Changing States: Melting, Freezing, Evaporating

Everything is a solid, liquid or gas

Look around you. A wooden table is a solid. The milk in your cup is a liquid. The air you breathe is a gas. These three are called the states of matter, and almost everything in the world is one of them. You can learn more about each one in solids, liquids and gases.

Here is the exciting part: a material can change from one state to another! Ice can become water, and water can become steam. These are called changes of state, and the secret behind all of them is heat.

Adding heat or taking it away

Tiny particles make up everything. In a solid, the particles are packed tightly and can only wobble. In a liquid, they have a bit more room to slide around. In a gas, they zoom about freely with lots of space.

• Add heat → particles gain energy and move faster → solid melts to liquid, then liquid evaporates to gas.
• Take heat away → particles slow down → gas condenses to liquid, then liquid freezes to solid.

So heating and cooling is how we change states. Let's meet each change.

Melting: solid to liquid

Melting is when a solid turns into a liquid because it is heated.

Ice is solid water. Leave an ice cube in a warm room and it will melt into liquid water. Chocolate melts in your warm hand. Butter melts in a hot pan. Even metal melts if it gets hot enough — that is how blacksmiths shape iron.

The temperature where a solid melts is called its melting point. Ice melts at 0°C (zero degrees Celsius).

Freezing: liquid to solid

Freezing is the opposite of melting. It is when a liquid turns into a solid because it is cooled.

Put liquid water in a freezer and it gets so cold that it freezes into solid ice. Water freezes at 0°C — the very same temperature at which ice melts. That is no accident: melting and freezing are simply the same change going in opposite directions.

Evaporating: liquid to gas

Evaporating is when a liquid turns into a gas because it is heated.

When a puddle dries up on a sunny day, the water has not disappeared — it has evaporated into an invisible gas called water vapour, which floats up into the air. Wet clothes dry on a washing line the same way.

When a liquid gets hot enough, it evaporates very quickly and bubbles. We call this boiling. Water boils at 100°C, turning into the gas we call steam.

Condensing: gas to liquid

Condensing is the opposite of evaporating. It is when a gas turns into a liquid because it is cooled.

Have you seen tiny water drops on the outside of a cold drink, or mist on a cold window? That is condensation. Warm air holds invisible water vapour. When that vapour touches something cold, it cools down and turns back into little drops of liquid water. Clouds form this way too — and that is why it rains.

It is still the same material!

Here is the big idea to remember. When water changes state, it is still water the whole time:

• Ice is solid water.
• Water is liquid water.
• Steam is water as a gas.

Changing state does not make a brand-new material. You are only rearranging the same particles. Cool the steam and you get water again; freeze the water and you get ice again. Nothing is created and nothing is lost — that is why the same water on Earth keeps cycling around forever in what we call the water cycle.

Try it yourself! 🧪

Watch all the changes happen — with one ice cube!

You need an ice cube, a small saucer, and a cold metal spoon from the fridge.

1. Put the ice cube on the saucer in a warm room. Watch it slowly melt from solid ice into a puddle of liquid water. This is melting.
2. Leave the puddle for a few hours, or pop it on a sunny windowsill. The water slowly shrinks and disappears — it has evaporated into water vapour in the air. This is evaporating.
3. Now breathe out gently onto the cold metal spoon (the warm, damp air from your breath touches the cold metal). Watch tiny misty drops appear! Your breath's water vapour has condensed into liquid. This is condensing.

You have seen three changes of state in one go! For the last one, freezing, just put a little water back in the freezer overnight and check it in the morning.

Heat is the energy that makes all of these changes happen — discover more in heat and how it travels.