Question 1Multiple choice · 1 mark
A fixed mass of gas at constant temperature has pressure 200 kPa and volume 3.0 m³. Its volume is reduced to 1.5 m³.
What is the new pressure?
- A 100 kPa
- B 300 kPa
- C 400 kPa
- D 600 kPa
Show answer
Answer: C
Free Gases & absolute temperature questions for IGCSE Physics (0625), Extended tier (Supplement content), part of Thermal physics. Each question has the answer and the marking points an examiner would look for.
Generate a full Gases & Absolute Temperature worksheet →All Thermal physics questionsA fixed mass of gas at constant temperature has pressure 200 kPa and volume 3.0 m³. Its volume is reduced to 1.5 m³.
What is the new pressure?
Answer: C
Which term is described by this statement?
“For a fixed mass of gas at constant temperature, pressure is inversely proportional to volume.”
Answer: D
A diver is 20 m below the surface of a freshwater lake. Atmospheric pressure is 1.0 × 105 Pa and the density of water is 1000 kg/m³. (g = 9.8 N/kg)
Calculate the total pressure on the diver.
The diver releases an air bubble of volume 2.0 cm³. The temperature of the water is constant. Calculate the volume of the bubble as it reaches the surface.
Explain, in terms of the air molecules, why the pressure inside the bubble decreases as it rises and expands.
The diver rises from this depth to the surface at the same time as the bubble. Suggest why divers are trained to breathe out as they rise.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 2.96 × 105 Pa ρgh = 1000 × 9.8 × 20 = 1.96 × 105 Pa C1 total = 2.96 × 105 Pa A1 | 2 |
| (b) | 5.92 cm³ p1V1 = p2V2 with total pressures C1 5.92 A1 | 2 |
| (c) | Molecules collide with the walls; in a larger volume they hit the walls less often (per unit area); same average speed (constant temperature), so smaller force per unit area molecules collide with the walls of the bubble, exerting a force B1 larger volume so fewer collisions per second per unit area B1 temperature constant so average speed/kinetic energy unchanged B1 | 3 |
| (d) | Air in the lungs expands as the pressure falls and could damage the lungs air in lungs expands as pressure decreases (could burst/damage lungs) B1 | 1 |
A car tyre is checked before and after a long journey. The pressure of the air in the tyre is greater after the journey.
Suggest why the air in the tyre is hotter after the journey.
Explain, in terms of molecules, why the pressure increases.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | work done by friction/flexing friction with the road / flexing of the tyre heats the air B1 | 1 |
| (b) | faster molecules, more frequent and harder collisions the molecules have more kinetic energy/move faster B1 they collide with the walls more often B1 and with greater force (change of momentum) B1 | 3 |
Convert between the Celsius and kelvin temperature scales.
State the value of absolute zero in °C.
Convert 27 °C to kelvin.
Describe what happens to the particles of a substance as its temperature approaches absolute zero.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | −273 °C −273 °C B1 | 1 |
| (b) | 300 K 300 K B1 | 1 |
| (c) | kinetic energy approaches minimum the particles have the least possible kinetic energy (they almost stop moving) B1 | 1 |
This question is about the behaviour of gases.
Describe the motion of the molecules of a gas.
A gas syringe contains 800 cm³ of air at 100 kPa. The air is compressed slowly at constant temperature to 300 cm³. Calculate the new pressure.
Explain, in terms of molecules, why the pressure increases when the gas is compressed at constant temperature.
Convert 17 °C and 87 °C to kelvin.
A sealed can of gas at 17 °C is heated to 87 °C. Its volume does not change. Describe and explain, in terms of particles, what happens to the pressure of the gas.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | random; high speed; collide with each other and the walls move randomly / in all directions B1 at high speeds, colliding with each other and the container walls B1 | 2 |
| (b) | 267 kPa p₁V₁ = p₂V₂ C1 267 kPa A1 | 2 |
| (c) | more molecules per volume; more frequent collisions with walls the molecules are closer together / more per unit volume B1 so they hit the walls more frequently (more collisions per second per unit area) B1 | 2 |
| (d) | 290 K and 360 K 290 K and 360 K B1 | 1 |
| (e) | pressure increases: particles faster, more frequent and harder collisions pressure increases B1 particles move faster / have more kinetic energy B1 so they hit the walls more frequently and with more force (greater force per unit area) B1 | 3 |
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