The Kinetic Theory of Matter


In solids the molecules are close together and the attractive and repulsive forces between neighboring molecules balance. Each molecule vibrates to and fro about each position. Solids therefore have a regular, repeating molecular pattern that is they are crystalline and their shape is definite.


In liquids the molecules are usually slightly farther apart than in solids and, as well as vibrating, they can at the same time move rapidly over short distances. However they are never near each other long enough to get trapped in a regular pattern and a liquid can flow.


In gases the molecules are farther apart than in solids or liquids. They dash around at very high speed in all the space available. It is only during the brief spells when they collide with other molecules or with the walls of the container that the molecular forces act.