What happens to volume if pressure doubles at constant temperature?

It halves, because PV is constant and the two are inversely proportional.

Why must absolute temperature be used?

Gas law ratios need kelvin; degrees Celsius cannot be substituted directly.

S5 Physics Gas Laws, Paper 1: Boyle law and pressure-volume relations

2026-11-01

Tutor Prep Guide · Physics

At constant temperature PV stays fixed, and at constant volume the pressure is proportional to absolute temperature.

Focus　Write PV as a constant, substitute both sets of data, and check units.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10
Suggested time	25
Difficulty mix	Foundation 3 · Intermediate 4 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Physics: Gas Laws
Level	S4 to S5

02　Questions (10 items)

FoundationA fixed mass of gas at constant temperature occupies 2.0 L at 100 kPa. It is compressed to 1.0 L. Find the new pressure.
FoundationA gas of 300 cm3 at 150 kPa expands to 900 cm3 at constant temperature. Find the pressure.
FoundationAt constant temperature, what shape is the graph of pressure against 1/volume?
Intermediate500 cm3 of gas at 120 kPa is compressed to 250 cm3 at constant temperature. Find the pressure.
Intermediate4.0 L of gas at 1.5 atm is compressed to 3.0 atm at constant temperature. Find the volume.
Intermediate8.0 L of gas at 90 kPa expands to 12 L at constant temperature. Find the pressure.
IntermediateAt constant volume a gas at 300 K and 100 kPa is heated to 400 K. Find the pressure to 3 significant figures.
ChallengeAt constant volume 80 kPa of gas at 250 K is heated to 500 K. Find the pressure.
ChallengeA bubble rises from deep water to the surface and its volume triples. Ignoring temperature change, what fraction of the original pressure remains?
Error redoCorrect the statement: “If the pressure doubles the volume doubles too, because PV is constant.”

03　Answers and one-line notes

#	Answer	One-line note
1	200 kPa	At constant temperature P1V1 = P2V2, so 100*2.0/1.0 gives 200.
2	50 kPa	Triple the volume and the pressure falls to a third: 150/3 = 50.
3	A straight line through the origin	P is proportional to 1/V, so the graph is a straight line through the origin.
4	240 kPa	Halving the volume doubles the pressure: 120*2 = 240.
5	2.0 L	Using P1V1 = P2V2, (1.5*4.0)/3.0 = 2.0 L.
6	60 kPa	PV stays at 720, so 720/12 gives 60.
7	133 kPa	At fixed volume P is proportional to T: 100*400/300 = 133 kPa.
8	160 kPa	Doubling the absolute temperature doubles the pressure to 160 kPa.
9	One third	With PV fixed, tripling the volume cuts the pressure to one third.
10	It should read: doubling the pressure halves the volume.	A constant PV means the two are inversely proportional, so one doubling halves the other.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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