ACARA v9 CONTENT DESCRIPTION “explain observable properties of solids, liquids and gases by modelling the motion and arrangement of particles”
Builds on knowing that solids, liquids and gases look and act differently. Here we explain why, by zooming in to the tiny particles that everything is made of and looking at how they are arranged and how they move in each state.
Everything is made of particles
All matter is made of particles far too small to see. We cannot watch the real particles, so scientists use a model: we picture them as tiny balls and ask how they are arranged and how they move. With this particle model we can explain the things we do see, such as why a solid is firm, why a liquid pours and why a gas spreads out. The same particles can be a solid, a liquid or a gas depending only on how they sit and move.
How the particles are arranged
Everything is made of tiny particles. Switch state and watch how the particles sit together.
In a solid the particles are packed tightly in a neat, ordered pattern. In a liquid they are still close together but jumbled, so they can slide around. In a gas there are big gaps: the particles are spread far apart and fill the whole space. The arrangement of the particles is what makes each state look and behave the way it does.
The particles are always moving
In every state the particles are moving, but by different amounts. In a solid they only vibrate gently while staying in their fixed spots, so the solid holds together. In a liquid they have more energy and slide past one another. In a gas they have the most energy and move freely and fast in all directions. How much the particles move is what sets the three states apart.
How fast the particles move
Pick a state, then step the motion forward. Watch how far the particles jump each step.
The particles are always moving. In a solid they only vibrate gently in their fixed spots, so the shape stays put. In a liquid they have enough energy to slide past one another. In a gas they move freely and fast, bouncing all over the space. More motion means a looser, freer state.
Why a solid holds its shape and a gas does not
Because solid particles hold tightly together in a fixed pattern, a solid keeps its own shape wherever you put it. A liquid particles can slide, so a liquid settles and takes the shape of the bottom of its container while keeping the same amount. A gas particles fly apart in every direction, so a gas spreads out until it fills the whole container. The way the particles are arranged and how freely they move explains exactly what we observe.
Why each state keeps its shape or not
Put the same particles in a box. Watch a solid hold firm, a liquid settle flat, a gas fill up.
In a solid the particles hold tightly to each other, so the block keeps its own shape no matter what holds it. In a liquid the particles can slide, so they settle and take the shape of the bottom of the container. In a gas the particles fly apart in every direction, so they spread out and fill the whole container. The particle picture explains what we see.
Why a gas can be squeezed
A gas has large gaps between its particles, so when you press on it the particles can be pushed much closer together and the gas takes up less space. A solid and a liquid already have their particles touching, with almost no gaps, so they barely squeeze at all. This is why you can pump air into a tyre but you cannot squash a block of wood or a cup of water.
A gas can be squeezed
Choose a state, then push the lid in. See how far you can squeeze the particles together.
A gas has big gaps between its particles, so when you push the lid in, the particles pack much closer and the gas is squeezed into a smaller space. A solid and a liquid have their particles already touching, with almost no gaps, so they barely squeeze at all. That is why you can compress air but not a block of wood or a cup of water.
Heating gives the particles more motion
Heat is energy that makes the particles move more. When you heat a solid, its locked particles gain enough motion to break free and slide, so the solid melts into a liquid. Heat the liquid more and the particles move fast enough to fly apart, so the liquid becomes a gas. Cooling does the reverse. The particles are never created or destroyed, they simply move more when heated and less when cooled, which is the cause of every change of state.
Heating changes the state
Add heat step by step. Watch the particles gain motion and turn solid into liquid, then gas.
Heat is energy that makes the particles move more. A cold solid has its particles locked in place. Adding heat gives them enough motion to break free and slide, so the solid melts into a liquid. Adding even more heat lets them fly apart completely, so the liquid becomes a gas. It is the same particles all along, just moving more as they warm up.
Why this matters
The particle model is one of the most useful ideas in science. It lets you explain melting ice, boiling water, the air you breathe, and why steam takes up so much more room than the water it came from. Once you can picture the arrangement and motion of particles, the observable properties of solids, liquids and gases stop being separate facts and become one clear story, and it sets you up for chemistry and physics in the years ahead.
Quick self-check
1. How are the particles arranged in a solid?
2. How do the particles in a gas move compared with those in a solid?
3. Using the particle picture, why does a liquid take the shape of its container?
4. Why can a gas be squeezed into a smaller space, but a solid cannot?
5. When you heat a solid until it becomes a liquid and then a gas, what is happening to the particles?