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5.3.5Chemical Energetics

The Pressure Law — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 11 of 12 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5The Pressure Law

5.3.5 The Pressure Law

When the volume of a gas is held constant, its pressure depends on its kelvin temperature.

Pressure law
  • For a fixed mass of gas at constant volume, pressure is directly proportional to the kelvin temperature.
Equation
  • The ratio p ÷ T is constant, so p₁ ÷ T₁ = p₂ ÷ T₂, with T in kelvin.
Heating a gas
  • Heating a gas in a sealed container increases the speed and collision rate of its particles, so the pressure rises.
Cooling a gas
  • Cooling a sealed gas reduces particle speed and the force of collisions, so the pressure falls.

Part 2 of 5Particle Explanation

The pressure law follows directly from the kinetic theory of gases.

1
At constant volume the container walls stay the same distance apart, so the volume of the gas cannot change.
2
A higher temperature means the particles have more kinetic energy and move faster.
3
Faster particles strike the walls with greater force and more often, which raises the pressure.
4
Doubling the kelvin temperature doubles the pressure, which is the pressure law in action.

Part 3 of 5Applications

Pressure changes with temperature in many practical devices and situations.

Applications
Aerosol cans
  • Warnings on aerosol cans advise against heating, because the pressure of the gas inside rises sharply.
Car tyres
  • Tyre pressure increases as the tyres warm up on a hot day, so drivers check the pressure regularly.
Pressure cooker
  • A sealed cooker raises the pressure inside, which raises the boiling point of the water.
Limitation
  • The law only applies to a fixed mass of gas at constant volume, with temperatures in kelvin.

Part 4 of 5Worked Example

Question

  • A sealed container holds a gas at a pressure of 2.0 × 10⁵ Pa and a temperature of 280 K.
  • Calculate the pressure when the temperature rises to 350 K, with the volume kept constant.

  1. Write the equationp₁ ÷ T₁ = p₂ ÷ T₂.
  2. Rearrange to give p₂ = p₁ × T₂ ÷ T₁ = 2.0 × 10⁵ Pa × 350 K ÷ 280 K.
  3. p₂ = 2.5 × 10⁵ Pa.

Answer The pressure rises to 2.5 × 10⁵ Pa.

Part 5 of 5Exam Checklist & Tips

5.3.5 – Key Points for the Exam

  • Pressure law For a fixed mass of gas at constant volume, pressure is directly proportional to kelvin temperature.
  • Constant volume A volume that does not change during the process being studied.
  • Kelvin The SI unit of temperature, in which the pressure law is applied.
  • Directly proportional Two quantities that increase at the same rate, so doubling one doubles the other.
  • Collision An impact between a gas particle and another particle or the walls of the container.

Common Mistakes

  • Using degrees Celsius in the equation, which gives a wrong answer because the ratio of temperatures is not meaningful in Celsius.
  • Saying pressure is inversely proportional to temperature;
    • it is directly proportional when temperature is in kelvin.
  • Forgetting to convert the temperatures into kelvin before substituting into the equation.
  • Applying the pressure law when the volume of the gas also changes, which needs a different method.

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