Kinetic Molecular Theory of Matter
Matter consists of very small particles that we cannot see, called atoms and
We use theories models of what we think matter looks like.
The Kinetic Molecular Theory of Matter states that:
All matter occurs in one of three phases, namely solid, liquid and gas
The properties of the three phases of matter are explained with the kinetic
particle model of matter.
Matter is made up of particles that are constantly moving.
All particles have energy, but the amount of energy depends on whether
the substance is a solid, liquid or gas.
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 these become stronger
as the particles move closer together. However, when the particles are
extremely close, repulsive forces start to increase.
To prove that matter has a particle nature:
Brownian motion and diffusion proves that matter consists of particles.
Brownian motion
Brownian motion is the random movement of microscopic particles suspended in a
liquid or gas. The movement is caused by collisions between the molecules of the liquid
or gas.
Diffusion is the spontaneous movement of a substance from an area of high
concentration of that substance to an area of lower concentration.
Characteristics of the three phases using the kinetic molecular theory:
Physical state of a substance
The physical state (or phase) that a substance is in at a certain
temperature depends on its boiling point, melting point and freezing point.
The freezing point is the temperature at which a liquid changes it’s
physical state (or phase) and becomes a solid.
The temperature at which a solid changes completely into a liquid is the
melting point.
The temperature at which a liquid changes into a gas (or vapour), is called
the boiling point of a substance.
Temperature is measured with a thermometer. Temperature can be
measured using different units of measurement. The most common unit of
temperature in South Africa is the degree Celsius (°C).
Examples of boiling points and melting points
Water has a melting point of 0 °C and a boiling point of 100 °C.
Nitrogen gas boils at -196 °C and it is a gas at room temperature
Ethanol is a liquid at room temperature and be cooled to below -114 °C to
change to the solid phase.
Phase changes
A phase change occurs when a substance moves from one physical state
(solid, liquid or gas phase) to another.
A phase change is accompanied by:
A change in the arrangement of the particles
The absorption or release of energy.
The change of state from solid phase to liquid is called melting.
When a solid is heated, the particles vibrate more vigorously.
For a solid to melt, the energy of the particles must increase enough to overcome the
intermolecular forces holding the particles together.
A solid held together by strong intermolecular forces will have a higher melting point
than one where the forces are weak.
Evaporation and boiling
The process whereby a liquid changes to a gas at temperatures below its boiling point is
called evaporation.
Some of the particles near the surface of the liquid have sufficient energy to break away
from the surface of the liquid and enter the gas phase.
A liquid is boiling if there are changes from the liquid to the gas phase occurring
throughout the liquid.
At this temperature the particles have sufficient energy to break the intermolecular
The change of state from gas to liquid is called condensation.
The intermolecular forces of attraction become stronger.
The change of state from liquid to solid state is called freezing.
When temperature is at the freezing point of the particles, the intermolecular forces of
attraction between them become much stronger.
The direct change from solid to gas phase on heating and from a gas to a solid on
cooling is called sublimation. The substance does not go the liquid phase.
Dry ice (solid carbon dioxide) sublimes directly from solid to gas

Grade 10 Notes (Kinetic Molecular Theory of Matter)