3.1 Solids, Liquids, and Gases
Kinetic Theory
Kinetic energy:
Energy an object has due to its motion.
Kinetic theory of matter:
All particles of matter are in constant motion.
3.1 Solids, Liquids, and Gases
Kinetic Theory
A ball thrown at 137 kilometers per
hour has more kinetic energy than a
ball thrown at 100 kilometers per
hour.
The kinetic theory was developed to
explain the behavior of gases.
It can also help to explain the
behavior of liquids and solids.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Gases
Motion of billiard balls can be compared to motion
of particles in a gas.
• Cue stick strikes a ball 
Ball moves in straight line until it strikes something
(side of table or another ball)
• When moving ball strikes a ball at rest 
First ball slows down; second ball begins to move.
Kinetic energy is transferred during collisions.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Gases
Photograph taken just after
cue stick struck the white
ball, which began to move.
White ball moved in
straight line until it collided
with dark blue ball.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Gases
Motion in Gases
Unlike billiard balls, particles in a gas are never at rest.
• Avg speed of gas particles at room temp = ~1,600 km/hr
(1,600 km/hr = ~1,000 mph)
(Speed of sound is ~1,250 km/hr = ~770 mph)
• Not all particles are moving at same speed.
• Particles are constantly changing speed and direction
through collisions
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Gases
Illustration shows paths of two atoms in
a container of gas.
• Atoms move in straight lines
until colliding
(with each other or container)
• If one atom loses kinetic energy,
other atom gains kinetic energy 
no net change no overall energy.
• Total kinetic energy of the
atoms remains the same.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Gases
There are forces of attraction among the
particles in all matter.
Between collisions, why doesn’t one particle in a
gas attract the other particles in the gas?
• Gas particles far apart and moving fast 
attractions are too weak to have an effect.
– Under ordinary conditions, scientists ignore the forces
of attraction in a gas.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Gases
Motion in Gases – Summary
1) Particles in a gas are never at rest.
2) Atoms move in straight lines until colliding.
3) Total kinetic energy of atoms remains same.
4) Gas particles far apart and moving fast 
attractions are too weak to have an effect.
•
http://en.wikipedia.org/wiki/Image:Translational_motion.gif
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Gases
Kinetic Theory of Gases
Kinetic theory explains general properties of gas:
– Constant motion of particles
+
– Minimal attraction between particles

– Gas fills a container of any shape or size.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Liquids
•Particles in liquids have kinetic energy, too
•All materials at a given temperature have
the same average kinetic energy.
So, at room temp, why is mercury a liquid & helium a gas?
3.1 Solids, Liquids, and Gases
Why Mercury a liquid instead of a gas at room temp
At room temp, why is mercury a liquid & helium a gas?
– Reminder: liquids have definite volume and gasses
expand to fill the container
•Mercury atoms much heavier than helium atoms
• He atomic weight ~4 atomic units
• Hg atomic weight ~200 atomic units (50X heavier)
To have same kinetic energy, mercury must move
much slower than helium.
• Think of impact from bowling balls vs tennis balls
• To have same impact, bowling balls move much slower
• Average speed of mercury atom can only be 1/50th
speed of helium atom to have same kinetic energy.
3.1 Solids, Liquids, and Gases
Why Mercury a liquid instead of a gas at room temp
•To have same kinetic energy, mercury atoms
must move much slower than helium atoms.
• Particles in liquid move slower than particles in gas.
• Particles in liquid are much closer than particles in gas.
•Unlike gasses, attractions between
particles in a liquid DO affect movement of
the particles.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Liquids
VARIABLE SHAPE (not definite) because particles
• Can flow to new locations.
• Move in straight lines until they collide
(with walls or each other) - just like gasses
FIXED VOLUME because
• Forces of attraction keep particles close
together - unlike gasses
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Liquids
Kinetic theory explains general properties of liquids:
– Constant motion of particles
+
– Forces of attraction limiting motion of particles

– Liquids:
• Conform to container’s shape
• Can’t expand volume to completely fill large container
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Solids
Solids have definite volume and definite
shape because particles vibrate around fixed
locations.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Solids
Vibration is a repetitive back-and-forth motion.
• Strong attractions restrict atoms’ motion
• Each atom stays in a fixed location relative
to its neighbors.
• Each atom vibrates around its location, but it
does not exchange places with a neighboring
atom.
3.1 Solids, Liquids, and Gases
Explaining the Behavior of Solids
During a movie people move around in their seats, but each
person remains in essentially the same location.
“Particles” have “fixed” locations - total volume doesn’t change.
3.1 Solids, Liquids, and Gases
Exit Slip Questions WRITE OUT QUESTIONS AND ANSWERS
1. Materials are classified as solids, liquids, or
gases based on their:
a) temperature and density characteristics.
b) chemical composition.
c) shape and volume characteristics.
d) reactivity.
3.1 Solids, Liquids, and Gases
Exit Questions
2. Describe gasses based on the kinetic theory:
a) Particles of gas are in motion part of the time
and stationary part of the time.
b) Particles in a gas are arranged in an orderly
fashion.
c) Gas particles are not affected by collisions
with other gas particles.
d) Forces of attraction between particles can be
ignored under ordinary conditions.
3.1 Solids, Liquids, and Gases
Exit Questions
3. When gas particles collide with each other:
a)
b)
c)
d)
Particles change their paths.
One of the particles stops moving.
Net kinetic energy is changed.
All of the above.
3.1 Solids, Liquids, and Gases
Exit Questions
4. Particles in a liquid:
a) All have exactly the same kinetic energy.
b) Are not very strongly attracted to each other.
c) Move in straight lines until they collide with walls or
each other.
d) Stay in fixed locations relative to their neighbors.
3.1 Solids, Liquids, and Gases
Exit Questions
5. Particles in a solid:
a) All have exactly the same kinetic energy.
b) Are not very strongly attracted to each other.
c) Move in straight lines until they collide with walls or
each other
d) Stay in fixed locations relative to their neighbors.