Required Practicals / AQA / Practical 8
8 A2 3.6.2.1

Boyle's Law and Charles's Law

Verify Boyle's Law (pV = constant at constant T) and Charles's Law (V/T = constant at constant p).

Apparatus

  • Boyle's Law apparatus (sealed air column with pressure gauge and pump)
  • Ruler to measure gas column length (proportional to volume)
  • Capillary tube sealed at one end with a mercury plug (for Charles's Law)
  • Thermometer and water bath with heater
  • Vernier calipers (to measure capillary tube diameter)

Safety

  • Do not exceed the maximum rated pressure of the Boyle's Law apparatus.
  • Take care when heating the water bath; handle hot glassware with care.

Method

  1. Boyle's Law: use the pump to set the pressure. Read pressure p and measure the length L of the trapped air column. Volume is proportional to L for a uniform tube.
  2. Repeat for at least eight pressure settings spanning a wide range. Allow time for the temperature to stabilise after each pressure change.
  3. Plot p against 1/V (or 1/L): straight line through origin confirms pV = constant.
  4. Charles's Law: immerse the sealed capillary tube in the water bath. Heat the bath slowly from ~10 degrees C to ~80 degrees C.
  5. At each temperature (measured in kelvin), record the length L of the trapped air column. Plot L against T (K).

Key Variables

Independent Pressure p (Boyle's) or temperature T in kelvin (Charles's)
Dependent Volume V (Boyle's) or volume V (Charles's)
Controlled Temperature constant for Boyle's Law; Pressure constant (atmospheric) for Charles's Law

Analysis and Results

  • Boyle's Law: $pV = $ constant. Plot p vs $1/V$: straight line through origin.
  • Charles's Law: $V \propto T$ (kelvin). Plot L vs T (K): straight line that extrapolates to zero at T = 0 K, confirming absolute zero.
  • Temperature must be in kelvin for Charles's Law: $T\,/\text{K} = T\,/\degree\text{C} + 273$.

Common Errors

  • Not waiting for thermal equilibrium after changing pressure (Boyle's Law) or temperature (Charles's Law).
  • Plotting L against temperature in degrees C rather than kelvin for Charles's Law.
  • Gas leaking past the mercury plug in the capillary tube.
  • Plotting p against V (a curve) instead of p against 1/V (a straight line).

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

Q1 3 marks

A trapped gas has volume $240$ cm$^3$ at pressure $1.0 \times 10^5$ Pa. Calculate its pressure when compressed to $60$ cm$^3$ at constant temperature.

Q2 3 marks

A student plots gas column length against temperature in degrees Celsius for a Charles's Law experiment. The graph is a straight line but does not pass through the origin. Explain why and how to correct this.

Q3 2 marks

State two conditions under which a real gas does not obey the ideal gas laws.