Energy and particles · GCSE Physics
Changes of state
Teacher-written GCSE Physics revision on changes of state: melting, freezing, boiling, condensing and sublimation in the particle model, with internal energy, mass conservation and heating curves.
Changing state does not change mass or the substance — it changes the arrangement and the potential store. Temperature can stay constant while that happens.
The important bits
What you need to know
- 1
The particle model: solids — regular, vibrating about fixed positions; liquids — close, able to flow; gases — far apart, moving randomly at high speed.
- 2
Melting and freezing happen at the melting point. Boiling and condensing happen at the boiling point. Evaporation can occur below boiling point at the surface.
- 3
Internal energy is the total kinetic and potential energy of the particles. Heating can raise the kinetic store (temperature) or the potential store (change of state).
- 4
Mass is conserved. The substance is still the same: ice, water and steam are all H₂O. Density usually falls as particles spread out, except ice/water.
- 5
Boiling happens throughout the liquid at a fixed temperature. Evaporation is a surface process that can cool the remaining liquid because the fastest particles leave.
- 6
Sublimation is solid to gas without a liquid stage (for example, iodine or solid carbon dioxide in demonstrations).
- 7
A change of state is a physical change: it is reversible, and no new substance is made. Contrast that with a chemical reaction.
- 8
Use mcΔθ when temperature changes and mL when state changes. A full heating curve needs both, in order.
Quotations worth analysing
Short evidence. Real method.
“Internal energy is the total kinetic and potential energy of all the particles that make up a system.”
Temperature tracks the kinetic part. A change of state at constant temperature is a change in the potential part as forces between particles change.
“Mass is conserved when a substance changes state.”
If 50 g of ice melts, you have 50 g of water. Volume and density change; the number of particles does not.
Go deeper
Arrangement, not “heat particles getting bigger”
Particles themselves do not swell like balloons when a solid melts. The spacing and the freedom to move change. In a solid they vibrate about fixed positions in a regular pattern. At the melting point they have enough energy to break out of that pattern and slide, which is a liquid. At the boiling point they separate completely. Draw three boxes in revision: regular grid, clustered but disordered, sparse and random. Then attach melting, boiling, freezing and condensing as arrows between those boxes. If a question asks why a gas is easy to compress, the answer is the empty space between particles, not that the particles are soft. Solids and liquids are almost incompressible because the particles are already close.
Go deeper
Evaporation is not boiling, and both still conserve mass
Boiling needs the boiling point and happens throughout the liquid, with bubbles of gas forming inside. Evaporation happens at any temperature at the free surface: the particles with the most kinetic energy escape, so the average kinetic energy of those left behind falls and the liquid cools. Sweat works that way. In both cases the water that left is still water — now a gas — so mass of the closed system is unchanged. An open puddle loses mass because the vapour mixes into the room; the particles have not been destroyed. Exam answers that say “the water disappears” lose the particle-model mark. Say where the particles went.
See the idea in action
A 0.050 kg ice cube at 0 °C melts in a drink, then the water warms from 0 °C to 8.0 °C. L_f = 3.3 × 10⁵ J/kg, c_water = 4200 J/kg°C. Energy to melt = mL = 0.050 × 3.3 × 10⁵ = 16 500 J, still at 0 °C. Energy to warm = mcΔθ = 0.050 × 4200 × 8.0 = 1680 J. Total energy taken from the drink = 18 180 J. The mass of H₂O is still 0.050 kg; only the state and then the temperature changed. Using mcΔθ for the melting stage would miss most of the cooling effect.
Exam technique
Turn knowledge into marks
Name the change of state, say whether temperature is constant, and use the particle arrangement in the explain sentence. Keep mass conserved. Split heating-curve calculations into mcΔθ and mL chunks.
Common mistakes
Do not give these marks away
- 01
Saying particles in a solid do not move, or that they get bigger when heated.
- 02
Claiming mass is lost when a liquid boils in a closed container.
- 03
Using one energy formula for a journey that both warms a substance and changes its state.
What happens to the mass of 20 g of ice when it melts to water in a closed flask?
AIt decreases because ice is denser
BIt stays 20 g; only the arrangement of particles changes
CIt increases because liquids are heavier
DIt becomes zero because the ice disappears
Show the answer
It stays 20 g; only the arrangement of particles changes. Mass is conserved at a change of state. Density and volume change as the particle arrangement changes, but the amount of substance does not.
Quick questions
If this is the bit you searched
What is internal energy?
The total kinetic and potential energy of the particles in a system. Heating can change either store, depending on whether temperature or state is changing.
What is the difference between boiling and evaporation?
Boiling occurs at the boiling point throughout the liquid. Evaporation is a surface process that can happen at lower temperatures and cools the remaining liquid.
Why can you compress a gas more easily than a liquid?
Gas particles are far apart with large empty spaces. Liquid particles are already close, so there is little space to squeeze.
Is melting a chemical change?
No. It is a physical change: reversible, same substance, mass conserved. A chemical change would make a new substance.