Kinetic Particle Theory – IGCSE Chemistry Definition

The kinetic particle theory (also called the particle model) underpins almost every topic in the states of matter section of the 0620 syllabus. Examiners expect you to use it to explain diffusion, changes of state, gas pressure, and why substances have different physical properties. If a question says “explain in terms of particles,” the kinetic particle theory is what you need.

The 0620 key ideas

1. All matter is made of particles (atoms, molecules, or ions)
2. Particles are in constant motion — the higher the temperature, the more kinetic energy they have
3. The three states of matter differ in the arrangement, spacing, and motion of their particles

Particles in the three states

Property	Solid	Liquid	Gas
Arrangement	Regular, ordered pattern	Irregular, close together	Random, widely spaced
Spacing	Very close (touching)	Close (almost touching)	Very far apart
Motion	Vibrate about fixed positions	Slide past each other	Move rapidly and randomly in all directions
Energy	Lowest kinetic energy	Moderate kinetic energy	Highest kinetic energy
Shape	Fixed shape	Takes shape of container	Fills the container
Volume	Fixed volume	Fixed volume	No fixed volume
Compressibility	Cannot be compressed	Almost incompressible	Easily compressed (large gaps between particles)

Using kinetic particle theory in exams

The theory is not tested in isolation — it is the explanation tool. You apply it whenever a question asks you to explain:

• Diffusion: particles move randomly from high to low concentration
• Gas pressure: particles collide with container walls; heating increases collision force and frequency
• Changes of state: heating gives particles more energy to overcome intermolecular forces
• Compression of gases: large spaces between gas particles allow them to be pushed closer
• Why solids have fixed shapes: particles locked in position by strong intermolecular forces

Worked exam question

Use the kinetic particle theory to explain why gases can be compressed but solids cannot. (3)

Mark scheme

In gases, particles are far apart / there are large spaces between particles [1]; when compressed, particles are pushed closer together into the available space [1]; in solids, particles are already very close together / touching, so there is no space to compress into [1]

Common exam mistakes

• Writing “particles expand” when heated. Particles do not expand — they gain kinetic energy and move faster or vibrate more vigorously. It is the spaces between particles that increase when a gas is heated in an expandable container.
• Using the word “atoms” when the substance is molecular. If the question is about water, refer to “water molecules”, not “water atoms”.
• Forgetting that all particles have some motion, even in solids. Solid particles vibrate — they are not completely still.