ACARA v9 CONTENT DESCRIPTION “use particle theory to describe the arrangement of particles in a substance, including the motion of and attraction between particles, and relate this to the properties of the substance”
Builds on earlier work sorting materials by their observable properties. Particle theory goes underneath those properties: it pictures all matter as tiny particles, and explains what we can see from how those particles are arranged and how they move.
Everything is made of particles
Particle theory says that all substances are made of tiny particles, far too small to see, that are always moving. Three things about those particles, how they are arranged, how much they move, and how strongly they attract one another, decide whether a substance is a solid, a liquid or a gas, and how it behaves.
How particles are arranged
The same particles, packed three ways. Spacing and attraction explain how each state behaves.
In a solid, particles are packed in a fixed, regular pattern. Strong attraction holds them in place, so they only vibrate; a solid keeps its shape.
Energy changes the state
The particles do not change when a substance melts or boils; their energy does. Heating gives particles more energy to move, until they can break away from the attraction holding them in place. That is why adding or removing heat moves a substance between solid, liquid and gas without making any new substance.
Heating adds energy
Adding heat gives particles more energy to move, until they break free of their fixed positions.
Heating gives particles more energy to move. With enough energy they overcome the attraction holding them together: a solid melts to a liquid, then a liquid boils to a gas. Cooling reverses each step.
Properties follow the particles
Once you can picture the particles, the properties of each state stop being a list to learn and become something you can predict. Shape, whether a substance flows, and whether it fills a container all follow from how close the particles are and how freely they move.
Why each state behaves as it does
Shape and volume are not extra facts to memorise; they follow directly from how the particles are arranged.
A solid holds its shape because its particles are locked in place. A liquid keeps the same volume but takes the container shape because its particles stay close yet move. A gas spreads to fill the container because its particles are free and far apart.
Dissolving spreads particles, it does not destroy them
When a solute such as salt seems to disappear in water, its particles are not gone. They spread out and slot in between the moving solvent particles to make a solution. The same particles are still there, so the mass does not change, and a solution is still a mixture: the solute can be recovered by evaporating the solvent away.
Dissolving makes a solution
Drop a solute, such as salt, into a solvent, such as water. Watch where the particles go, and count them.
The solute particles start clumped together in the solvent. They have not gone anywhere yet, they are just grouped.
Where gas pressure comes from
A gas pushes on whatever holds it. That push is gas pressure, and the particle model explains it: the fast-moving particles are constantly striking the container walls, and each tiny impact pushes outward. Pack the same particles into a smaller space and they hit the walls more often, so the pressure rises.
Gas pressure comes from collisions
Gas particles move fast in every direction and fill their container. Squeeze them into less space and see what happens to the wall hits.
The fast-moving particles spread out to fill the whole container. Each time one hits a wall it pushes on it; all those tiny pushes together are what we measure as gas pressure.
Why this matters
The particle model is one of the most useful ideas in science. It explains melting, boiling, dissolving, gas pressure and much more, all with the same simple picture of moving particles. Later topics on elements, compounds and chemical reactions all build on this foundation.
Quick self-check
1. In which state are particles packed in a fixed, regular pattern?
2. Why can a liquid be poured while a solid cannot?
3. What happens to particles when a substance is heated?
4. Which state has particles that are far apart with very weak attraction?
5. A substance takes the shape of its container but keeps the same volume. It is a...