3.7 Particle motion and gas pressure
9 exam questions worth 21 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Explain how the particles of a gas produce a pressure on the walls of a container.
Mark scheme
the particles move randomly and collide with the walls (1); each collision exerts a force on the wall (1); pressure is the force per unit area, caused by a very large number of collisions (1)
Model answer
The gas particles move randomly and collide with the walls. Each collision exerts a force on the wall. The very large number of collisions causes a steady force on the walls, and pressure is this force for each unit of area.
A sealed container of gas is heated. The volume of the container does not change. Explain why the pressure of the gas increases.
Mark scheme
the particles gain kinetic energy / move faster (1); they collide with the walls more often (1); and with greater force (1)
Model answer
The particles gain kinetic energy and move faster. They hit the walls more often and with a greater force, so the pressure increases.
Convert 27 °C to kelvin, and 250 K to degrees Celsius.
Mark scheme
27 + 273 = 300 K (1); 250 − 273 = −23 °C (1)
Model answer
27 °C = 27 + 273 = 300 K. 250 K = 250 − 273 = −23 °C.
State what is meant by absolute zero.
Mark scheme
the lowest possible temperature / 0 K / −273 °C (1); the particles have the lowest possible kinetic energy (1)
Model answer
Absolute zero is the lowest possible temperature, 0 K or −273 °C, where the particles have the least possible kinetic energy.
Describe the motion of the particles in a gas.
Mark scheme
random motion / move in random directions (1); in straight lines until they collide with each other or with the container walls (1)
Model answer
The particles move randomly, in straight lines, until they collide with other particles or with the walls of the container.
What happens to the average speed of the particles in a gas when its temperature increases?
Choose one answer.
Correct Not quite: the answer is It increases
Mark scheme
A: it increases
Model answer
It increases (A), because the temperature is related to the average kinetic energy of the particles.
A gas is sealed in a rigid container. Its pressure is 120 kPa at 300 K. It is heated to 450 K. Calculate the new pressure, assuming that the pressure is directly proportional to the temperature in kelvin.
Mark scheme
p₂ = p₁ × T₂ ÷ T₁ (1); 120 × 450 ÷ 300 (1); 180 kPa (1)
Model answer
p₂ = 120 × 450 / 300 = 180 kPa.
Explain why the pressure in a car tyre increases after a long journey.
Mark scheme
the air in the tyre is warmed (by friction with the road) (1); the particles move faster (1); they hit the walls harder and more often, so the pressure increases (1)
Model answer
The tyre warms up as it travels, so the air particles move faster. They collide with the inside of the tyre harder and more often, so the pressure goes up.
Convert 25 °C to kelvin and 500 K to degrees Celsius.
Mark scheme
25 + 273 = 298 K (1); 500 − 273 = 227 °C (1)
Model answer
25 °C = 298 K. 500 K = 227 °C.
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