Lesson 1 of 4
States and changes of state
Explain what particles do and where energy goes.
O-Level 6092 (2026) / SEC G3 K324 (2027)
I can explain states and their interconversion.Syllabus
Syllabus K324, 2.1(a). Explain states and their interconversion
States and changes of state
Explain what particles do and where energy goes.
Make a guess. You are not marked.
Crushed ice at 0 degrees C is heated steadily and stirred. What does the thermometer show while the ice is melting?
Show the answer
It stays at 0 degrees C while ice melts
The energy supplied overcomes the attractions between particles instead of making them move faster, so the temperature stays constant.
Matter consists of particles. In a solid, particles remain close together in an ordered arrangement and vibrate about fixed positions. In a liquid, they remain close but move past one another. In a gas, they are widely spaced and move rapidly in random directions. A gas fills its container because its particles spread throughout the available space; compression mainly reduces the empty spaces between particles.
| State | Arrangement and movement | Observable behaviour |
|---|---|---|
| Solid | Close, regular; vibrate in fixed positions | Fixed shape and volume |
| Liquid | Close, irregular; move past one another | Fixed volume; takes container shape |
| Gas | Far apart; random movement | No fixed shape or volume; readily compressed |
- Solid to liquid: melting
Energy is absorbed to overcome enough attraction for particles to move past each other.
- Liquid to gas: boiling or evaporation
Energy is absorbed to separate particles much further. Evaporation occurs at the surface and can occur below the boiling point.
- Gas to liquid: condensation
Particles come closer and energy is transferred to the surroundings.
- Liquid to solid: freezing
Particles settle into fixed positions and energy is released. Sublimation changes solid directly to gas; deposition is the reverse.
During heating within one state, average particle kinetic energy increases and temperature rises. During the melting or boiling of a pure substance at constant pressure, supplied energy changes particle separation rather than raising temperature, so a plateau can occur. Cooling reverses these energy transfers. Particles themselves do not become larger or disappear.
Heating a pure substance
A rising line alternates with two horizontal plateaus: melting then boiling. Heat continues to enter during each plateau.
Check your understanding
Ice is melting while its temperature stays constant. Is it absorbing energy?
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Yes. The energy overcomes the attractions between particles instead of raising their speed.
During melting, energy is absorbed to break up the fixed arrangement, not to raise the average kinetic energy. A constant temperature does not mean no energy is transferred.