in the mixture of gases in the atmosphere, which is composed of about 78% nitrogen, 21% oxygen, 1% argon, and other minor constituents by volume.

Units of pressure

A number of different units are used to measure pressure. Because instruments used for measuring pressure, such as the barometer and manometer, often contain mercury, the most commonly used units for pressure are based on the height of the mercury column (in millimeters) that the gas pressure can support.

Standard atmospheric pressure (1 atm) is equal to the pressure that supports a column of mercury exactly 760 mm (or 76 cm) high at 0°C at sea level. The mmHg unit is also called the torr, after the Italian scientist Evangelista Torricelli, who invented the barometer. Thus,

1 torr = 1 mmHg and 1 atm = 760 mmHg.

The SI unit of pressure is the pascal (Pa), named after French scientist Blaise Pascal.

Where N is newton, m is meter, and s is second.

The relationship between atmosphere and pascal is

1atm = 101, 325Pa = 1.01325 × 10⁵Pa

We will usually express gas pressure in atmosphere, pascal, kilopascal, or torr, so you should be familiar in converting pressures from one unit to another.