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
- 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 $\propto L$).
- Take readings on both increasing and decreasing pressure.
- Record room temperature before and after; it should remain constant.
- Plot p vs $1/L$: straight line through origin confirms $pV = $ constant.
- 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.
Q13 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.
Q22 marks
A student notices that the calculated value of pV slightly increases at high pressures. Suggest why.