Kinetic theory of matter

The kinetic theory of matter is the idea that matter is made of tiny particles in constant, random motion that possess kinetic energy, which increases with temperature. Malay: teori kinetik jirim · Chinese: 物质动理论.

This model explains a great deal at once. The three states of matter differ in how their particles are arranged and how freely they move; when a substance is heated, its particles gain kinetic energy and move or vibrate faster; and because the particles are always moving, they spread out and mix on their own.

Example. The kinetic theory explains why a solid melts when heated: the particles gain enough energy to break free of their fixed positions and slide past one another as a liquid. It also explains diffusion, the way the purple colour of potassium manganate(VII) spreads through water as its particles move randomly among the water particles, and why a gas fills any container it is put in.

Confusion to avoid. Do not confuse the kinetic theory (the model of matter) with kinetic energy (the energy of motion the particles have). A second common slip is to say that particles “expand” or “get bigger” when heated, they do not; it is the spacing and speed of the particles that change, not the size of each particle.

In the exam you are expected to use the theory to explain observations rather than just state it. A strong answer names the particles, says what happens to their energy and movement, and links that to what is seen, for example, “the particles gain kinetic energy, move faster and spread further apart.” Marks are usually awarded for this chain of reasoning, not for the conclusion on its own.

The kinetic theory is the foundation of the first half of the Matter and the Atomic Structure chapter. Every explanation you give for states of matter, changes of state, heating and cooling curves and diffusion comes back to this one idea, so learning to apply it in words is a skill our teachers build from the first lesson.