Required Practicals / Edexcel / Practical 11
11 A2 CP11

Boyle's Law (CP11)

Verify that pressure and volume of a fixed mass of gas are inversely proportional at constant temperature.

Apparatus

  • Boyle's Law apparatus with sealed air column and pressure gauge
  • Ruler to measure gas column length
  • Thermometer
  • Oil-sealed pump

Safety

  • Do not exceed the rated pressure of the apparatus.
  • Ensure all connections are tight before pressurising.

Method

  1. Use the pump to set a range of pressures from ~50 kPa to ~300 kPa. At each setting, allow equilibrium and record pressure p and gas column length L (volume $\propto L$).
  2. Take readings on both increasing and decreasing pressure.
  3. Record room temperature before and after; it should remain constant.
  4. Plot p vs $1/L$: straight line through origin confirms $pV = $ constant.
  5. Alternatively, plot pL vs p: horizontal line confirms the relationship.

Key Variables

Independent Pressure p
Dependent Volume V (proportional to column length L)
Controlled Temperature; Mass of trapped gas (sealed column)

Analysis and Results

  • Boyle's Law: $pV = $ constant. Plot p vs $1/V$: straight line through origin.
  • Calculate pV for each reading; if constant (within uncertainty), the law is verified.
  • A significant deviation at high pressures indicates non-ideal gas behaviour.

Common Errors

  • Not allowing time for thermal equilibrium after compressing (compression heats the gas).
  • Plotting p vs V (a curve) rather than p vs $1/V$ (a straight line).
  • Reading pressure from an analogue gauge with parallax error.

Exam-style questions on this practical. Click Show mark scheme to reveal the answer after attempting each question.

Q1 3 marks

Gas is trapped in a syringe at pressure $1.0 \times 10^5$ Pa and volume $80$ cm$^3$. The plunger is pushed in until the volume is $50$ cm$^3$ at constant temperature. Calculate the new pressure.

Q2 2 marks

A student notices that the calculated value of pV slightly increases at high pressures. Suggest why.