## Postulates of the kinetic theory of matter

The postulates of kinetic theory of matter are used to explain the existence of matter in diﬀerent phases (i.e. solid, liquid and gas), and how it is able to change from one phase to the next.

The kinetic theory of matter also helps us to understand the macroscopic properties of matter.

Kinetic Theory of Matter

• Matters are made up of particles and they are in constant random motion.
• The particles collides with each other and with the wall of the container.
• All particles have energy, but the energy differs depending on the state they are in. Solid particles have the least energy and gas particles have the most amount of energy.
• The temperature of a substance is a measure of the average kinetic energy of the particles.
• A change in phase may occur when the energy of the particles is changed.
• There are spaces between the particles of matter.
• There are attractive forces between particles and the closer the distance between the particles, the stronger the force of attraction between them.

The attractive forces can either be intramolecular forces (if the particles are atoms) or intermolecular forces (if the particles are molecules).

The three phases of matter

Let us look at these states of matter and their characteristics

In a solid (e.g. ice), the molecules are held close together in a regular pattern called a lattice the water molecules have very little energy and can’t move away from each other.

If the ice is heated, the kinetic energy of the molecules increases and the intermolecular forces holding them together will become broken. This is why liquid water is able to ﬂow, because the molecules are freer to move than they were in the solid lattice.

If the molecules are heated further, the molecule will gain more kinetic energy and the liquid water will become water vapour, which is a gas.

Gas particles have the highest kinetic energy and are far away from each other. That is why it is diﬃcult to keep a gas in a speciﬁc area! The attractive forces between the particles are very weak and they are only loosely held together.

Changes in phase

In the next tutorial let we are going to use what we have learnt about chemical bonds, intermolecular forces and the kinetic theory of matter to understand the properties of matter.

SUMMARY

• The kinetic theory of matter attempts to explain the behaviour of matter in diﬀerent phases.
• The theory says that all matter is composed of particles which have a certain amount of energy which allows them to move at diﬀerent speeds depending on the temperature (energy). There are spaces between the particles, and also attractive forces between particles when they come close together.
• Understanding chemical bonds, intermolecular forces and the kinetic theory of matter, can help to explain many of the macroscopic properties of matter