The kinetic molecular theory of matter explains how matter can change
among the phases of solid, liquid, and gas.
LEARNING OBJECTIVE [ edit ]
Describe the kinetic molecular theory of matter.
KEY POINTS [ edit ]
All particles have energy, and the energy varies depending on the temperature the sample
of matter is in, which determines if 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 averagekinetic energy of the particles. A
change in phase may occur when the energy of the particles is changed.
There are spaces between particles of matter. The average amount of empty space between
molecules gets progressively larger as a sample of matter moves from the solid to the liquid and
gas phases.
TERMS [ edit ]
phase
A component in a material system that is distinguished by chemical composition and/or physical
state. Matter can exist in the solid, liquid, and gaseous phases.
kinetic molecular theory
Theory of treating samples of matter as a large number of small particles (atoms or molecules), all
of which are in constant, random motion
lattice
A regular spacing or arrangement of atoms/molecules within a crystal.
kinetic
Of or relating to motion.
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TheKineticTheory: A
Microscopic Description
of Matter
The kinetic molecular 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). An
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application of the theory is that it helps to explain why matter exists in
different phases(solid, liquid, and gas) and how matter can change from one phase to the
next.
The three phases of matter
Notice that the spacing between atoms or molecules increases as we move from a description of the solid
phase to the gaseous one.
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 the temperature the sample
of matter is in. This in turn determines whether the substance exists in the solid, liquid,
or gaseous state. Molecules in the solid phase have the least amount of energy, while gas
particles have the greatest amount of energy.
The temperature of a substance is a measure of the averagekinetic energy of the particles.
A change in phase may occur when the energy of the particles is changed.
There are spaces between particles of matter. The average amount of empty space
between molecules gets progressively larger as a sample of matter moves from the solid to
the liquid and gas phases.
There are attractive forces between atoms/molecules, and these become stronger as the
particles move closer together. These attractive forces are called intermolecular forces.
Example: Water
Let's take water as an example. We find that in its solid phase (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 close together, and the molecules move
further apart, forming liquid water. This is why liquid water is able to flow: the molecules
have greater freedom to move than they had in the solid lattice. If the molecules are heated
further, the liquid water will become water vapor, which is a gas. Gas particles have more
energy and are on average at distances from each other which are much larger than the size of
the atoms/molecules themselves. The attractive forces between the particles are very weak
given the large distances between them.
Changes in phase
A change in phase may occur when the energy of the particles is changed.
Diffusion
The kinetic theory of matter is also illustrated by the process of diffusion. Diffusion is the
movement of particles from a highconcentration 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.
Interactive: Diffusion of a Drop
Click in the model to add a drop of dye. Watch how the molecules move through the water. Trace an
individual molecule to see how it moves through the liquid.
How diffusion works [HD animation]
A description of what happens during the process of dissolution of a solid in a liquid and diffusion.