Heating: temperature or state? (spec 5.8P, 5.9P)
Two ways for energy to enter a system.
Spec 5.8P principle. When you heat a system, energy is transferred TO the particles. That energy can do one of two things:
- Raise the temperature — increases the average kinetic energy of the particles (and they vibrate or move faster).
- Cause a change of state — increases the potential energy of the particles by separating them against the intermolecular forces; temperature stays CONSTANT during the change.
Spec 5.9P — the three changes.
- Solid → liquid (melting). Particles gain enough KE to break out of the rigid lattice. They can now slide past each other. Temperature stays constant at the MELTING POINT.
- Liquid → gas (boiling). Particles gain enough KE to escape the liquid surface and move freely. Temperature stays constant at the BOILING POINT.
- Liquid → gas (evaporation). Slower process — fastest particles escape from the surface AT ANY TEMPERATURE below boiling. Causes cooling of the remaining liquid (the average KE of the remainder is lower because the fast ones left).
Reverse changes.
- Gas → liquid (condensation): particles slow enough for attractive forces to pull them together. Releases energy (= the latent heat absorbed during boiling).
- Liquid → solid (freezing): particles slow enough to lock into a regular lattice. Releases energy.
Mark-scheme phrasing. Edexcel rewards descriptions that mention BOTH the energy change AND the structural rearrangement.
- Heat → either T rises OR state changes (not both at once).
- Melting / boiling: T constant; energy breaks bonds (PE rise).
- Evaporation: surface escape at any T; cools the remaining liquid.