Kinetic Molecular
Theory of Matter
Have you ever bought an ice
cream on a hot summer day? If yes,
then you must have noticed how ice
cream, when taken out of the
freezer, takes a definite shape, but
when exposed in the summer heat,
slowly loses its form and melts.
In this lesson, you will be learning
about
theories governing
the
different states of matter. You will
learn why solids have definite shapes,
but liquids and gases take up the
shape of their container. You will also
learn the relationship of the different
macroscopic properties of matter to
that of its molecular interactions.
Kinetic Molecular Theory of Matter
The kinetic molecular theory of matter provides an overview of the microscopic
properties of molecules or atoms and their interactions. These concepts, when
combined, lead to the macroscopic behavior and properties of matter. The kinetic
molecular theory describes the microscopic properties of matter and how they translate
to the state and other properties of matter. The kinetic molecular theory states that:
(1) Matter is composed of small particles.
(2) The molecules interact with one another through attractive forces. The
strength of these forces is related to the distance between the particles.
(3) These molecules are always in constant random motion.
(4) The temperature of a substance is a measure of the average kinetic energy
of the molecules.
Matter Is Composed of
Molecules Interact through
Attractive Forces
Small Particles
Atoms are the building blocks of
matter. Matter can exist as a group of
individual atoms or as a group of atoms
bonded together called molecules. A
pure gold bar, for example, is
composed of many gold atoms.
However, some elements can exist as
molecules. For example, oxygen gas
exists as a diatomic molecule, O2 .
Matter can also come in the form of
compounds.
Compounds
are
produced when two or more atoms of
different
elements
combine
chemically.
Water
(H2O)
is
a
compound composed of molecules
made up of an oxygen atom
covalently bonded to two hydrogen
atoms. Sodium chloride is a compound
made up of sodium and chloride ions
interacting.
The attractive forces between
molecules
are
known
as
the
intermolecular forces. These forces
affect the spacing between molecules.
The stronger the interaction between
two molecules, the smaller their
distance will be. The strength of the
attractive force at a given temperature
can be used to differentiate the states
of matter. Solids have particles with
strong intermolecular forces such that
their particles are very close to one
another. Liquids have intermediate
intermolecular forces. This makes liquid
particles farther from one another
compared to those in solids. Gases, on
the other hand, have particles that are
very far apart from one another due to
weak intermolecular forces.
Molecules Are in Constant Random Motion
All molecules are in constant random motion. The extent of their motion varies
depending on the temperature and strength of the interaction between the particles.
Solids, due to their strong intermolecular forces, have restricted motion. The particles of
solids are only able to vibrate back and forth around a specific point or location. Liquids,
having intermediate intermolecular forces, are able to move past each other. Since
their particles are still close to one another, the motion is restricted to small distances as
they will collide with another molecule. Gases, having weak intermolecular forces, are
able to move in relatively long distances before colliding with another molecule. This
happens because the particles are very far apart from one another.
Temperature Is a Measure of
States of Matter
the Average Kinetic Energy
Kinetic energy refers to the energy
of particles in motion. Since all
molecules are in constant random
motion, they contain kinetic energy.
The higher the kinetic energy, the more
active the particles are. Temperature is
a measure of the average kinetic
energy of molecules. Increasing the
temperature will result in a faster motion
of the particles. This can be used to
describe phase changes.
of
Matter can be described in terms
its physical state. A solid is
characterized by having a rigid shape
and fixed volume. Unlike solids, liquids
and gases do not have a definite
shape. They take the shape of their
container. Liquids are similar to solids in
such a way that their volumes do not
change significantly with varying
temperatures and pressure. Gases, on
the other hand, have volumes that
depend on temperature and pressure.
How can the kinetic molecular theory explain the
properties of each state of matter?
Based on the kinetic molecular theory of matter, the state of a matter is
determined by two factors—temperature and strength of intermolecular forces. At lower
temperatures, intermolecular forces determine the state of a substance. Substances
with intermediate to strong intermolecular forces will form a condensed phase, either
solid or liquid. Those with weak intermolecular forces will be in the gaseous state.
How does the kinetic molecular theory explain phase
changes?
If we take into consideration the same set of molecules, then its state will be
determined by the temperature. Recall that a higher temperature means a higher
average kinetic energy. In other words, the particles will have enough energy to move
around faster. Let’s take ice as an example. The water molecules in ice are only capable
of vibrating back and forth to a specific location since it is in the solid phase. Increasing
the temperature will provide enough kinetic energy to overcome their strong
intermolecular forces. This will allow the molecules to move past one another. In this
process, the solid becomes a liquid in a process called melting. In the same manner,
when liquid water is heated, its particles are able to move past one another in relatively
longer distances. The process in which liquid is converted to a gas is called vaporization.