The kinetic theory of matter offers a description of the microscopic properties of atoms (or molecules) and their interactions, leading to observable macroscopic properties (such as pressure, volume, temperature). One important application of this theory is that it helps to explain why matter exists in the three different phases as discussed above and how matter can change from one state to the next.

This theory is based on the following assumptions:

• Matter is made up of very tiny particles that are constantly moving.
• All particles in a sample substance possess kinetic energy that varies depending on the temperature of the sample. This in turn determines whether the substance exists in the solid, liquid, or gaseous state. Particles in the solid phase have the least amount of kinetic energy, while gas particles have the greatest amount of kinetic energy.
• The temperature of a substance is a measure of the average kinetic energy of the particles.
• A change in phase undertakes when the average kinetic energy of the particles is changed.
• There are spaces between particles of any matter. The average amount of empty space between molecules gets progressively larger as a sample of matter changes from the solid to the liquid and then to gas phases.

There are attractive forces between atoms/molecules, which get stronger and stronger as the particles move closer together. These attractive forces are called van der Waals forces or van der Waals bonds, or attractive intermolecular forces.