Kinetic TheoryOxford AQA IGCSE Physics: Revision notes
Section 1
How does kinetic theory explain solids, liquids and gases?
Kinetic theory models matter as tiny particles that are always moving, and it explains why solids, liquids and gases behave so differently.
| State | Arrangement | Movement | Energy |
|---|---|---|---|
| Solid | Regular, close together | Vibrate around fixed positions | Least |
| Liquid | Close together, irregular | Move around each other, slide | Medium |
| Gas | Far apart, random | Move freely at high speed in all directions | Most |
As a substance is heated, particles gain kinetic energy and move faster; this is what drives changes of state.
Section 2
What is specific heat capacity?
Specific heat capacity is the energy required to change the temperature of one kilogram of a substance by one degree Celsius.
Calculate using: E = m × c × Δθ
- E = energy transferred (joules)
- m = mass (kilograms)
- c = specific heat capacity (joules per kilogram per degree Celsius)
- Δθ = change in temperature (degrees Celsius)
Substances with a high specific heat capacity need a lot of energy to change temperature — this is why water is used in heating systems.
To raise the temperature of 2 kg of water (c = 4200 J/kg°C) by 10°C: E = 2 × 4200 × 10 = 84,000 J.
Section 3
What is specific latent heat?
Specific latent heat is the energy needed to change the state of one kilogram of a substance, without any change in temperature.
There are two types:
- Specific latent heat of vaporisation — energy to change 1 kg from liquid to vapour at constant temperature. Calculate using: E = m × Lv
- Specific latent heat of fusion — energy to change 1 kg from solid to liquid at constant temperature. Calculate using: E = m × Lf
During a change of state, all the energy supplied goes into breaking or forming bonds between particles, not into raising temperature.
Students often think temperature keeps rising during melting or boiling — it stays constant while the state changes, because the energy is being used to change state, not to heat the substance further.
Section 4
How do impurities affect melting and boiling points?
The melting point of a solid and the boiling point of a liquid are affected by impurities.
Impurities generally:
- Lower the melting point of a solid
- Raise the boiling point of a liquid
This is why salt is spread on icy roads — it lowers the melting point of the ice, causing it to melt at temperatures below 0°C.
Section 5
What does a temperature-time graph for heating/cooling look like?
When a substance is heated steadily through a change of state, a temperature-time graph shows:
- A rising slope (temperature increasing) as the solid warms up
- A flat, horizontal section at the melting point, while the solid melts into a liquid
- A rising slope again as the liquid warms up
- A flat, horizontal section at the boiling point, while the liquid boils into a gas
- A rising slope as the gas continues to warm
The flat sections show energy being used for the change of state (breaking bonds) rather than raising temperature. This can be investigated experimentally using cooling curves for stearic acid.
When describing a temperature-time graph in an exam, explicitly link the flat sections to the state change and explain that energy is still being transferred even though temperature is not changing.
Must Know
- Kinetic theory explains solids, liquids and gases in terms of particle arrangement, movement and energy
- Specific heat capacity: E = m × c × Δθ — energy to change temperature of 1 kg by 1°C
- Specific latent heat of vaporisation and fusion: E = m × L — energy to change state of 1 kg at constant temperature
- Impurities lower the melting point and raise the boiling point of a substance
- On a temperature-time graph, flat sections occur during a change of state, where energy is used to change state rather than raise temperature
- The cooling curve of stearic acid is a required practical example of this
That's the notes covered.
Carry on to the next subtopic.