Changes of StateEdexcel IGCSE Physics: Revision notes
Section 1
Heating, energy and change of state
Heating a system transfers energy to it, increasing the internal energy stored within its particles. Depending on the situation, this extra energy either:
- raises the temperature of the system (particles move/vibrate faster), or
- produces a change of state (particles rearrange, breaking or forming bonds between them), without changing the temperature.
During a change of state, all the energy supplied goes into changing the arrangement of particles (breaking bonds when melting/evaporating, or overcoming/forming bonds), not into raising temperature — this is why temperature stays constant during melting or boiling.
Section 2
Melting, evaporating and boiling
- Melting: a solid changes to a liquid at its melting point; particles gain enough energy to break free of their fixed positions but remain close together.
- Evaporation: a liquid changes to a gas at the liquid's surface, at any temperature below the boiling point, as the fastest-moving particles escape.
- Boiling: a liquid changes to a gas throughout the whole liquid, at a fixed temperature (the boiling point).
In all these changes, particles gain enough energy to overcome the forces of attraction holding them in a more ordered arrangement.
Section 3
Particle arrangement and motion
| State | Arrangement | Motion |
|---|---|---|
| Solid | Regular, closely packed | Vibrate about fixed positions |
| Liquid | Randomly arranged, close together | Move around each other, sliding past |
| Gas | Randomly arranged, far apart | Move freely at high speed in all directions |
Going from solid → liquid → gas, particles gain energy, move faster, and the forces of attraction between them are progressively overcome.
When describing particle arrangement, always mention both spacing (close/far apart) and motion (vibrating/sliding/moving freely) — examiners credit each separately.
Section 4
Specific heat capacity
Specific heat capacity is the energy required to raise the temperature of one kilogram of a substance by one degree Celsius, measured in J/kg °C.
ΔQ = m × c × ΔT
where ΔQ is energy transferred (J), m is mass (kg), c is specific heat capacity (J/kg °C), and ΔT is the change in temperature (°C).
This is investigated practically by heating a known mass of material (e.g. water or a metal block) with a known electrical power for a measured time, recording temperature rise, and calculating c. A temperature–time graph through a change of state shows a flat section (constant temperature) while the substance melts or boils, even though energy is still being supplied.
2 kg of water (c = 4200 J/kg°C) is heated from 20°C to 40°C. ΔQ = 2 × 4200 × 20 = 168,000 J.
Must Know
- heating raises temperature OR causes a change of state, depending on whether energy goes into particle speed or into breaking/forming bonds
- melting (solid→liquid) and boiling (liquid→gas, whole liquid, fixed temperature) occur at fixed temperatures; evaporation (liquid→gas, surface only) can occur below the boiling point
- solid: particles vibrate in fixed positions; liquid: particles move/slide, close together; gas: particles move freely, far apart
- temperature stays constant during a change of state, even while energy is still supplied
- ΔQ = m × c × ΔT, where c is specific heat capacity in J/kg°C
- a temperature-time graph shows a flat plateau during melting or boiling
That's the notes covered.
Carry on to the next subtopic.