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

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

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.

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.

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.

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