States of matter: solids, liquids, gases and changes of state
Use the particle model to explain melting, boiling and condensing, and why temperature stays constant during a change of state.
KS3GCSEMiddle SchoolHigh SchoolCurriculum fit: GCSE Chemistry and Combined Science (states of matter). US: middle and high school physical science (structure and properties of matter).
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What is it?
Matter exists mainly as a solid, a liquid or a gas. The particle model explains the differences by how the particles are arranged and how they move.
In a solid, particles are packed closely in a regular pattern and vibrate. In a liquid they are close but free to move past each other. In a gas they are far apart and move quickly in all directions.
Key formulas and ideas
- Melting: solid → liquid. Freezing: liquid → solid.
- Boiling or evaporating: liquid → gas. Condensing: gas → liquid.
- Sublimation: solid → gas directly (for example, dry ice).
- Energy is needed to overcome forces between particles when a substance melts or boils, so the temperature stays constant during a change of state.
Worked examples
Example 1: State at room temperature
Question: A substance melts at −10 °C and boils at 60 °C. What state is it at 25 °C?
- 25 °C is above the melting point (−10 °C).
- 25 °C is below the boiling point (60 °C).
Answer: Liquid
Example 2: Heating curve
Question: Why does the temperature stay constant while ice is melting?
- Energy supplied is used to break the forces holding the particles in the solid.
- It does not increase the kinetic energy of the particles, so the temperature does not rise.
Answer: Energy is used to change state, not to raise temperature
Example 3: Particle arrangement
Question: Describe the movement of particles in a gas.
- Particles are far apart with weak forces between them.
- They move randomly and quickly.
Answer: Random, fast movement with large spaces between particles
Common mistakes
- Saying particles themselves melt or expand. They stay the same and the arrangement changes.
- Saying that a gas has no particles in the gaps. The gaps are empty space.
- Confusing evaporation and boiling.
- Thinking the temperature keeps rising during a change of state.
Practice questions
Frequently asked questions
What is the difference between evaporation and boiling?
Evaporation happens at the surface of a liquid at any temperature. Boiling happens throughout the liquid at the boiling point.
What are the limitations of the particle model?
It treats particles as hard spheres with no forces shown and no sense of the space between them or the real structure.
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KS3GCSEMiddle SchoolHigh SchoolSources and review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.