The three states and the six changes
Matter has three common states. In a solid the particles are packed and only shake. In a liquid they are close but slide past each other. In a gas they are far apart and fly freely.
Changes of state:
- Melting: solid to liquid (heat in).
- Freezing: liquid to solid (heat out).
- Boiling and evaporation: liquid to gas (heat in). Boiling happens throughout the liquid at a fixed temperature (100 °C for water). Evaporation happens only at the surface and at any temperature.
- Condensation: gas to liquid (heat out).
Finding state changes in cooking
| In the kitchen | Change | Heat |
|---|---|---|
| Butter or ghee melts | Melting (solid → liquid) | In |
| Water boils in a pot | Boiling (liquid → gas) | In |
| Drops on the lid | Condensation (gas → liquid) | Out |
| Ice tray in the freezer | Freezing (liquid → solid) | Out |
| Wet cloth or pickle drying | Evaporation (liquid → gas) | In |
Ask two questions: What state did it start in? What state did it end in? If the substance is the same at the end, it is a state change.
Cooking that is NOT a state change
Some cooking changes make new substances. This is a chemical change.
- An egg turns from runny to firm. It cannot go back to raw.
- Bread and cakes rise and turn brown when baked.
- Milk turning sour or toast burning.
Signs: new colour, new smell, gas or heat released, and you cannot reverse it. State changes are physical and can be reversed: melted butter can set again, steam can condense into water again.
Try it: kitchen detective
With an adult, look for five changes in your kitchen today. Make a table with columns: what happened, start state, end state, heat in or out, physical or chemical. Put a spoon in the freezer for a minute, then breathe on it and watch drops form: which change is it? In the 3D, pick each event and compare the particle box.
Key formulas and definitions
- Heat in: solid → liquid → gas (melting, boiling, evaporation)
- Heat out: gas → liquid → solid (condensation, freezing)
- Physical (state) change: same substance, can be reversed
- Chemical change: new substance, usually cannot be reversed
- Water boils at 100 °C and freezes at 0 °C (normal pressure)
Worked examples
1. Ghee is solid in winter and liquid in summer. Name the change when it gets warm and whether heat goes in or out.
Melting (solid to liquid). Heat goes in.
2. Steam from a boiling pot makes drops on the lid. Name the two state changes in the story.
Boiling (liquid water to steam, heat in) and then condensation (steam to drops, heat out).
3. Is frying an egg a state change? Give two reasons for your answer.
No, it is a chemical change. The egg turns firm and changes colour, forming new substances, and it cannot go back to raw egg.
Common mistakes
- Thinking steam is the white cloud you see. Real steam (water vapour) is invisible; the white cloud is tiny drops that have already condensed.
- Calling frying an egg melting or freezing. The egg makes new substances; it is a chemical change.
- Mixing up the heat direction: melting and boiling take heat in, freezing and condensation give heat out.
- Saying evaporation happens only at 100 °C. It happens at any temperature, from the surface.