ACARA v9 CONTENT DESCRIPTION “investigate the observable properties of solids and liquids and how adding or removing heat energy leads to a change of state”
Builds on sorting everyday materials by what you can see and feel. Here we look closely at two states, solids and liquids, and find out how adding or taking away heat can turn one into the other.
Solids and liquids look and act differently
A solid, like an ice cube or a wooden block, keeps its own shape. You can pick it up and hold it, and it stays the same wherever you put it. A liquid, like water or juice, does not keep a shape of its own. It takes the shape of its container and it pours. Looking at how something behaves, whether it holds a shape or flows, helps you tell a solid from a liquid.
A solid keeps its shape
An ice cube is a solid. Move it to a different place and watch its shape.
A solid keeps the same shape wherever you put it. The ice cube stays a cube on a hand, in a cup or on a plate. You can pick a solid up and hold it because it stays in one firm piece.
A liquid flows and fits its container
Pour the same water into a tall glass, a wide bowl or a jug and it looks different each time, because a liquid spreads to fit whatever holds it. That is why you keep liquids in cups and bottles. A solid does not need a container to keep its shape.
A liquid changes shape and pours
Water is a liquid. Pour the same water into different things and watch its shape change.
A liquid takes the shape of whatever holds it. The same water looks tall in a glass, wide in a bowl, and runs out in a stream when you tip the jug. A liquid flows and pours, so you cannot hold it the way you hold a solid.
Adding heat can change the state
Heat is a kind of energy. When you add heat to a solid like ice, it warms up. If it gets warm enough it melts, turning from a solid into a liquid. This is called a change of state, because the water is still water, it has just changed from ice into liquid.
Adding heat melts a solid
Add heat to the ice, step by step. Watch the temperature climb and the ice turn to water.
Each time you add heat, the temperature rises. When the ice gets warm enough it melts: the solid turns into liquid water. Adding heat energy is what changes the state from solid to liquid.
Removing heat can change it back
Taking heat away does the opposite. When liquid water gets cold enough it freezes, turning from a liquid into solid ice. So adding heat melts a solid, and removing heat freezes a liquid. Heat energy is the key to changing state in both directions.
Removing heat freezes a liquid
Take heat away from the water, step by step. Watch the temperature fall and the water turn to ice.
Taking heat away makes the temperature fall. When the water gets cold enough it freezes: the liquid turns into solid ice. Removing heat energy changes the state from liquid back to solid.
You can go back and forth
Melting and freezing are opposites of each other, and you can do them as many times as you like. The same water can melt, freeze, melt and freeze again. Changing state does not make a new material, it just changes whether the water is a solid or a liquid.
The change can go both ways
Turn the heat on and off. The same water swaps between solid and liquid as often as you like.
Melting and freezing are opposites, and you can repeat them again and again. Add heat and the ice melts to water. Take the heat away and the water freezes back to ice. It is the same water all along, just changing state.
Why this matters
Changing state is happening all around you. Ice melts in a warm drink, water freezes into ice in a freezer, and puddles dry up in the sun. Knowing that adding or removing heat changes a solid into a liquid, and back again, helps you make sense of these everyday things and sets you up for more science about heat and materials later on.
Quick self-check
1. Which of these is true about a solid such as an ice cube?
2. You pour water into a tall glass, then into a wide bowl. What happens to the water?
3. What do you need to add to melt an ice cube into water?
4. How can you change liquid water back into solid ice?
5. You melt ice into water, then freeze it again. What does this show?