IGCSE Chemistry States of Matter Notes
The kinetic particle model explains why solids, liquids and gases behave differently — and lets you predict what happens when we heat, cool or mix substances.
Key concepts
- Solid
- Particles packed in a fixed pattern, vibrating in place. Fixed shape and volume.
- Liquid
- Particles close together but able to move past one another. Fixed volume, takes container shape.
- Gas
- Particles far apart, moving quickly and randomly. Fills its container.
- Diffusion
- Net movement of particles from high to low concentration; faster in gases than in liquids, and faster at higher temperatures.
- Melting and freezing occur at the melting point; boiling and condensing at the boiling point.
- Sublimation goes directly from solid to gas (e.g. iodine, CO₂).
- At higher temperature, particles have more kinetic energy and move/vibrate faster.
- Lighter gas particles diffuse faster than heavier ones at the same temperature.
Worked example
Explain, using the kinetic particle model, why a gas fills its container.
- 1Gas particles are far apart with negligible forces between them.
- 2They move quickly in random directions.
- 3They spread out until they hit the container walls.
- 4So a gas has no fixed shape or volume; it fills the whole space available.
Answer: Random rapid motion of widely spaced particles spreads them evenly through the container.
Common mistakes
✗ Saying particles themselves melt or expand.
✓ The arrangement and motion of particles change; the particles do not change size.
✗ Confusing evaporation with boiling.
✓ Evaporation happens at any temperature at the surface; boiling happens throughout the liquid at the boiling point.
✗ Ignoring temperature when comparing diffusion.
✓ Always state that higher temperature means faster particle movement and so faster diffusion.
Quick check
Try these, then reveal the answers.
Which state has the most kinetic energy at the same temperature?Reveal answer
Gas — particles move fastest.
Name a substance that sublimes at room temperature.Reveal answer
Solid carbon dioxide (dry ice) or iodine when warmed gently.
Why does bromine gas diffuse faster than nitrogen dioxide at the same temperature?Reveal answer
It doesn't — lighter particles diffuse faster; compare Mr values before answering.
Exam tips
- State three things about particles in each state: arrangement, movement and forces between them.
- Use 'particles' rather than 'atoms' or 'molecules' unless the question specifies.
- Sketch heating/cooling curves with clearly labelled flat sections at melting and boiling points.
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Frequently asked questions
What is diffusion?+
The net movement of particles from an area of higher concentration to lower concentration, caused by their random motion.
Why does a solid have a fixed shape?+
Its particles are held in a regular pattern by strong forces; they can only vibrate, not move past each other.
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