States of matter

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Almost all matter can be classified as a solid. a liquid or a gas. These are called the three states of matter.

(The fourth state of matter is called ‘plasma: It only exists at high temperatures seldom seen on Earth, and so we won’t consider it further here. even though most of the matter in the universe and most stars are made of plasma.)

As you will know, in general solids can be turned into liquids by heating, and with more heating liquids can be turned into gas. We are all familiar with water in all three states, but less so with other materials. Solid air is uncommon simply because it exists only at extremely low temperatures, and iron in the form of a gas only exists at very high temperatures.

All materials are made of tiny particles called atoms. The atoms attract each other, and the particles in a solid are locked together by the forces between them. But even in a solid the articles are not completely still. They vibrate constantly about their fixed positions. If the material is heated, it is given more internal energy, and the particles vibrate faster and further.

If the temperature is increased more, the vibrations of the particles increase to the point where the forces are no longer strong enough to hold the structure together. The forces are no longer enough to prevent the particles moving around, but they do prevent the particles from flying apart from each other. This is the liquid state. A liquid can flow and takes the shape of whatever container it is in. Its volume does not change much.

If the temperature is increased even more, then the particles do indeed fly apart. They now form a gas. The particles fly around at high speed (several hundred kilometres per hour) and if they are in a container, they travel all over the container, bouncing off the walls. The volume of a gas is not fixed, it just depends on the size of the container that the gas is put in.


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