Gas Particles & Temperature

IGCSE Edexcel Physics
5.15–5.19 Kinetic theory of gases, pressure and absolute zero
Key Concepts: Gas molecules move rapidly and randomly. They exert pressure by colliding with container walls. Higher temperature → greater average kinetic energy → faster molecules → greater pressure (at constant volume). Absolute zero (0 K = −273 °C) is the temperature at which molecules have minimum kinetic energy. Kelvin scale: $T(\text{K}) = T(°\text{C}) + 273$.

Section A — Kinetic Theory of Gases

1. Describe the motion of gas molecules in a sealed container. [2]
2. Explain how gas molecules exert pressure on the walls of their container. [2]
3. Explain what happens to the pressure of a fixed volume of gas when the temperature is increased. Use ideas about molecules in your answer. [3]
4. Explain what happens to the pressure of a gas when its volume is decreased at constant temperature. Use ideas about molecules in your answer. [3]

Section B — Temperature Scales and Absolute Zero

5. State the temperature of absolute zero in degrees Celsius and in kelvin. [2]
6. Explain why there is a lowest possible temperature (absolute zero). [2]
7. Convert the following temperatures to kelvin: [3]

a) 20 °C

b) 100 °C

c) −50 °C

8. Convert 350 K to degrees Celsius. [1]

Section C — Linking Pressure and Temperature

9. A graph of pressure against temperature (in kelvin) for a fixed volume of gas is plotted. Describe the shape of the graph and state what happens when the line is extrapolated back to zero pressure. [3]
10. A bicycle tyre is pumped up on a cold morning at 5 °C. Explain why the pressure in the tyre increases after a long cycle ride when the temperature rises to 35 °C. [3]

Total marks: 24

Mark Scheme

1. Gas molecules move rapidly [1] in random directions (random motion) [1] [2]
2. Gas molecules collide with the walls of the container [1]; each collision exerts a force on the wall; pressure = total force ÷ area [1] [2]
3. Increasing temperature increases the average kinetic energy of the molecules [1]; molecules move faster and hit the walls more frequently [1]; each collision also exerts a greater force, so overall pressure increases [1] [3]
4. Smaller volume means molecules travel shorter distances between collisions [1]; they hit the walls more frequently [1]; so the pressure increases [1] [3]
5. −273 °C [1]; 0 K [1] [2]
6. Temperature is a measure of average kinetic energy of particles [1]; kinetic energy cannot be negative, so there is a minimum temperature below which energy cannot decrease further [1] [2]
7. a) 20 + 273 = 293 K [1]; b) 100 + 273 = 373 K [1]; c) −50 + 273 = 223 K [1] [3]
8. 350 − 273 = 77 °C [1]
9. Straight line through the origin (directly proportional) [1]; extrapolating back gives zero pressure at 0 K [1]; this confirms the existence of absolute zero [1] [3]
10. Temperature increases from 278 K to 308 K [1]; gas molecules move faster and collide with the tyre walls more frequently and with greater force [1]; so pressure increases [1] [3]