What are materials made of?
            1. Solids, liquids and gases
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Photo of iceberg
Picture 1.3. An iceberg - solid water.
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Solid to a liquid to a gas
The materials around us are at room temperature. When we describe a material as being a solid, liquid or gas, we are usually describing them at room temperature. We can change a material into its other states by heating it up or cooling it down.

For example, water is a liquid at room temperature.
If we cool it down, it changes into a solid – ice.
I
f we heat it up, it changes into a gas – steam.

Melting Point and Freezing Point
A particular material changes from being a solid to a liquid at a fixed temperature. This is its melting point.

Ice melts to form water at 0 °C. So its melting point is 0 °C.

We can easily reverse this process. If we cool water below 0 °C, it freezes to form ice. So we also call this temperature the freezing point of water.

The melting point and freezing point for any material are the same.

Evaporating and Condensing
Have you noticed that puddles gradually disappear after it has stopped raining? If a glass of water is left in a warm room, again the water will disappear. We say that the water has evaporated. All liquids evaporate, although some of them evaporate more quickly than others. Petrol and nail varnish remover evaporate more quickly than water. Cooking oil evaporates much more slowly than water.

If a vapour is cooled down it will usually change into a liquid. On a cold day water vapour in the air will change into water on a cold window. You may have written your name or drawn a face using this water! We say that the vapour has condensed into a liquid.

Boiling point
A liquid can evaporate at any temperature. However, if we heat it enough it will eventaully boil and turn into a gas. This happens at its boiling point.

If we cool a gas down below its boiling point, it will condense back into a liquid. We use the word condense to describe what happens when a vapour or a gas turn back into a liquid.

Photo showing steam and water droplets
Picture 1.4. Photo of kettle boiling with steam gap clearly visible.
What is steam?
Sometimes people mix up steam with water droplets. Let's see how.

When water reaches its boiling point (100 °C), the liquid water changes into steam. We can’t see steam because it is a colourless gas.

The steam comes out of the kettle and meets cooler air. The temperature of the steam drops below 100 °C and it condenses into a liquid. It forms tiny droplets that make up the white cloud above the spout. Unfortunately, it is this cloud that many people call steam when it is actually water!

Although you can't see steam, you can see where it is - between the spout and the cloud of water droplets.

Question 2
Look at the diagram below. It shows water in its three states. The arrows need to be labelled with the correct words. Choose the correct label by clicking on each arrow. Once you have chosen all the labels, click the Show answer button.
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