What Is a Phase Change? Melting, Boiling, Sublimation
Put a thermometer in a pan of melting ice and watch it. You're pouring heat in, and the reading refuses to move off 0 °C until the last ice cube is gone. Where is all that energy going?
The short answer: a phase change is a physical change in which a substance moves between solid, liquid and gas — melting, freezing, boiling, condensing, subliming or depositing. During a phase change the temperature stays constant, because the energy goes into breaking the attractions between particles rather than speeding them up.
The six phase changes
There are three states, so there are six one-way trips between them:
| Change | From → To | Everyday example |
|---|---|---|
| Melting (fusion) | Solid → Liquid | Ice turning to water |
| Freezing (solidification) | Liquid → Solid | Water turning to ice |
| Vaporisation (boiling/evaporation) | Liquid → Gas | Water becoming steam |
| Condensation | Gas → Liquid | Mist forming on a cold window |
| Sublimation | Solid → Gas | Dry ice fogging without melting |
| Deposition | Gas → Solid | Frost forming on grass overnight |
Three of them (melting, vaporisation, sublimation) absorb energy — they're endothermic, because you're breaking attractions. The other three (freezing, condensation, deposition) release energy — exothermic, because attractions are re-forming.
What's happening to the particles
Every phase change is the same tug-of-war resolving one way or the other.
Heating adds kinetic energy. The particles vibrate and move faster. At some point they have enough energy to escape the positions their neighbours were holding them in — and the substance changes state.
Crucially, the particles themselves are unchanged. Ice, water and steam are all H₂O molecules. No bonds within the molecules break; only the forces between the particles do — the intermolecular forces in a molecular substance like water, or the ionic and metallic bonding in a giant structure like salt or tungsten. That's exactly why a phase change is a physical change, not a chemical one, and why it's fully reversible.
Why the temperature goes flat
This is the part worth understanding properly, because it's a classic exam question.
Temperature measures the average kinetic energy of the particles. While ice is melting, every joule you add is being spent breaking hydrogen bonds — pulling molecules out of the crystal — not making them move faster. Average kinetic energy doesn't change, so the thermometer doesn't move.
That energy is called latent heat ("hidden heat"), and for water the numbers are striking:
- Latent heat of fusion: about 334 J to melt 1 g of ice at 0 °C.
- Latent heat of vaporisation: about 2,260 J to boil 1 g of water at 100 °C.
Compare that with the 4.18 J it takes to warm 1 g of liquid water by a single degree. Boiling water off takes roughly 540 times more energy than raising it one degree — which is why a pan takes minutes to reach the boil and then ages to boil dry.
The vaporisation figure dwarfs the fusion figure for a good reason: melting only loosens the molecules, while boiling separates them completely.
What sets a melting or boiling point
Stronger attractions between particles mean more energy needed, so a higher melting and boiling point.
- Water melts at 0 °C and boils at 100 °C (at 1 atm) — high for such a small molecule, thanks to hydrogen bonding.
- Sodium chloride melts at 801 °C, because you're fighting full ionic charges.
- Oxygen boils at −183 °C — nonpolar molecules with only feeble dispersion forces.
- Tungsten melts at 3,422 °C, the highest of any metal, because its metallic bonding is exceptionally strong.
Pressure matters too. Water boils at 100 °C at sea level but only about 71 °C on the summit of Everest, where there's less air pressure to push against. Carbon dioxide skips the liquid state entirely at 1 atm — dry ice sublimes at about −78.5 °C, which is why it fogs instead of puddling.
Worked examples
Predict before you read.
- Steam burns are worse than boiling-water burns of the same temperature. Why? Steam condensing on skin releases its 2,260 J/g of latent heat on top of the heat from cooling — far more energy delivered.
- Frost on a car windscreen: which phase change? Deposition — water vapour going straight to solid, skipping liquid.
- Ice at 0 °C is added to water at 0 °C. Does the temperature drop? No. Both are at 0 °C; the ice will melt using heat from the surroundings while the mixture holds at 0 °C.
- Why does sweating cool you down? Evaporating sweat absorbs latent heat of vaporisation from your skin.
- Does melting ice change its chemical formula? No — it's H₂O before and after. Physical change only.
Common mistakes to avoid
- Thinking heat always raises temperature. During a phase change it doesn't. Energy in, thermometer still — every time.
- Saying boiling "breaks the water molecule apart." Boiling separates whole H₂O molecules from each other. Splitting H₂O into hydrogen and oxygen is electrolysis, a chemical change needing far more energy.
- Confusing melting point with melting. The melting point is a fixed property of the substance; melting is the process. Adding more ice doesn't change the melting point.
FAQ
What is a phase change in chemistry?
A physical change where a substance moves between solid, liquid and gas — melting, freezing, vaporisation, condensation, sublimation or deposition. The substance's identity and chemical formula stay the same.
Why doesn't the temperature rise during melting?
Because the heat energy is used to break the attractions between particles, not to speed them up. Temperature measures average kinetic energy, which stays constant until the phase change finishes.
What is sublimation?
Sublimation is a solid turning directly into a gas without becoming a liquid first. Dry ice (solid CO₂) is the classic example — at normal pressure it sublimes at about −78.5 °C.
Is a phase change a physical or chemical change?
Physical. Only the intermolecular forces between particles change; no bonds within the molecules are broken, and the process is fully reversible.
The takeaway
A phase change is the moment kinetic energy finally beats the attractions holding particles together — or loses to them on the way back down. The substance never changes identity, and while the change is happening the thermometer flatlines because the energy is buying freedom, not speed. Learn that one idea and heating curves, latent heat and the whole melting-point-trends topic stop being separate things to memorise.
Foundations → [What Are the States of Matter?] (sibling post) and Intramolecular vs Intermolecular Forces — the forces you're breaking. Why this is physical, not chemical → Physical vs Chemical Changes. Energy in and out → Endothermic vs Exothermic Reactions Explained. Next up → [Evaporation vs Boiling].
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