The kinetic molecular theory of matter helps us to explain why matter exists in different phases (solid, liquid, and gas, as shown in Figure 1) and how matter can change from one phase to the next.
Broadly, the kinetic molecular theory of matter states that:
- Matter is made up of particles that are constantly moving.
- All particles have energy, but the energy varies depending on whether the substance is a solid, liquid, or gas. Solid particles have the least amount of energy, and gas particles have the greatest amount of energy.
- The temperature of a substance is a measure of the average kinetic energy of the particles.
- A change in phase (Figure 2) may occur when the energy of the particles is changed.
- There are spaces between particles of matter.
- There are attractive forces between particles, and these become stronger as the particles move closer together. These attractive forces will be either intramolecular forces (if the particles are atoms) or intermolecular forces (if the particles are molecules). When the particles are extremely close, repulsive forces start to act.
Let's take water as an example. We find that in its solid phase, which we call ice, the water molecules have very little energy and cannot move away from each other. The molecules are held closely together in a regular pattern called a lattice. If the ice is heated, the energy of the molecules increases. This means that some of the water molecules are able to overcome the intermolecular forces that are holding them together, and the molecules move further apart, forming liquid water. This is why liquid water is able to flow: the molecules are more free to move than they were in the solid lattice. If the molecules are heated further, the liquid water will become water vapor, which is a gas. Gas particles have a lot of energy and are far away from each other. The attractive forces between the particles are very weak and they are held together only loosely.
The kinetic theory of matter is also illustrated by the process of diffusion. Diffusion is the movement of particles from a high concentration to a low concentration. It can be seen as a spreading-out of particles resulting in their even distribution. Placing a drop of food coloring in water provides a visual representation of this process -- the color slowly spreads out through the water. If matter were not made of particles, then we would simply see a clump of color, since there would be no smaller units that could move about and mix in with the water.