States of Matter & Thermal CapacityEdexcel GCSE Physics: Revision notes
Section 1
How does the kinetic theory model explain solids, liquids and gases?
The kinetic theory models matter as particles in constant motion:
| State | Arrangement | Movement |
|---|---|---|
| Solid | Regular, closely packed | Vibrate about fixed positions |
| Liquid | Randomly arranged, close together | Move around each other, slide |
| Gas | Randomly arranged, far apart | Move fast in random directions |
Density differences between states arise from how closely packed the particles are: ρ = m ÷ V. Solids and liquids are much denser than gases because their particles are packed much more closely together.
Core Practical: densities of solids and liquids are found by measuring mass (balance) and volume (displacement for irregular solids, or measuring cylinder for liquids), then calculating ρ = m/V.
Section 2
What happens to mass during changes of state?
When substances melt, freeze, evaporate, boil, condense or sublimate, mass is conserved — no matter is created or destroyed. These are physical changes: the material recovers its original properties if the change is reversed (e.g. melted ice can be refrozen into the same water). This differs from many chemical changes, where new substances are formed and the change usually cannot be simply reversed.
Section 3
What is specific heat capacity?
Heating a system transfers energy to it, which either raises its temperature or causes a change of state, depending on whether the substance is already at its melting/boiling point.
Specific heat capacity (c) is the energy required to raise the temperature of 1 kg of a substance by 1 °C. It is used in:
ΔQ = m × c × Δθ
where ΔQ is the change in thermal energy (J), m is mass (kg), c is specific heat capacity (J/kg°C), and Δθ is the change in temperature (°C).
Heating 2 kg of water (c = 4200 J/kg°C) by 10 °C needs ΔQ = 2 × 4200 × 10 = 84 000 J.
Section 4
What is specific latent heat?
Specific latent heat (L) is the energy required to change the state of 1 kg of a substance without changing its temperature. It is used in:
Q = m × L
where Q is the thermal energy needed (J), m is mass (kg), and L is specific latent heat (J/kg).
The key difference from specific heat capacity: specific heat capacity relates to a temperature change with no change of state; specific latent heat relates to a change of state with no change in temperature — during melting or boiling, the temperature stays constant while energy is absorbed to break bonds between particles.
Core Practical: specific heat capacity of water can be found by heating a known mass of water with an immersion heater and measuring energy supplied (using P and t) against temperature rise. A temperature-time graph for melting ice shows a flat section (constant temperature) while the ice melts, corresponding to the latent heat being absorbed.
A common error is assuming temperature keeps rising during melting or boiling — it stays constant until the change of state is complete.
Must Know
- Kinetic theory: solids vibrate in fixed positions, liquids slide past each other, gases move freely and fast
- Density: ρ = m ÷ V; density differs between states due to particle spacing
- Changes of state conserve mass and are reversible physical changes
- ΔQ = m × c × Δθ for temperature change; specific heat capacity is energy to raise 1 kg by 1 °C
- Q = m × L for change of state; specific latent heat is energy to change the state of 1 kg with no temperature change
- Temperature stays constant during melting/boiling while latent heat is absorbed
That's the notes covered.
Carry on to the next subtopic.