An animation showing the movement of molecules. Five are coloured red, so their movement can be followed more easily by eye.All matter is made up of molecules or atoms that are in constant motion. This view of the way matter behaves is called kinetic theory ("kinetic" means moving). Ever since the 17th century, scientists have made observations and devised laws that prove kinetic theory. The amount the molecules or atoms that make up matter can move around depends on the strength of the forces that hold them together. Temperature also has an effect. The molecules in solids, which are closely packed, can be made to vibrate and take up more space by heating. This is why metal objects expand at higher temperatures.
Air molecules (shown in inset) move rapidly farther apart as they are heated. The air inside the balloon is therefore lighter...Read More >>Air molecules (shown in inset) move rapidly farther apart as they are heated. The air inside the balloon is therefore lighter than the air outside. This causes the balloon to rise.
Behaviour of gases
The forces between the molecules in liquids are weaker than in solids, so the molecules can move about more freely on heating, even while still remaining close together. In gases, the molecules are more widely spaced, and so move about much more quickly. Because of this, dramatic effects may be produced if a gas's temperature, volume or pressure is changed in some way. Heating a gas inside a fixed space—a can or glass jar for example—causes its pressure to increase. This is because the gas's freely-moving molecules collide with one another and the walls of the container more frequently as the heat rises. The container may eventually explode if the pressure reaches a certain point.
The average speed of molecules of nitrogen and oxygen in the air (they make up the vast majority of air) is about 450 m (about 1500 ft) per second.
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