The particle theory of matter

How come water freezes when you put it in a freezer and evaporates if you leave it at room temperature for long enough? Why can different materials change their physical state between solid, liquid, and gas? Why does the volume of materials change if we raise or lower their temperatures? One way to explain all this is to picture that everything around us is made up of tiny particles. This is the basic idea behind the particle theory of matter.

This theory can be summarised in five main points. Point one: all matter is made up of particles, such as atoms, molecules, or ions. Different types of matter — different substances — are made of different particles. For instance, gold contains only atoms of gold. Water is made up of water molecules that contain one oxygen and two hydrogen atoms.

The salt you use on your food consists of sodium ions and chloride ions. Point two in this particle theory of matter is that particles are attracted to each other by forces. These attractive forces pull together particles that are close by. Particles that are closer, are attracted to each other more strongly, than those that are further apart. Point three tells us that there is always space between particles.

In some substances, particles can be packed very tightly together, but they never actually touch. The space between the particles is empty. The fourth point states that all particles move constantly. This is true for all particles in all substances, in all states of matter. It might seem to you that solid objects, such as books or walls, are still and motionless.

They appear so, because the particles inside them form rigid structures, held together tightly by the attractive forces. But even in rigid structures, particles vibrate slightly in their positions. Even though an object itself is not in motion, the particles inside it move. This means that the particles have energy related to that motion - kinetic energy. High kinetic energy means a lot of movement, and low kinetic energy means little movement.

And this brings us to point number five in the particle theory of matter: there is a relationship between temperature and the energy of the moving particles. When a substance is heated, its temperature rises. Its particles move faster, and their kinetic energy increases. The fast-moving particles can more easily overcome the forces that attract them to surrounding particles. So, if possible, the particles spread out.

And that’s how heating up a substance can make it change its state — from solid to liquid or from liquid to gas! When heat is removed from the substance, the opposite happens. The temperature decreases, particles slow down, and their kinetic energy drops. Particles don’t move fast enough to overcome the attractive forces, so these forces pull them closer together. This is why, when a substance cools down, it can change its state — from gas to liquid or from liquid to solid!

So, this is the particle theory of matter. These five points explain what everything is made of. They help us better understand why there are different states of matter, and why substances behave the way they do.