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Kinetic Model of Matter

Relate molecular structure (arrangement, force, motion and distance between particles) and
properties of solids, liquids and gases
Solid

Force and arrangement of particles
Particles are closely packed
Held by strong attractive forces
Vibrate about fixed positions

Liquid

Particles are slightly farther apart compared to in solids
and randomly arranged
Free to move over one another
Strong attractive forces

Gas

Particles are far apart and randomly arranged
Free to move at high speeds
Weak forces of attraction between particles

Evidence for movement of molecules (Brownian motion)

1


Properties
High density
Fixed shape and
volume
Incompressible
High density
Fixed volume but no
fixed shape
Incompressible
Low density
No fixed volume and
shape
Compressible

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Experiment: observe smoke trapped in the glass cell under a microscope

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Observations:

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smoke particles appear as bright dots because they scatter the light that shines on
them

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smoke particles move in random directions

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the larger the particles the less agitated the motion

Deduction:
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The smoke particles moved randomly because they were being bombarded by
many air molecules in all directions
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g
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This random motion is due to the fluid particles moving about randomly and
bombarding the suspended particles
...

The thermal energy gained increases the random kinetic energy of the molecules
Hence when temperature increases, kinetic energy and speed of molecules increase

Assumptions of the kinetic model of an ideal gas (not tested)
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•
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The gas consists of a very large number of identical molecules
The volume of the molecules is negligible compared with the total volume occupied by the
gas
The molecules are moving in completely random directions, at a wide variety of speeds
There are no forces between the molecules
The total kinetic energy of the molecules remain constant at the same temperature

2


Pressure of a gas (Recall P = F/A)

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Moving gas molecules collide with the inner wall of the container and exert a force on it
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[Note: Each and every collision can result in a different sized force, and it is not realistic to know all
the forces that individual molecules exert on the walls
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]

Relationship between Pressure, Volume and Temperature of a gas
1
...

• The air molecules will then bombard the
walls of their container more forcefully and
more frequently
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The pressure p of a fixed mass of gas is
directly proportional to its temperature T
at constant volume
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Volume and temperature (keeping pressure constant)

• The glass tube allows the gas in the roundbottomed flask to expand
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• When the air is heated, the increased temperature causes the air molecules to move at
higher speeds
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In other words, the gas pressure increases
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As the air molecules occupy a larger volume, they collide with the walls less
frequently, resulting in a decrease in pressure
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Before heating
Constant
Higher

After heating
Increases
Lower

Force of
collision

Lower

Higher

Gas Pressure

Atmospheric
pressure

Atmospheric
pressure

Volume
Frequency of
collision

4


Explanation and conclusion
Molecules move faster and
farther apart hence colliding with
the walls less frequently
Temperature increases so KE
increases and force of impact
with walls increases
Remains unchanged

The volume V of a fixed mass of gas is
directly proportional to its temperature T
at constant pressure
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Pressure and volume (temperature remains constant)

The pressure of the gas inside the
syringe is increased by slowly
pushing the piston inwards (to
compress the gas)
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• The air molecules will therefore bombard the walls of the container more
frequently
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The pressure p of a fixed mass of gas is
inversely proportional to its volume V at
constant temperature
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Use the kinetic model of matter to explain and predict the
changes in the gas pressure for the following cases
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1
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e
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2
...
e
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3
...
e
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Question 2
The figure shows a syringe that contains gas at the same pressure as the air outside
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No gas escapes
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It stops moving when the
temperature is steady
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For each quantity, you should write greater, the same
or less

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