Melting, freezing, vaporisation, and condensation

Ice, water, and steam are all made of the same thing — water molecules. But in each of them, the molecules are arranged in a different way. This makes ice, water, and steam look and behave differently from each other. They are three states, or phases of water. You have probably noticed that water doesn’t stay in the same state forever.

It can change between different states. We call these changes phase transitions. Take this ice cube. Its temperature, straight from the freezer, is -10°C. The water molecules are close together.

They form a fixed structure. They can’t move freely, they just vibrate slightly in place. Let’s add heat and see what happens. The temperature rises. The molecules in their positions start vibrating more and more.

When the temperature reaches 0°C, the structure starts breaking apart, and the molecules can move around more freely. The ice cube is turning into water! This is what we call melting, one type of phase transition — a solid turning into a liquid. No matter how much heat we add, as long as there is any ice left, the temperature of the water remains unchanged: 0°C. This is the melting point of water.

Here, all the ice has melted into water. The molecules no longer form a fixed structure, but they are still close together. Let’s add more heat! The temperature rises. Molecules move faster, bump into each other, start to spread out … … until some of them break off and float away.

Water turns into water vapour — steam. This is an example of another phase transition — vaporisation. A liquid turns into a gas — it vaporises. Liquids can vaporise in two different ways. Here’s a puddle of water.

A few days pass and the puddle is gone. How come? It is easy for molecules on the surface to break away from the liquid. Little by little, all the water turns into vapour, and the puddle disappears. This is one type of vaporisation — evaporation.

Evaporation happens only on the surface of the liquid. Now, let’s heat water from below in a kettle. The water molecules on the bottom, heat up faster than those on the surface. When the temperature reaches 100°C water starts turning into gas. The gas forms bubbles, which rise to the surface of the water.

This is the second type of vaporisation — boiling. No matter how much heat we add, as long as the water is boiling it stays at 100°C — until all the liquid has turned into gas. 100°C is the boiling point of water. Ice melts and water vaporises when we add heat to water molecules. What happens when we remove heat?

We let the steam cool down. As the temperature drops, molecules slow down. When the slow-moving molecules get close to each other, they clump together. So, water vapour turns into liquid water. It condenses.

Condensation is another type of phase transition, when a gas turns into a liquid. Let’s keep removing heat to bring the temperature down. The molecules slow down even more. At 0°C, the molecules start forming an organised, fixed structure. Liquid water turns into solid ice!

This is an example of freezing — a phase transition during which a liquid turns into a solid. Water comes in three different states and changes between those states through phase transitions. When we add heat ice melts into water; and water vaporises into steam. When we remove heat, steam condenses into water; and water freezes into ice.