Particles, states and changes of state
Use particle spacing and movement to explain the everyday behaviour of solids, liquids and gases.
Core and Extended · Chemistry 0620, 2026–2028.
Before you begin
- Know that a substance is made of particles.
- Distinguish the size of a particle from the space between particles.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which quantity describes how much space a sample occupies?
What you will learn
- Compare the arrangement and motion of particles in each state.
- Name changes of state and describe how heating or compression changes a gas.
Three states, the same particles
In a solid, closely packed particles vibrate about fixed positions. The solid keeps its shape and volume. In a liquid, particles are still close together but can move past one another, so its shape follows the container while its volume stays almost fixed. Gas particles are far apart and move rapidly in many directions; a gas fills the available space.
A syringe of gas can be compressed because much of its volume is space between particles. Liquids and solids are much harder to compress because their particles are already close together. Compression does not shrink the particles themselves.
Name the direction of a change
Melting changes solid to liquid; freezing changes liquid to solid. Boiling and evaporation both change liquid to gas. Boiling occurs throughout a liquid at its boiling point for the stated pressure; evaporation occurs at the surface and can happen below that temperature. Condensation changes gas to liquid.
Heating a fixed quantity of gas increases its volume if pressure can stay constant, for example in a cylinder with a freely moving piston. Increasing pressure reduces its volume if temperature stays constant. Always state what stays fixed: a sealed rigid container cannot expand, so heating it increases pressure instead.
Pause and explain
Why can air be compressed much more easily than water?
Worked example
A sealed syringe contains air. With its temperature unchanged, the plunger is pushed inward. Describe the particle change.
Show the worked solution
- The same particles remain inside because the syringe is sealed.
- The available volume falls, reducing the average separation of particles.
- Particles do not become smaller; the gas pressure rises.
Answer Particles are closer together; pressure increases.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare the particle arrangement, movement, shape and volume of ice and liquid water.
Give me a hint
Use one statement about particles and one about the sample for each state.
Compare my reasoning
- Ice has close particles vibrating about fixed positions.
- Liquid water has close particles able to move past each other.
- Ice keeps shape and volume; water keeps nearly fixed volume but takes its container's shape.
Fixed-position vibration in ice; movement past neighbours in liquid water.
Look for these in your work
- Movement is stated for both states.
- Particle behaviour is linked to shape and volume.
A wet towel dries at 25°C. Name the change and explain why boiling is not needed.
Give me a hint
Consider the surface of the water.
Compare my reasoning
- Liquid water becomes water vapour.
- This is evaporation from the liquid surface.
- Evaporation can occur below the boiling point; the whole liquid need not boil.
Evaporation; it can occur at the surface below the boiling point.
Look for these in your work
- Liquid-to-gas is identified.
- Evaporation is distinguished from boiling.
Air is heated in a cylinder with a freely moving piston and unchanged outside pressure. Predict the volume change, then compare a rigid sealed container.
Give me a hint
Ask whether the boundary can move.
Compare my reasoning
- The movable piston allows the gas volume to increase.
- The pressure can remain equal to the outside pressure.
- A rigid sealed container has fixed volume, so heating instead raises the gas pressure.
Movable piston: volume increases. Rigid container: volume stays fixed and pressure rises.
Look for these in your work
- The fixed condition is identified.
- The answer changes appropriately for a rigid boundary.
Common mistakes
- Saying solid particles do not move: they vibrate.
- Describing melting as particles growing or turning into a different substance.
What can you explain now?
Water droplets form outside a cold glass. Name the change of state.
Compare with the explanation
Condensation
Water vapour in the surrounding air becomes liquid at the cold surface; it has not passed through the glass.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Tomorrow, sketch three boxes of particles and explain why only the gas is readily compressed. Name all five changes of state from memory.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.