IGCSE 0625 / Section 2.1 / Extended
Supplement (Extended) content

Squeeze it, and it pushes back.

Trap some air in a syringe, seal the end, and press. The further you push, the harder it resists. Halve the space and the pressure doubles. The same particles are simply crowded into less room, so they strike the walls twice as often.

The key idea

For a fixed mass of gas at constant temperature, the pressure multiplied by the volume is constant: pV = constant, so p1V1 = p2V2. Halving the volume doubles the pressure.

Definition · learn the exact words

Boyle’s law states that for a fixed mass of gas at constant temperature, the pressure multiplied by the volume is constant.

p V = constant, so p₁V₁ = p₂V₂

True only at constant temperature for a fixed mass of gas.

Section 01

Trade volume for pressure.

Change the volume of a trapped gas at constant temperature and watch the pressure respond.

Stage 1 · Learn

Check boyles law

Four quick checks. Each correct answer earns XP and lights this skill on your star map.

Quick check+10 XP

Boyle’s law applies to a fixed mass of gas at constant...

Quick check+10 XP

For a fixed mass of gas at constant temperature, the product pV is...

Quick check+10 XP

If the volume of a gas is halved at constant temperature, the pressure...

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Compressing a gas at constant temperature makes the particles hit the walls...

Section 02

The inverse relationship.

Pressure and volume move in opposite directions so that their product stays fixed.

p V = constant  (fixed mass of gas, constant temperature)
p₁ V₁ = p₂ V₂
Worked example

A gas at 100 kPa occupies 2.0 m³. It is compressed to 0.50 m³ at constant temperature. Find the new pressure.

Step 1 · Write the lawp₁V₁ = p₂V₂
Step 2 · Rearrangep₂ = p₁V₁ ÷ V₂
Step 3 · Substitute and solvep₂ = (100 × 2.0) ÷ 0.50 = 400 kPa
Examiner trap

At constant temperature the particles do not speed up when you compress the gas. The pressure rises only because the same particles are packed into a smaller space, so they strike the walls more often per second.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 2.

Finish the four checks above to unlock the exam questions
Exam style+20 XP

A gas at 200 kPa occupies 3.0 m³. Compressed to 1.0 m³ at constant temperature, its pressure becomes...

Exam style+20 XP

A gas at 150 kPa and 4.0 L expands to 6.0 L at constant temperature. Its new pressure is...

Exam style+20 XP

The relationship p₁V₁ = p₂V₂ can be used only when the gas...

Skill unlocked

Boyles law, mastered.

That completes the particle model. Keep the chain going.

-Rank -Level -Score -Topics
Go deeper · practice
Six original Cambridge-style questions
The pV = constant relationship, using p1V1 = p2V2, and the particle explanation for the pressure change. Attempt each, then reveal the worked solution.
Stage 3 · master the unit
Thermal physics challenge
Mixed questions across the whole unit, each one worth XP. Start this only when you feel confident across every topic in the unit, not just boyles law.
Start the unit challenge →