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A-level Physics required practicals

Gas laws — Boyle's law and Charles's law

AQA 3.6.2.2 · RP8

A-level Physics (7408) · Required practical 8 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.

Board and spec code confirmed against AQA 7408 · registry checked 2026-07-11How this checking works

Test Boyle's law at constant temperature and Charles's law at constant pressure using two clearly separated experimental branches.

Apparatus

  • Boyle branch: trapped-gas apparatus with calibrated volume scale and pressure gauge
  • Charles branch: gas volume apparatus that can move at approximately constant pressure
  • Water baths, thermometer, heater and heatproof container
  • Metre rule or travelling scale where volume is inferred from gas-column length
  • Eye protection and clamps to secure the apparatus

Apparatus & techniques (AT)

AT a · analogue measurement

Read pressure, gas volume and temperature from calibrated analogue scales at equilibrium in the two gas-law branches.

Safety

Hazard

Hot water, heaters and hot glass can cause burns.

Control

Wear eye protection, use a heatproof mat, handle hot apparatus with tongs or thermal gloves and allow it to cool before dismantling.

Hazard

Over-pressurising trapped gas can eject a fitting or break apparatus.

Control

Clamp the apparatus, stay within its rated range, change volume slowly and never heat a rigid sealed vessel.

Method

  1. 1Boyle branch — trap a fixed sample of gas, change its volume slowly and wait after each adjustment until the pressure and temperature readings have settled.
  2. 2Boyle branch — record pressure and volume over a broad safe range while keeping the gas amount and temperature constant; return to one setting to check for leakage or drift.
  3. 3Charles branch — keep a fixed gas sample at approximately constant pressure and place it in water baths spanning a useful temperature range.
  4. 4Charles branch — allow the gas and thermometer to reach thermal equilibrium at each temperature, then record the gas volume from the same scale reference.
  5. 5For both branches, record instrument resolution and any dead volume that is not included in the visible gas-column measurement.

CPAC focus (editorial)

This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.

  • CPAC 2: Editorial focus: design two valid branches, naming the constant condition and choosing a useful measurement range for each.
  • CPAC 3: Editorial focus: secure pressurised and heated apparatus and manage hot-water hazards.
  • CPAC 4: Editorial focus: record settled pressure, volume and temperature readings with scale resolutions.
  • CPAC 5: Editorial focus: linearise Boyle data, extrapolate Charles data and evaluate the limits of both models.

Variables

Independent

Boyle branch: gas volume; Charles branch: gas temperature

Dependent

Boyle branch: gas pressure; Charles branch: gas volume

Control

  • Amount of trapped gas in both branches
  • Temperature during the Boyle branch
  • Pressure during the Charles branch
  • The same apparatus, scale reference and equilibration criterion

Results & processing

  • Boyle branch: plot pressure p against 1/V for a straight-line test, and calculate pV at each setting to check whether it is constant within uncertainty.
  • Charles branch: plot volume V against Celsius temperature. Extrapolate the best-fit line to V = 0 to estimate absolute zero, while recognising that this is well beyond the measured range.
  • Use residual scatter and error bars to decide whether deviations are random or suggest leakage, dead-volume error or incomplete thermal equilibrium.

Analysis skills

  • Transform Boyle data by plotting p against 1/V and compare pV values with their uncertainty.
  • Fit V against Celsius temperature for the Charles branch and obtain the V = 0 intercept.
  • Explain why an absolute-zero estimate from a long extrapolation carries greater uncertainty than readings inside the measured range.

Uncertainty

Sources

  • Parallax and finite divisions on pressure, volume and temperature scales
  • Dead volume in tubing and connectors
  • Leaks, compression heating and thermal lag before equilibrium

Calculations

  • Combine percentage uncertainties in p and V when assessing a calculated pV value.
  • Use extreme acceptable best-fit lines to estimate uncertainty in the Charles-law intercept.
  • Quote volume uncertainty from the scale resolution and propagate it through 1/V where needed.

Interpretation

  • Repeated readings reduce random reading scatter but cannot correct an unmeasured dead volume.
  • An apparently straight graph does not prove the control condition was maintained; stable temperature or pressure readings provide that evidence.

Exam angles

  • State what is held constant in the Boyle and Charles branches.
  • Choose and justify p against 1/V rather than p against V for a linear test.
  • Use a Charles-law intercept to estimate absolute zero and explain the extrapolation limitation.
  • Identify thermal lag, leakage and dead volume from a pattern in the data.

Where students lose marks

Taking a reading immediately after compressing the gas or moving it into a new water bath.

Fix: Change the condition slowly and wait until the pressure, volume and temperature readings are stable before recording them.

  • Examiner report: P3-22 · PDF p. 5

Writing 'room temperature' as though it proves temperature stayed constant in the Boyle branch.

Fix: Monitor temperature, adjust volume slowly and allow re-equilibration so compression heating does not bias the pressure reading.

Calling one mixed data set a gas-law test without separating the constant-temperature and constant-pressure conditions.

Fix: Label the Boyle and Charles branches separately and name the controlled variable for each one.

Improve the method

  • Use several readings across a broad safe range rather than relying on two endpoints.
  • Immerse the relevant gas volume sufficiently in a stirred water bath and wait for thermal equilibrium.
  • Estimate or calibrate dead volume and check joints for leaks before collecting the main data set.

Source references

  • Specification: AQA 7408 specification §3.6.2.2 · PDF p. 32

Try it — exam-style

Easy
ORIGINAL

A trapped gas has pressure 105 kPa at volume 80.0 cm3. Its temperature is constant. Calculate the pressure when its volume is 50.0 cm3.

[2 marks]

Total for this question: 2

Medium
ORIGINAL

At constant pressure, a gas occupies 50.0 cm3 at 27.0 °C and 60.0 cm3 at 87.0 °C. Assuming a straight line, estimate the Celsius temperature at which the extrapolated volume is zero.

[3 marks]

Total for this question: 3

Easy
ORIGINAL

Explain why pressure should be recorded only after the gas has settled following a reduction in volume in the Boyle branch.

[2 marks]

Total for this question: 2

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

A-level Physics spec coverage

Thermal physics notes and exam-style practice for 7408.

Stuck on gas laws — boyle's law and charles's law?

Keep the two gas-law branches separate, then make every control and graph earn its mark.

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