AQA GCSE Physics Foundation (8463), Foundation tier · Particle model of matter › Particle model and pressure
Practise Pressure in gases. 8 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
What happens to the pressure of a gas when its volume changes at constant temperature. You need to explain this with the particle model and use pV = constant (on the equation sheet). This subtopic is in GCSE Physics only, not in Combined Science. Expect particle explanations and 2 to 4 mark calculations, sometimes from a table of results.
Key facts
pV = constant (fixed mass of gas at constant temperature; on the equation sheet)
p in Pa, V in m3; 1 kPa = 1000 Pa
p1V1 = p2V2
Volume up → pressure down (at constant temperature)
Bigger volume → molecules collide with the walls less often → lower pressure
Gas pressure produces a net force at right angles to any surface
Notes
Gas pressure and force
A gas can be compressed (squashed into a smaller volume) or expanded by changes in pressure.
The pressure of a gas produces a net force at right angles to the wall of its container, or to any surface the gas touches.
That is why the air in a balloon pushes outwards on every part of the rubber.
Changing the volume at constant temperature
diagram
For a fixed mass of gas at a constant temperature, increasing the volume decreases the pressure.
Why: the molecules have more space, so they travel further between collisions with the walls. They collide with the walls less often, so there is a smaller force on each unit area: the pressure is lower.
The temperature is constant, so the average kinetic energy (and speed) of the molecules does not change. It is the frequency of the collisions that changes.
Decreasing the volume does the opposite: the molecules collide with the walls more often, so the pressure increases.
Constant temperature: smaller volume, more frequent collisions, higher pressure.
pV = constant
For a fixed mass of gas at constant temperature: pressure × volume = constant, pV = constant (on the equation sheet). p in pascals (Pa), V in m3.
To compare two situations use p1V1 = p2V2. Any units work, as long as both pressures are in the same unit and both volumes are in the same unit.
Pressure and volume are inversely proportional: halve the volume and the pressure doubles; triple the volume and the pressure falls to a third.
How to answer each type of question
Recall: the force from a gas and the effect of volume
1 mark eachGrade 5
The force from gas pressure is always at right angles (perpendicular) to the surface.
At constant temperature, pressure × volume stays the same: if one doubles, the other halves.
Example. A fixed mass of gas is held in a closed container. (a) Describe the direction of the force that the gas exerts on the walls of the container. (b) The volume of the container is doubled and the temperature of the gas does not change. What happens to the pressure of the gas?
Show the model answerHide the model answer
(a) At right angles (perpendicular) to the walls (1) (b) The pressure halves (1)
Don’t lose marks
Using pV = constant when the temperature changes. It needs a fixed mass of gas at constant temperature.
Mixing units, e.g. one pressure in kPa and the other in Pa.
Saying the molecules hit the walls 'with less force' when the volume increases. At constant temperature their speed is unchanged; they hit the walls less often.
Rearranging the wrong way up: p2 = p1V1 ÷ V2, not p1V2 ÷ V1. Use the sense check.
Saying gas pressure acts 'downwards' or 'along' a wall. It acts at right angles to the surface.
More tips
Memory tricks
Squash a gas and its pressure goes up: halve V, double p.
Units shortcut: for p1V1 = p2V2 you don't need Pa and m3; just use the same units on both sides.
Sense check: if the volume got smaller, the new pressure must be bigger (and the other way round).
To test data, multiply p × V for every row: the products should all be about the same.
Exam technique
pV = constant is on the equation sheet. Write p1V1 = p2V2 with the numbers substituted to earn the first mark.
In particle explanations for a volume change at constant temperature, the key phrase is 'collide with the walls less (or more) frequently'.
If a question says a gas was compressed or expanded slowly, that tells you its temperature stayed constant.
Give the unit of your answer: it is the unit you used for that quantity (e.g. kPa or cm3).
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
State the direction of gas pressure forcesThe pressure of a gas produces a net force at right angles to the wall of its container (or any surface).
Grade 5
Describe how volume affects gas pressureAt constant temperature, increasing the volume of a gas decreases its pressure, and decreasing the volume increases it.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
A fixed mass of gas has a pressure of 100 kPa and a volume of 0.030 m3. The gas is compressed to a volume of 0.010 m3. The temperature of the gas does not change. Calculate the new pressure of the gas. Use the equation: pressure × volume = constant
300 kPa
State what happens to the volume of a fixed mass of gas, at constant temperature, if its pressure is increased.
The volume decreases.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) The pressure of a gas produces a net force on the walls of its container. What is the direction of this force? Tick (✓) one box.[1]
At right angles to the wall
Parallel to the wall
Always downwards
Always upwards
(b) A fixed mass of gas has a pressure of 100 kPa and a volume of 0.030 m3. The gas is compressed to a volume of 0.010 m3. The temperature of the gas does not change. Calculate the new pressure of the gas. Use the equation: pressure × volume = constant[2]
(c) What happens to the pressure of a fixed mass of gas at constant temperature when its volume is increased?[1]
Show the answer and mark scheme
(a)Answer: At right angles to the wall
(b)Answer: 300 kPa
100 × 0.030 = p × 0.010
p = 300 (kPa)
(c)Answer: It decreases.
it decreases
Question 2Medium6 marks
A student sealed the end of a plastic syringe. The syringe contained 60 cm3 of air at a pressure of 100 kPa. The student slowly pushed the plunger in until the volume of the air was 40 cm3. The temperature of the air did not change.
(a) Calculate the pressure of the air when its volume was 40 cm3. Use the Physics Equations Sheet.[2]
(b) The student then pulled the plunger out so that the volume of the air was greater than 60 cm3. The temperature of the air stayed the same. Explain, in terms of the air molecules, why the pressure of the air decreased.[3]
(c) Explain why the student pushed the plunger in slowly.[1]
Show the answer and mark scheme
(a)Answer: 150 kPa
100 × 60 = p × 40
p = 150 (kPa)
(b)Answer: The molecules move at the same average speed but are more spread out, so they hit each unit area of the walls less often and the pressure falls.
the (average) speed / kinetic energy of the molecules does not change (because the temperature is constant)
the molecules are more spread out / travel further between collisions with the walls
so there are fewer collisions with each unit area of the walls each second, so the pressure decreases
(c)Answer: Pushing quickly would do work on the air and warm it; pushing slowly keeps its temperature constant.
so that the temperature of the air stayed constant / pushing quickly would do work on the air and raise its temperature