Boyle's law
4 marks · NormalA student investigates Boyle's law using a sealed syringe containing air. She changes the volume and measures the pressure at constant temperature.
Describe one step the student should take to make sure the temperature of the gas stays constant during the experiment.
The student records the following results:
| Volume (cm³) | Pressure (kPa) |
|---|---|
| 50 | 80 |
| 40 | 100 |
| 32 | 125 |
Show that the product of pressure and volume is roughly constant for these readings.
Suggest why the results might not be perfectly constant.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Allow time for the syringe and air to return to room temperature between measurements OR keep the experiment in a temperature-controlled room.
(b) 80 × 50 = 4000, 100 × 40 = 4000, 125 × 32 = 4000; so pV ≈ constant.
(c) There may be heat transfer to/from the environment, or measurement errors.
Common mistakes: Ignoring the need for thermal equilibrium.; Mathematical errors in pV calculation.; Unrealistic sources of error.
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Suitable step described (e.g. waiting for thermal equilibrium, carrying out in constant temp room, not compressing too quickly).
M1 (b) · 1 mark · Multiplies pressure × volume for all readings.
A1 (b) · 1 mark · Shows that the values are all the same or nearly the same (e.g. comments on rounding).
B1 (c) · 1 mark · Plausible suggestion (e.g. heat transfer, gas leaks, measurement error).
Worked solution
(a) She should wait for the syringe to reach room temperature after each volume change, or keep the whole setup at the same temperature.
(b) Calculating p × V for each row:
This shows the products are the same to within rounding error.
(c) The results may not be exactly constant due to heat loss/gain, leaks in the system, or pressure gauge inaccuracy.