3.3.2 Pressure in Gases

In this lesson, we will explore how gases can be compressed or expanded by pressure changes and how these changes affect the behaviour of gases. We will discuss the net force exerted by the pressure of a gas, the relationship between pressure and volume at constant temperature, and how to calculate changes in pressure and volume.

Compression and Expansion of Gases

Pressure in a gas produces a net force perpendicular to the walls of the gas container or any surface it encounters. By changing the pressure applied to a gas, we can compress or expand it.

Using the particle model, we can explain the relationship between pressure, volume, and the behaviour of gas molecules.

• Increased Volume: Increasing the volume of a gas at constant temperature results in a decrease in pressure.
• Decreased Volume: Conversely, decreasing the volume of a gas at constant temperature leads to an increase in pressure.

Boyle's Law

Now, let's explore the quantitative relationship between pressure and volume at constant temperature, known as Boyle's Law.

For a fixed mass of gas held at a constant temperature, the pressure and volume are inversely proportional, following the equation:

p V = constant

Variables:

• p = Pressure in pascals, Pa
• V = Volume in metres cubed, m³

Boyle's Law applies when the temperature remains constant. Being an inverse relationship, as the volume of a gas increases, the pressure decreases, and vice versa, while keeping the product of pressure and volume constant.

Pressure Changes:

• Pressure Increase: If the pressure of a gas is increased, the volume will decrease proportionally, assuming constant temperature and a fixed mass of gas.
• Pressure Decrease: Conversely, if the pressure of a gas is decreased, the volume will increase proportionally.

Volume Changes:

• Volume Increase: Increasing the volume of a gas at constant temperature results in a decrease in pressure, following Boyle's Law.
• Volume Decrease: Decreasing the volume of a gas at constant temperature leads to an increase in pressure.

Conclusion

We explored the concept of pressure in gases and its effects on compression and expansion. Gases can be compressed or expanded through changes in pressure, and pressure exerted by gases produces a net force perpendicular to the container walls. Boyle's Law describes the inverse relationship between pressure and volume at constant temperature.