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Kinetic-molecular theory of matter


Solids, liquids and gases each have their own characteristic properties. In order to explain how these properties come about, the kinetic-molecular theory has been developed.

According to this theory, all matter is made up of extremely small particles (atoms, molecules or ions), which are in constant motion:

  • In solids, these particles are tightly packed together, usually in a regular array, and vibrate back and forth.
  • In liquids, the particles are still tightly packed, but as well as vibrating, they can move over and in between one another.
  • In the gaseous state, the particles are spaced out relative to one another and are moving around with rapid, random motion.

    Relationships between states of matter

    The classical states of matter (solid, liquid and gas) can transition directly into any of the other classical states. Either adding heat to the system or removing heat from the system can achieve this.

    Two other states of matter

    When a gas is heated to very high temperatures, the gaseous atoms are stripped of their outer electrons. This creates ‘ionised’ gas that consists of a highly energetic mix of positive ions, electrons and atoms. The gas has been converted into plasma, often referred to as the ‘fourth state of matter’.

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