Kinetic nature of matter

We already know that matter is composed of atoms, ions or molecules. We have not yet considered the reason why the same substance, say water, can exist in more than one form, for example as solid ice, liquid water, and gaseous steam. But does matter behave like that?



The kinetic theory of matter has been used to explain the way in which the arrangement of the particles of a substance can determine the properties of that substance, and particularly the state in which it is likely to be found under a given set of conditions. The idea is that all matter is made up of tiny moving particles.


The main points of the theory are as follows:

1. All matter is made up of tiny particles (atoms and molecules) that are invisible to the naked eye and to most microscopes.

2. The particles are moving all the time. The higher the temperature is, the higher the average energy of the particles.

3. Heavier particles move more slowly than lighter particles at the same temperature.

4. Each substance has unique particles that are different from the particles of other substances.

5. The particles of matter are held together by strong electrostatic forces.

6. There are empty spaces between the particles of matter that are very large compared to the particles themselves.


The solid state

In the solid state, the particles are so closely packed (see figure bellow. The particles are held together by strong forces of attraction that act like a chemical glue. Free movement of particles cannot take place. 

They cannot move around freely in this arrangement. Instead, they vibrate about a fixed position. They are arranged in a fixed pattern which form a cluster of vibrating masses. This makes a solid to have a fixed shape, which cannot be changed except by applying strong external forces.


The liquid state

The particles of a liquid are also closely packed but the forces of attraction between them are weaker than of a solid. These forces of attraction tend to bind them together. The particles have more kinetic energy and they can move around each other. The binding forces are strong when particles come close to one another. 

It is thought that the particles of a liquid are fairly randomly arranged but consist of "clusters" closely packed together. This property makes a liquid to have a definite volume. However, since the particles are fairly free to move a liquid does not have any characteristic shape (see figure 5.5(b). Thus, a liquid will always take the shape of its container.


The gaseous state

The gaseous state is one in which the particles are moving independently of each other in all directions and at great speeds. The particles of a gas are relatively far apart. They exert no force of attraction on each other. They have more energy than the particles of solids and liquids. They move rapidly and randomly, colliding with each other and with the walls of the container. 

A typical speed for a molecule of hydrogen in air at ordinary temperature and pressure has been found to be approximately 500 ms-1. It has been estimated that a nitrogen molecule makes collisions each second. Thus, a gas will rapidly spread out to fill any container in which it is placed. A gas cannot have any shape of its own.