Boyle's law
4 marks · NormalFigure 1 shows a diagram of an air-filled syringe with its tip sealed. The plunger is pressed in to reduce the volume, while the temperature is kept constant.
Explain what happens to the pressure inside the syringe as the plunger is pushed in (the volume decreases and temperature stays the same).
Describe the motion of the air particles inside the syringe as the plunger is pressed in at constant temperature.
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.
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(a) The pressure inside increases. As volume decreases at constant temperature, particle collisions with the walls become more frequent and pressure rises (Boyle's law: p increases as V decreases).
(b) Particles move at the same average speed (since temperature does not change) but are now closer together, so they collide with the walls and each other more often.
Common mistakes: Suggesting temperature changes due to compression; Wrongly describing change in speed
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States that pressure increases.
M1 (a) · 1 mark · Explains pressure rise due to more frequent collisions or links to Boyle's law.
B1 (b) · 1 mark · States correctly that speed/kinetic energy does NOT change.
M1 (b) · 1 mark · States that particles are closer and collide more often.
Worked solution
(a) As the plunger is pushed in, the air is compressed, reducing the volume. According to Boyle's law, at constant temperature, a reduction in volume causes the pressure to increase, because the particles collide with the walls more often.
(b) The temperature is constant, so particles move at the same average speed. However, since there is less space, the particles are closer together and they hit the walls and each other more frequently.