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pV = constant means that for a fixed mass of gas at constant temperature, pressure times volume stays the same. Free GCSE Physics notes and 14 FAQs.
Calculating Pressure and Volume Changes is part of Particle model and pressure in GCSE AQA Higher Physics.
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GCSE AQA Higher
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pV = constant means that for a fixed mass of gas at constant temperature, pressure times volume stays the same. Free GCSE Physics notes and 14 FAQs.
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Use this lesson to stay inside Particle model and pressure while connecting the detail back to the wider Particle model of matter topic.
Read the core explanations from Calculating Pressure and Volume Changes before testing yourself from memory.
When you squeeze a balloon, it gets harder to compress. When a diver goes deeper underwater, the air in their lungs compresses. These everyday experiences reveal a fundamental relationship between pressure and volume that affects everything from car engines to breathing apparatus.
Understanding this relationship lets you predict how a gas will compress under pressure. This helps explain everyday situations such as syringes, lungs and diving.
Picture gas particles bouncing around inside a container like tiny ping-pong balls. They constantly collide with the walls, creating pressure. The more collisions per second, the higher the pressure.
Now imagine squashing the container to half its volume. The same number of particles are now crammed into half the space. They hit the walls twice as often, so the pressure doubles. This is the key insight behind Boyle's Law.
Robert Boyle discovered that for a fixed mass of gas at constant temperature, pressure and volume are inversely proportional. When one doubles, the other halves. When one triples, the other becomes one-third.
**Trap**: Many students think pressure and volume are directly related - that increasing pressure increases volume. Actually, they're inversely related. Higher pressure means particles are squeezed into smaller volume.
Let's see how to use Boyle's Law in exam-style calculations. First identify the initial and final pressure and volume values, then substitute them into p₁V₁ = p₂V₂. If the gas is compressed, volume falls and pressure rises. If the gas expands, volume rises and pressure falls. This proportional reasoning is a quick check on whether your final answer makes physical sense.
Worked example: Calculating Final Pressure
A gas has initial pressure 100,000 Pa and volume 0.02 m³. It is compressed to 0.005 m³ at constant temperature. Calculate the final pressure. [3 marks]
Answer: 400,000 Pa
Notice how the volume decreased by a factor of 4 (from 0.02 to 0.005), so the pressure increased by the same factor (from 100,000 to 400,000). This confirms our inverse relationship.
Students often make predictable errors with Boyle's Law calculations. Here are the main traps and how to avoid them.
**Unit confusion**: Always check your pressure units. Exam questions might give pressure in kPa (kilopascals) but expect answers in Pa. Remember: 1 kPa = 1,000 Pa. Volume units must match too - don't mix m³ with cm³.
**Rearrangement errors**: When finding final volume, students often write $V_2 = \frac{p_1V_1}{p_2}$ incorrectly as $V_2 = \frac{p_2V_1}{p_1}$. Always check your algebra by substituting simple numbers.
Worked example: Unit Conversion Example
A gas at 150 kPa and volume 20 cm³ is compressed to 5 cm³. Find the final pressure in Pa. [3 marks]
Answer: 600,000 Pa
Boyle's Law questions are common in GCSE physics papers. To maximise marks, state the law clearly: for a fixed mass of gas at constant temperature, pressure and volume are inversely proportional. Then show the substitution and the rearrangement. Mention the particle model in explain questions by saying that the same particles are squeezed into a smaller space so they collide with the walls more often.
**Command word focus**: 'Calculate' questions need formula, substitution, and answer with units. 'Explain' questions need the particle model - describe how particle collisions change when volume changes.
Published by Revise Right. Use the official specification reference to check the requirements for your course. Content updated: 2026-03-20.
Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
pV = constant means that for a fixed mass of gas at constant temperature, pressure times volume stays the same, so pressure is inversely proportional to volume. It is used as p₁V₁ = p₂V₂ to calculate changes in pressure or volume.
Use p₂ = p₁V₁/V₂. A gas at 100,000 Pa with volume 0.02 m³ compressed to 0.005 m³ at constant temperature has p₂ = 2,000 ÷ 0.005 = 400,000 Pa; the volume fell by a factor of 4, so the pressure rose by 4.
The final volume is found by rearranging p₁V₁ = p₂V₂ to V₂ = p₁V₁/p₂. A common error is writing V₂ = p₂V₁/p₁, so the algebra should always be checked by substituting simple numbers.
Convert kPa to Pa when the answer is needed in Pa, using 1 kPa = 1,000 Pa, and make sure both volumes use the same units. A gas at 150 kPa (150,000 Pa) and 20 cm³ compressed to 5 cm³ rises to 150,000 × 20 ÷ 5 = 600,000 Pa.
Pressure and volume are inversely proportional for a fixed mass of gas at constant temperature, not directly related. If the gas is compressed, volume falls and pressure rises; if it expands, volume rises and pressure falls, because the same particles collide with the walls more or less often.
Gas at 150 kPa and 20 cm³ compressed to 5 cm³ ends at a final pressure of 600,000 Pa. Converting first gives p₁ = 150,000 Pa, then p₂ = p₁V₁/V₂ = 150,000 × 20 ÷ 5 = 600,000 Pa; the volume units cancel because both are in cm³.
Boyle's law applies to a fixed mass of gas at constant temperature. In this case the pressure is inversely proportional to the volume, so pV = constant.
A Boyle's law answer is checked with proportional reasoning: if the gas is compressed, volume falls and pressure rises; if it expands, volume rises and pressure falls. When volume falls by a factor of 4, from 0.02 to 0.005 m³, pressure rises by 4.
When a gas expands at constant temperature, its volume rises and its pressure falls, because pressure and volume are inversely proportional. The particles have more space, so they collide with the walls less often.
The volume units must match in Boyle's law: m³ should not be mixed with cm³. When both volumes are in cm³, the units cancel, and pressure units must also match, with 1 kPa = 1,000 Pa.
Boyle's law is explained by the particle model: when a gas is compressed, the same particles are squeezed into a smaller space, so they collide with the walls more often. More collisions per second means higher pressure.
Inversely proportional means that when the volume of a gas doubles, its pressure halves, and when the volume becomes one-third, the pressure triples. For a fixed mass of gas at constant temperature, pressure × volume stays constant.
Gas at 100,000 Pa and 0.02 m³ compressed to 0.005 m³ at constant temperature ends at 400,000 Pa. Using p₂ = p₁V₁/V₂, p₂ = 100,000 × 0.02 ÷ 0.005 = 2,000 ÷ 0.005 = 400,000 Pa.
When a diver goes deeper underwater, the pressure on the air in their lungs increases, so the air compresses into a smaller volume. Boyle's law predicts this, because pressure and volume are inversely proportional for a fixed mass of gas at constant temperature.