Changes of State
What Is a Change of State?
A change of state (or phase change) happens when a substance moves between the solid, liquid and gas states. It is a physical change — the substance is still the same chemical, just in a different form. Melted ice is still water; it can be frozen back again.
Changes of state happen when particles gain or lose energy. Adding energy makes particles move faster and spread apart; removing energy makes them slow down and come together.
The Six Changes of State
| Change | Name | Energy |
|---|---|---|
| Solid → Liquid | Melting | Absorbed |
| Liquid → Solid | Freezing (solidification) | Released |
| Liquid → Gas | Evaporation / Boiling | Absorbed |
| Gas → Liquid | Condensation | Released |
| Solid → Gas | Sublimation | Absorbed |
| Gas → Solid | Deposition | Released |
Changes that separate particles (melting, evaporation, sublimation) always absorb energy. Changes that bring particles together (freezing, condensation, deposition) always release energy.
What Happens to the Particles
- Melting: particles in a solid gain energy, vibrate more, and break out of their fixed positions to flow as a liquid.
- Freezing: particles in a liquid lose energy, slow down, and settle into fixed positions in a regular arrangement.
- Evaporation/boiling: particles in a liquid gain enough energy to escape completely and move freely as a gas.
- Condensation: gas particles lose energy, slow down, and clump together into a liquid.
Temperature Stays Constant During a Change
A surprising fact: while a substance is changing state, its temperature does not change, even though energy is still being added or removed.
This is because the energy is used to break or form the bonds between particles rather than to make them move faster. This energy is called latent heat. Only once the change of state is complete does the temperature start to change again.
This is why a mix of ice and water stays at 0°C until all the ice has melted.
Melting and Boiling Points
Every pure substance has its own fixed melting point and boiling point.
| Substance | Melting point | Boiling point |
|---|---|---|
| Water | 0°C | 100°C |
| Oxygen | −219°C | −183°C |
| Iron | 1538°C | 2862°C |
A high melting or boiling point means strong forces between particles that need a lot of energy to overcome.
Evaporation vs Boiling
These are both changes from liquid to gas, but they are not the same:
- Evaporation happens only at the surface of a liquid and can occur at any temperature. A puddle dries up on a cool day by evaporation.
- Boiling happens throughout the liquid and only at the boiling point. Bubbles of gas form inside the liquid.
What speeds up evaporation?
- Higher temperature (particles have more energy)
- Larger surface area (more particles exposed)
- Moving air / wind (removes vapour so more can escape)
- Lower humidity (drier air holds more water vapour)
Everyday Examples
- Puddles drying after rain — evaporation
- Dew or mist forming on cool mornings — condensation
- Ice cubes melting in a drink — melting
- Water freezing into ice in a freezer — freezing
- Dry ice giving off "smoke" — sublimation (solid CO₂ to gas)
- Frost forming on a cold window — deposition (water vapour straight to ice)
Worked Example — Heating Ice
A student heats ice from −10°C until it becomes steam. What happens?
- Temperature rises from −10°C to 0°C — ice warms up
- Temperature stays at 0°C — ice melts to water (latent heat absorbed)
- Temperature rises from 0°C to 100°C — water warms up
- Temperature stays at 100°C — water boils to steam (latent heat absorbed)
- Temperature rises above 100°C — steam warms up
The two flat sections are where the changes of state happen.
Key Terms
- Change of state — a physical change between solid, liquid and gas
- Melting point — the temperature at which a solid turns to liquid
- Boiling point — the temperature at which a liquid turns to gas throughout
- Evaporation — change from liquid to gas at the surface, at any temperature
- Condensation — change from gas to liquid
- Sublimation — change directly from solid to gas
- Latent heat — energy absorbed or released during a change of state with no temperature change
Common Mistakes
- Saying temperature rises during melting or boiling — it stays constant until the change is complete
- Thinking evaporation and boiling are identical — evaporation is surface-only and can happen at any temperature
- Believing a change of state creates a new substance — it is a physical change; the chemical is unchanged
- Forgetting that freezing and condensation release energy
Tips and Tricks
- Spreading apart (melting, boiling, sublimation) → energy in; coming together (freezing, condensation, deposition) → energy out
- On a heating graph, the flat parts are always the changes of state
- Remember dry ice for sublimation — it skips the liquid stage entirely
- Wind + warmth + big surface area = fastest drying