# States of matter

Solids, liquids and gases are called the three states of matter.

Materials can be changed from one state to another by heating or cooling.

## Heating

• If ice (solid) is heated, it changes to water (liquid). This change is called melting.

• Water (liquid) can change to water vapour (gas). This is called evaporation.

• If water (liquid) is heated until it boils, it changes to water vapour (gas) very quickly. Water boils at 100°C

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## Cooling

• If water vapour (gas) is cooled, it changes to water (liquid). This change is called condensing.

• If water (liquid) is cooled, it changes to ice (solid). This change is called freezing. Water freezes at 0°C

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# Solids and liquids

Heat melts a solid and turns it into a liquid. Cooling freezes a liquid into a solid.

Different solids melt at different temperatures, some high, some low. These are called their melting points.

## Heating

Heat can change solids into liquids or gases.

• Most solids melt into liquid when they are heated.

• A liquid evaporates into a gas when it is heated.

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## Changes of State

### Cooling

When we cool something we take heat away from it. Cooling changes a gas into a liquid, and a liquid into a solid.

• A gas condenses into a liquid when it is cooled.

• A liquid freezes into a solid when it is cooled.

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## Changes of State

### Melting points

Different solids melt at different temperatures. Ice melts at 0 degrees Celcius (0°C). Chocolate melts at about 35°C. We say that chocolate has a higher melting point than ice.

Metals, like aluminium and iron, also melt when we heat them. They have very high melting points. They have to be very hot to melt.

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# Temperature

Temperature is a measure of how hot or cold things are. You need a thermometer to measure temperature.

Temperature is measured in degrees Celsius (°C).

• Ice melts at exactly 0°C.

• A hot bath is about 40°C

• Water boils at exactly 100°C

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