Kinetic Molecular Theory of Matter
Booklet
The Kinetic Molecular Theory of Matter is a concept that basically states that atoms and
molecules possess an energy of motion (kinetic energy) that we perceive as temperature. In
other words, atoms and molecules are constantly in motion , and we measure the energy of
these movements as the temperature of that substance. This means if there is an increase in
temperature, the atoms and molecules will gain more energy (kinetic energy) and move even
faster.
This kinetic-molecular theory states (postulates) that:

All matter (solid, liquid, and gas) is made up of tiny particles called atoms, or atoms that
are joined to form molecules.

These particles are in constant motion.

Molecular motion is random.

Particles in motion possess kinetic energy.

Their motion increase as they gain energy.

There is an exchange (transfer) of energy between particles (atoms and molecules)
during a collision between them.

Particles (molecules) in gases do not exert large forces on each other, unless they are in
collision with each other.

Collisions between these particles are perfectly elastic.

Molecular motion is greatest in gases, less in liquids, and least in solids.

Solids retain a fixed volume and shape – particles are tightly packed, usually in a regular
pattern.

Liquids assume the shape of the container which it occupies but maintain their volume –
particles close together with no regular arrangement.

Gases assumes the shape and volume of its container and will expand to fill a container
of any size – particles are very well far apart with no regular arrangement.
The kinetic molecular theory is very useful in explaining or describing the forces
between molecules and the energy that they possess, as well as, the effects of thermal
energy, temperature and pressure on matter.
Video at: https://www.youtube.com/watch?v=_rsqBNhFG1Y
A: What Evidence Do We Have for the Kinetic Molecular Theory of Matter???



Show an understanding of the random motion of particles in a suspension as
evidence for the kinetic molecular model of matter
Describe this motion (sometimes known as Brownian motion) in terms of random
molecular bombardment
Show an appreciation that massive particles may be moved by light, fast moving
molecules
Brownian Motion
When viewed through a microscope, pollen grains in water, or smoke particles in air
seem to move about randomly. Why?
The pollen grains or smoke particles are being “pushed around”. Pushed around by what?
Pushed around by the air molecules or water molecules!
B: The Properties of Solids, Liquids, and Gases

State the distinguishing properties of solids, liquids and gases
Exercise 1: Complete the chart with the properties of solids, liquids, and gases.
Properties
Solids
Liquids
Gases
C: Describing the Structure of Solids, Liquids and Gases
(Arrangement, Separation, Motion, Force, and Distance)


Describe qualitatively the molecular structure of solids, liquids and gases in terms
of the arrangement, separation and motion of the molecules
Relate the properties of solids, liquids and gases to the forces and distances
between molecules and to the motion of the molecules
Let’s observe the behavior of the molecules of a solid with a simulation.
https://phet.colorado.edu/en/simulation/states-of-matter
Go to: https://phet.colorado.edu/en/simulation/states-of-matter
What do you notice about the:
a) arrangement of the molecules
b) separation of the molecules
c) motion of the molecules
What can you say about the force between the molecules?
Let’s observe the behavior of the molecules of a liquid with a simulation.
What do you notice about the:
a) arrangement of the molecules
b) separation of the molecules
c) motion of the molecules
What can you say about the force between the molecules?
Let’s observe the behavior of the molecules of a gas with a simulation.
What do you notice about the:
a) arrangement of the molecules
b) separation of the molecules
c) motion of the molecules
What can you say about the force between the molecules?
Exercise 2: Fill in the chart with the information about the arrangement, separation, and motion
of the molecules in solids, liquids, and gases.
Molecules
of:
arrangement
separation
motion
Solids
Liquids
Gases
D: Relationship between Temperature and the Motion of Gas Molecules

Interpret the temperature of a gas in terms of the motion of its molecules
1. What happens to the motion or speed of the molecules as we lower the
temperature?
2. What happens to the motion or speed of the molecules as we raise the temperature?
3. What is the relationship between temperature and the motion of gas molecules?
E: Gas Pressure and the Motion of Molecules


Describe qualitatively the pressure of a gas in terms of the motion of its
molecules.
Explain pressure in terms of the change of momentum of the particles striking the
walls creating a force.
Container B
Container A
Notice that the container on the left and the right are at two different pressures. Using
the simulation program, observe the motion of the gas molecules.
1. Can you see the molecules hitting the side of the container? In which container, A or
B, are the molecules hitting the sides more often and with greater force?
2. Describe the pressure of a gas in term of the motion of its molecules.
The particles striking the sides of the container have what we call momentum, p., and
⃗ =m𝒗
⃗
𝒑
If a molecule changes its speed or direction, then it changes its momentum.
When the molecules hit the side of the container, they bounce off in a different direction
changing their momentum.
𝛥𝑝
𝛥𝑡
= Force
So… the pressure of a gas is caused by the change in momentum of the particles
hitting the side of the container.
Exercise 3:
1. In the containers below, the black dots are gas molecules. Assuming the same
temperature for both containers, in which container, A or B, will the pressure be higher?
Why?
2. Assuming the same temperature for both containers, in which container, A or B, will the
pressure be higher? Why?
3.. In the containers below, container A is at a higher temperature. Which container has the
higher pressure? Why?
F: Evaporation



Describe evaporation in terms of the escape of more-energetic molecules from the
surface of a liquid
Relate evaporation to the consequent cooling of the liquid
Demonstrate an understanding of how temperature, surface area and draught over
a surface influence evaporation
The molecules in a liquid are in a state of continuous motion, some moving faster than others. In
the middle of the liquid they collide with each other but at the surface of the liquid some of them
are going fast enough to escape from the liquid altogether.
We call this EVAPORATION.
Of course the molecules need energy to escape and so the hotter the liquid becomes the more
evaporation there is. Notice that it is only the surface molecules that evaporate and there are no
bubbles of gas formed in the body of liquid
Evaporation is very important in the cooling of surfaces. If we put a drop of methylated spirit on
our hand the methylated spirit will soon evaporate and our hand feels cold. The molecules of
methylated spirit get the energy they need from our hand, leaving the molecules of the hand
with less energy and so colder. The same thing happens when you sweat, only in this case you
are losing water from your body by evaporation. The evaporation of perfume is why you can
smell it – no evaporation means no perfume molecules in the air and so no smell.
Exercise 3:
1. Explain why you feel cold on a windy day after getting out of the swimming pool.
2. Given what you know about evaporation:
a) What would raising the temperature of a liquid do to its evaporation rate?
Why?
b) What would increasing the surface area of the liquid to the evaporation rate?
Why?
c) What effect would air blowing across the top of the liquid have on the
evaporation rate?
Why?