GCSE Physics revision

Pressure in gases (physics only)

Particle model and pressure

AQA 4.3.3.2

Your specification

AQA student objectives

Learning pathway

ALL 🎯

Use the particle model to explain how increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure.

MOST 🎯🎯

Apply pressure in gases knowledge to a relevant numerical or graphical problem and show the working.

SOME 🎯🎯🎯

Analyse an unfamiliar problem involving pressure in gases and justify the method or conclusion.

Revision summary

Read on screen, then print for Cornell-style active revision.

Gas pressure is caused by gas particles colliding with the walls of their container.
Pressure can be increased by raising the temperature, adding more particles, or decreasing the volume of the container.

Pressure and volume are inversely related, meaning as one increases, the other decreases.
For a fixed amount of gas at a constant temperature, the product of pressure and volume always remains constant.
This relationship is expressed as P V = constant, which is known as Boyle's Law.

2. The Inverse Relationship Between Pressure and VolumeAQA

Pressure and volume are inversely related, meaning as one increases, the other decreases.
For a fixed amount of gas at a constant temperature, the product of pressure and volume always remains constant.
This relationship is expressed as P V = constant, which is known as Boyle's Law.

To solve problems, identify the initial pressure and volume, calculate the constant, then use it to find the unknown value.
Because the constant is the same for both states of the gas, you can write
P1 V1 = P2 V2 to directly compare two situations.
This rearranged form avoids needing to calculate the constant separately and is often quicker to use.

3. Using the PV = Constant EquationAQA

To solve problems, identify the initial pressure and volume, calculate the constant, then use it to find the unknown value.
Because the constant is the same for both states of the gas, you can write
P1 V1 = P2 V2 to directly compare two situations.
This rearranged form avoids needing to calculate the constant separately and is often quicker to use.

A gas at a volume of 1.5 m³ and pressure of 100 Pa is compressed to 0.3 m³ at constant temperature.
Using P1 V1 = P2 V2 : 100 × 1.5 = P2 × 0.3, giving P2 = 500 Pa.
As the volume decreases, the pressure increases, which is consistent with the inverse relationship.

1,800 L of atmospheric air at 101 kPa is compressed into a 12 L gas cylinder at constant temperature.
Using P1 V1 = P2 V2 : 101 × 1800 = P2 × 12, giving P2 = 15,150 kPa.
It is important to keep units consistent throughout the calculation so the answer is in the correct unit (kPa in this case).

5. Worked Example 2: Filling a Diving CylinderAQA

1,800 L of atmospheric air at 101 kPa is compressed into a 12 L gas cylinder at constant temperature.
Using P1 V1 = P2 V2 : 101 × 1800 = P2 × 12, giving P2 = 15,150 kPa.
It is important to keep units consistent throughout the calculation so the answer is in the correct unit (kPa in this case).

Boyle's Law only applies when both the temperature and the mass (amount) of gas remain constant.
If temperature or the number of particles changes, the simple P1 V1 = P2 V2 relationship no longer holds.

Always list your known values (P1 , V1 , V2 ) before substituting into the equation.
Check that pressure and volume units are consistent on both sides of the equation before calculating.
Rearrange P1 V1 = P2 V2 to isolate the unknown by dividing both sides by the known value on the same side.

7. Tips for Tackling Pressure–Volume QuestionsAQA

Always list your known values (P1 , V1 , V2 ) before substituting into the equation.
Check that pressure and volume units are consistent on both sides of the equation before calculating.
Rearrange P1 V1 = P2 V2 to isolate the unknown by dividing both sides by the known value on the same side.