What are materials made of?
           1. Solids, liquids and gases
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What are solids, liquids and gases made of?
To help explain the differences between solids, liquids and gases we need to look closely at what they are made of. Scientists have found that all materials are made of very, very tiny particles. These particles are so small that we can’t see them with our eyes or even a microscope.

We can explain why solids, liquids and gases are different from one another using ideas about particles:

  • how near they are to each other - when particles are close together they attract each other. This affects the behaviour of solids, liquids and gases.
  • the way that they move - when we heat a material, its particles move faster or more vigorously.
Animation of particles melting
Picture 1.5 Animation of a solid, a liquid and a gas. In the real thing, there would be billions of particles.

A solid
In a solid the particles fit very closely together. They are constantly vibrating and twisting. But they do not move past their neighbouring particles. Because the particles are close together:
  • they attract their neighbours - this is why solids tend to keep their own shape and to stay where they are put
  • solids are very difficult to compress - the gaps between particles are already very small.
In a liquid the particles are still close together but a little further apart than in a solid. The particles can move around and mix with other particles. Therefore, liquids can change shape to match their container. There is still very little space between particles, so liquids are also difficult to compress.
In gases the particles are much further apart than in solids or liquids. There is a lot of space in between the particles and they are constantly moving about. The particles collide with other particles and with the walls of the container.

Because the particles are moving about, a gas will fill any container that it is put into.
Because there is space between the particles, they can be squashed into a smaller volume when the gas is compressed.

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Question 3
Look at the descriptions below. Each one refers to a solid, liquid or gas. In each case, decide what it is referring to and check the correct box. Some of the descriptions may relate to more than one state of matter.
particle:   solid liquid gas
particles are close together
subastance is easy to compress
particles are fixed to near neighbour