Required Practicals/OCR A/Practical 8
8A2PAG 8

Boyle's Law (PAG 8)

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

Apparatus

  • Boyle's Law apparatus with sealed air column and Bourdon pressure gauge
  • Ruler (to measure column length)
  • Thermometer
  • Pump

Safety

  • Do not exceed rated maximum pressure.
  • Ensure all seals are intact before pressurising.

Method

  1. Set a range of pressures using the pump. At each pressure allow thermal equilibrium, then record p and column length L.
  2. Take readings on increasing and decreasing pressure.
  3. Plot p vs $1/L$: straight line through origin confirms $pV = $ constant.
  4. Calculate pL for each reading; values should be approximately constant.

Key Variables

Independent
Pressure p
Dependent
Volume V (proportional to L)
Controlled
Temperature; Sealed gas mass

Analysis and Results

  • $pV = $ constant. Plot p vs $1/V$: straight line through origin.
  • Any deviation at high pressure indicates non-ideal gas behaviour.
  • Temperature must be constant throughout.

Common Errors

  • Not waiting for thermal equilibrium after pressure change.
  • Plotting p vs V rather than p vs $1/V$.
  • Gas leaking from the sealed column.

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

Q13 marks
A student records the following values: $p = 1.5 \times 10^5$ Pa, $V = 40$ cm$^3$. They then compress to $V = 25$ cm$^3$ at constant temperature. Calculate the new pressure.
Q22 marks
Why must the temperature be kept constant for Boyle's Law to apply?