20.10 Pressure and Density

20.10.2 Explain what is meant by the ‘partial pressure’ of a gas.

Partial pressure is the pressure exerted by one individual gas within a mixture of gases.

The atmosphere is made up of several gases, mainly nitrogen and oxygen. Each gas contributes part of the total atmospheric pressure.

For example, at mean sea level where atmospheric pressure is about 1013 hPa:

• Nitrogen makes up about 78% → partial pressure about 790 hPa

• Oxygen makes up about 21% → partial pressure about 213 hPa

• Other gases make up the small remainder

The partial pressures of all the gases added together equal the total atmospheric pressure.

Key point

As altitude increases and atmospheric pressure decreases, the partial pressure of each gas also decreases.

The percentage of oxygen remains about 21%, but its partial pressure becomes lower. This means fewer oxygen molecules are available in each breath, which is important in aviation.

20.10.4 Explain the significance of air pressure with reference to:

(a) barometric tendency;

Barometric tendency is the change in atmospheric pressure over a period of time.