Changes of state
Matter exists in three common states: solid, liquid and gas. Energy transferred to or from a substance can cause it to change state — these are physical changes, not chemical changes.
The state changes
- Melting — solid → liquid.
- Freezing — liquid → solid.
- Boiling/evaporation — liquid → gas.
- Condensation — gas → liquid.
- Sublimation — solid → gas (e.g. dry ice).
- Deposition — gas → solid (e.g. frost).
Particle model of states
- Solid — particles in a fixed lattice, vibrating in place. Strong forces hold them. Fixed volume and shape.
- Liquid — particles touching but free to slide. Forces are weaker. Fixed volume but takes container's shape.
- Gas — particles far apart, moving rapidly. Negligible forces between them (in the model). No fixed volume or shape.
Why state changes are reversible
A physical change rearranges particles but doesn't break or form chemical bonds. Re-cooling a melted solid returns it to a solid; the substance keeps its chemical identity. Compare a chemical change like burning, which produces new substances.
Conservation of mass during a state change
When ice melts, the resulting water has the same mass as the ice. No particles are created or destroyed — just rearranged. Density usually changes (volume changes), but mass does not.
Heating curve — what happens to temperature
Plot temperature vs time as a solid is heated steadily.
- Solid heats up — temperature rises.
- Plateau at melting point — energy goes into breaking lattice forces, not raising temperature.
- Liquid heats up — temperature rises again.
- Plateau at boiling point — energy goes into breaking liquid bonds and pushing particles apart.
- Gas heats up — temperature rises further.
The plateaux occur because energy supplied is being used to change state rather than to increase kinetic energy of particles.
Specific latent heat
The energy per kg required to change state without changing temperature is the specific latent heat $L$:
$E = mL$
$L_f$ for melting (fusion); $L_v$ for boiling (vaporisation). $L_v$ is typically larger because total separation of particles is needed.
✦Worked example
How much energy melts 0.50 kg of ice at 0 °C? $L_f$ for water = 334 000 J/kg.
- $E = mL = 0.50 \times 334,000 = 167,000$ J.
⚠Common mistakes
- Saying "ice melts and becomes hotter" during the melting plateau — temperature stays constant during melting.
- Confusing melting and boiling on the curve.
- Forgetting that mass is conserved (a melted block of ice in a sealed container weighs the same).
- Treating sublimation as exotic — it's a normal state change for substances like CO₂.
AI-generated · claude-opus-4-7 · v3-deep-physics