The six changes of state
Matter is usually found as a solid, a liquid or a gas. These are called states (or phases). Heating or cooling can change one state into another.
| Change | From → to | Heat | Example |
|---|---|---|---|
| Melting (fusion) | solid → liquid | taken in | ice to water |
| Freezing (solidification) | liquid → solid | given out | water to ice in a freezer |
| Boiling / evaporation (vaporisation) | liquid → gas | taken in | water to steam |
| Condensation | gas → liquid | given out | drops on a cold glass |
| Sublimation | solid → gas | taken in | dry ice, naphthalene balls, camphor |
| Deposition | gas → solid | given out | frost on a cold window |
All of these are physical changes. The particles stay the same. No new substance is made. The mass stays the same: 100 g of ice melts into 100 g of water. The change can be reversed.
Melting and freezing
When a solid is heated, its particles shake more and more. At one fixed temperature, the melting point, they shake hard enough to break out of their rows. The solid becomes a liquid.
- A pure substance melts at one sharp temperature. Ice melts at 0 °C. Iron melts at about 1538 °C.
- A liquid freezes at the same temperature at which the solid melts. Water freezes at 0 °C.
- Substances that have a fixed melting point are called crystalline (ice, salt, metals). Amorphous solids like wax, glass and butter have no sharp melting point; they get softer bit by bit.
- Impurities change the melting point. Salt on an icy road makes ice melt below 0 °C.
Why the temperature stays still
While ice melts, the heat goes into breaking the pull between particles, not into making them move faster. So the thermometer stays at 0 °C until all the ice is water. On a graph this shows as a flat line.
Evaporation and boiling
Both turn a liquid into a gas, but they are not the same.
| Evaporation | Boiling | |
|---|---|---|
| Temperature | any temperature | only at the boiling point |
| Where | only at the surface | all through the liquid (bubbles) |
| Speed | slow | fast |
| Effect | cools the liquid left behind | temperature stays at the boiling point |
Why evaporation cools
Particles in a liquid move at different speeds. The fastest ones near the surface can escape. The slower ones stay, so the average energy of what is left drops. That is why sweat cools your skin.
What makes evaporation faster
- Higher temperature (more fast particles).
- Larger surface area (spread clothes out to dry).
- Wind (it carries the vapour away).
- Lower humidity (dry air takes in vapour more easily).
Boiling point and pressure
Water boils at 100 °C at sea-level air pressure. On a high mountain the air pressure is lower, so water boils below 100 °C and food cooks slowly. A pressure cooker raises the pressure, so water boils at about 120 °C and food cooks faster.
Sublimation and deposition
Some solids turn straight into a gas without becoming a liquid. This is sublimation. Examples: dry ice (solid carbon dioxide) at -78 °C, camphor, naphthalene balls in a cupboard, and iodine crystals when warmed.
The reverse, gas straight to solid, is deposition. Frost forming on a very cold window, or iodine vapour forming crystals on a cold surface, are examples.
Heating and cooling curves, and latent heat
If we heat ice steadily and plot temperature against time (or energy), we get a heating curve:
- Sloping part: ice warms to 0 °C.
- Flat part: ice melts at 0 °C.
- Sloping part: water warms to 100 °C.
- Flat part: water boils at 100 °C (this flat part is longer).
- Sloping part: steam gets hotter.
A cooling curve is the same picture run backwards: condensing and freezing show as flat parts.
The energy used during a flat part is called latent heat ("latent" means hidden: it does not show on a thermometer).
- Specific latent heat of fusion (melting): energy to melt 1 kg with no temperature change. For ice: about 334 000 J/kg.
- Specific latent heat of vaporisation (boiling): energy to boil 1 kg. For water: about 2 260 000 J/kg.
Energy for a change of state: E = m × L. Boiling needs far more energy than melting, because the particles must be pulled completely apart. That is also why a burn from steam is worse than a burn from boiling water: the steam gives out its latent heat when it condenses on your skin.
Try it at home
Put two equal wet handkerchiefs out: one folded, one spread flat. Predict which dries first, then check after 30 minutes. Then try one in the shade and one in the wind.
Key formulas and definitions
- Energy to change state: E = m × L
- Latent heat of fusion of ice: L ≈ 334 000 J/kg
- Latent heat of vaporisation of water: L ≈ 2 260 000 J/kg
- Energy to warm without changing state: E = m × c × ΔT (c of water ≈ 4200 J/kg °C)
- Heat in: melting, boiling/evaporation, sublimation. Heat out: freezing, condensation, deposition
Worked examples
1. Name the change: a puddle disappears on a sunny afternoon.
Evaporation (liquid → gas at the surface, below the boiling point).
2. Why do drops of water appear on the outside of a cold bottle?
Water vapour in the air touches the cold bottle, loses heat and condenses into liquid drops. The water comes from the air, not through the glass.
3. How much energy is needed to melt 2 kg of ice at 0 °C? (L = 334 000 J/kg)
E = m × L = 2 × 334 000 = 668 000 J = 668 kJ. The temperature stays at 0 °C the whole time.
4. How much energy turns 0.5 kg of water at 100 °C into steam at 100 °C? (L = 2 260 000 J/kg)
E = 0.5 × 2 260 000 = 1 130 000 J = 1130 kJ (1.13 MJ).
5. A heater gives 500 J each second to melting ice. How long does it take to melt 0.3 kg of ice at 0 °C?
E = 0.3 × 334 000 = 100 200 J. Time = 100 200 ÷ 500 = 200.4 s, about 3.3 minutes.
6. How much energy turns 1 kg of ice at 0 °C into water at 20 °C?
Melt: 1 × 334 000 = 334 000 J. Warm: 1 × 4200 × 20 = 84 000 J. Total = 418 000 J = 418 kJ.
Common mistakes
- Thinking the temperature keeps rising while ice melts. It stays at 0 °C until all the ice has melted.
- Mixing up evaporation and boiling. Evaporation happens at any temperature, only at the surface; boiling happens at one temperature, all through the liquid.
- Saying the drops on a cold glass came through the glass. They come from water vapour in the air that condenses.
- Thinking a change of state makes a new substance. Ice, water and steam are all water; it is a physical change.