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5.3.4Chemical Energetics

The Gas Laws — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 10 of 12 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Boyle's Law

5.3.4 The Gas Laws

For a fixed mass of gas at constant temperature, pressure and volume are inversely related.

Boyle's law
  • For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume.
Equation
  • The product p × V is constant, so p₁V₁ = p₂V₂ for any two states of the same gas.
Compression
  • Squeezing a gas into a smaller volume increases the rate of collisions with the walls, so the pressure rises.
Expansion
  • A gas expanding into a larger volume has fewer collisions per second with each surface, so the pressure falls.

Part 2 of 5Investigating Boyle's Law

A sealed syringe and a pressure gauge give a simple way to test the law.

1
Trap a fixed mass of air in a sealed syringe connected to a pressure gauge.
2
Record the pressure and the volume of the trapped air at several different positions of the plunger.
3
Plot pressure against 1/volume to obtain a straight line through the origin, which confirms an inverse relationship.
4
Avoid holding the syringe for long periods, so the temperature of the trapped air stays constant during the readings.

Part 3 of 5Applications

Boyle's law explains several everyday situations involving gases.

Applications
Breathing
  • The lungs expand to lower the pressure inside, drawing air in, then contract to push air out.
Syringe
  • Pushing the plunger in compresses the trapped air and raises its pressure.
Diving tanks
  • The air in a diver's tank is compressed, so a large amount of air can be stored in a small volume.
Limitation
  • Boyle's law only applies when the temperature and the mass of the gas are both constant.

Part 4 of 5Worked Example

Question

  • A fixed mass of gas has a pressure of 100 kPa and a volume of 0.60 m³ at constant temperature.
  • Calculate its pressure when the volume is compressed to 0.20 m³.

  1. Write the equationp₁V₁ = p₂V₂.
  2. Rearrange to give p₂ = p₁V₁ ÷ V₂ = 100 kPa × 0.60 m³ ÷ 0.20 m³.
  3. p₂ = 300 kPa.

Answer The new pressure is 300 kPa, three times the original pressure, because the volume has been reduced to one third.

Part 5 of 5Exam Checklist & Tips

5.3.4 – Key Points for the Exam

  • Boyle's law For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume.
  • Fixed mass A quantity of gas that does not change during the experiment.
  • Constant temperature A temperature that does not change throughout the experiment.
  • Inversely proportional Two quantities for which doubling one halves the other.
  • Pressure The force per unit area, measured in pascals.

Common Mistakes

  • Saying pressure and volume are directly proportional;
    • they are inversely proportional at constant temperature.
  • Forgetting that Boyle's law only applies when the mass and temperature of the gas are constant.
  • Using inconsistent units for the pressures or volumes on the two sides of the equation.
  • Rearranging p₁V₁ = p₂V₂ incorrectly, for example dividing by p₁ instead of by V₂.

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