The kinetic particle model describes matter as being made up of particles (atoms, molecules or ions) that are always moving. The state of a substance depends on how much energy the particles have and how strongly they attract each other.
Solid
Particles are close together in a regular, ordered arrangement. They are held strongly in position by forces of attraction and can only vibrate about fixed positions. A solid has a definite shape and a definite volume. It cannot be compressed because the particles are already touching.
Liquid
Particles are close together but in a random (irregular) arrangement. They have enough energy to move past each other and flow. A liquid has a definite volume (particles are still touching, so it cannot be compressed) but no fixed shape — it takes the shape of its container.
Gas
Particles are far apart and in a random arrangement. They move quickly in all directions with much more energy than in a liquid or solid. A gas has no fixed shape and no fixed volume — it fills its container completely. Gases can be compressed because there is a large amount of empty space between the particles.
Quick Check: A student says liquids and gases both have no fixed shape, but only a gas can be compressed. Are they correct?
Yes, correct. Both liquids and gases take the shape of their container, but only gases can be significantly compressed — liquid particles are too close together. Gases have large spaces between particles that can be reduced.
This deep dive covers Particle Behaviour in the Three States within States of Matter for GCSE Chemistry. Revise States of Matter in Atomic Structure for GCSE Chemistry with 12 exam-style questions and 10 flashcards. This is a high-frequency topic, so it is worth revising until the explanation feels precise and repeatable. It is section 3 of 13 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for States of Matter
Which of the following best describes the arrangement of particles in a solid?
A heating curve shows a flat section (plateau) when a solid is being heated to become a liquid. Explain why the temperature does not rise during this flat section. [3 marks]