Science
What are melting and freezing?
Quick answer
Melting is when a solid turns into a liquid as it heats up. Freezing is when a liquid turns into a solid as it cools. Water melts and freezes at 0 degrees Celsius.
In one sentence
Melting changes a solid into a liquid when it is heated, and freezing changes a liquid back into a solid when it is cooled.
Explained
Everything is made of tiny particles. In a solid, they are packed tightly and can only wiggle in place. In a liquid, they are still close together but can slide past each other. See solids, liquids and gases.
When you heat a solid, the particles gain energy and wiggle harder. At a certain temperature, they break free from their fixed places and the solid turns into a liquid. This is melting, and the temperature is the melting point. When a liquid cools, the particles slow down. At the freezing point they lock into place and the liquid becomes a solid. The melting and freezing point of a pure substance are the same temperature.
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Water freezes and ice melts at 0 °C. Different substances melt at different temperatures:
| Substance | Melting point (about) |
|---|---|
| Ice | 0 °C |
| Butter | 30 to 35 °C |
| Chocolate | 30 to 34 °C |
| Candle wax | 55 to 60 °C |
| Iron | 1,538 °C |
Water behaves in a special way. Most liquids shrink as they freeze, but water expands. Ice takes up about 9 percent more space than the same water in liquid form. That is why ice floats in a drink, and why a full water bottle can burst in the freezer. Frozen pipes can crack in winter for the same reason.
Salt lowers the freezing point of water. That is why roads are gritted with salt in winter, so that ice melts or does not form at 0 °C. Melting and freezing are changes of state, and you can reverse them. If you melt an ice cube and refreeze it, it is still water. The same idea explains how heat moves in, how a water cycle works and why evaporation (liquid to gas) is a related change.
Examples
| Change | Direction | Example |
|---|---|---|
| Melting | solid to liquid | ice cube in a glass |
| Freezing | liquid to solid | water in an ice tray |
| Melting | solid to liquid | candle wax near a flame |
| Freezing | liquid to solid | melted chocolate setting |
Common mistakes
- "Ice is not water." It is water in solid form.
- "Melting needs a flame." Any warmth above the melting point works.
- "Melting and dissolving are the same." Dissolving needs a liquid to mix into.
Quick quiz
Melting and freezing
Check what you know with 4 quick questions.
Question 1 of 4
At what temperature does water freeze?
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Related topics
Solids, liquids and gases
Solids, liquids and gases are the three main states of matter. Solids keep their shape, liquids flow and take the shape of their container, and gases spread out to fill any space.
The water cycle
The water cycle is the endless movement of water between the land, the sea and the sky. Water evaporates, condenses into clouds, falls as rain or snow and collects in rivers and oceans.
Evaporation
Evaporation is when a liquid slowly turns into a gas from its surface, like a puddle drying in the sun. It happens faster when it is warm, windy or the liquid is spread out.
Heat and temperature
Temperature tells you how hot or cold something is. Heat is energy that flows from a hotter thing to a cooler one. They are linked, but they are not the same.
Adaptation
An adaptation is a feature or behaviour that helps a living thing survive where it lives. A polar bear's thick fur and a cactus's spines are adaptations.
Amphibians
Amphibians are cold-blooded animals with thin, moist skin that live part of their lives in water and part on land. Frogs, toads, newts and salamanders are amphibians.