BLOWING AIR can return to the liquid. This increases the rate of evaporation.

1. PHYSICS – Simple kinetic molecular
model of matter (1)

2. LEARNING
OBJECTIVES
2.1.1 States of matter
Core • State the distinguishing properties
of solids, liquids and gases
2.1.2 Molecular model
Core • Describe qualitatively the molecular
structure of solids, liquids and gases in
terms of the arrangement, separation and
motion of the molecules
• Interpret the temperature of a gas in
terms of the motion of its molecules
• Describe qualitatively the pressure of a
gas in terms of the motion of its molecules
• 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
Supplement • Relate the properties of
solids, liquids and gases to the forces and
distances between molecules and to the
motion of the molecules
Explain pressure in terms of the change of
momentum of the particles striking the
walls creating a force
• Show an appreciation that massive
particles may be moved by light, fast-
moving molecules

3. Solid
Liquid
Gas
Changes of State

4. Solid
Liquid
Gas
{
melting
Changes of State

5. Solid
Liquid
Gas
{
melting
{
Boiling
(evaporating)
Changes of State

6. Solid
Liquid
Gas
{
melting
{
Boiling
(evaporating) } condensing
Changes of State

7. Solid
Liquid
Gas
{
melting
{
Boiling
(evaporating)
} condensing
} freezing
Changes of State

8. Solid
Liquid
Gas
Particles are fixed in place and cannot move
Changes of State

9. Solid
Liquid
Gas
Particles are fixed in place and cannot move
Particles are free to move within a
container
Changes of State

10. Solid
Liquid
Gas
Particles are fixed in place and cannot move
Particles are free to move within a
container
Particles are free to move about
Changes of State

11. SOLIDS
• Strong forces of
attraction
• held in fixed position
• lattice arrangement
• don’t move, so have
definite shape and volume
• vibrate

12. SOLIDS
• as they become hotter,
the particles vibrate more.
• so they expand
• can’t be compressed
• generally very dense

13. SOLIDS
• when heated, molecules
gain energy.
• they vibrate more and
more
• strong forces are
overcome, molecules start
to move = MELTED

14. LIQUIDS
• Some attraction between
molecules.
• free to move
• no definite shape, but
take shape of container
• molecules in constantly
random motion

15. LIQUIDS
• when heated, they move
faster and expand
• can’t be compressed
• quite dense

16. LIQUIDS
• heat makes the molecules
move faster as they gain
energy.
• fast moving molecules at
the surface will overcome
forces of attraction and
escape = EVAPORATION

17. GASES
• no force of attraction
• free to move, travel in
straight lines
• sometimes collide
• no definite shape or
volume, expand to fill space

18. GASES
• exert pressure on wall of
container
• constantly moving randomly
• move faster when heated
• can be compressed
• very low densities

19. GASES
• when heated enough,
molecules have enough speed
and energy to overcome
forces and escape each
other.
• molecules break away in big
bubbles of gas = BOILING

20. States of Matter
Solids, Liquids & Gases

22. The Forces & Distances between Molecules
•In a solid:
• The molecules are held in place by strong intermolecular bonds
• These bonds prevent the molecules from moving, giving the solid its rigid shape
and fixed volume
•In a liquid:
• The molecules have enough energy that they are able to break the bonds between
them
• The bonds are still there, but they no longer hold the molecules in place
• As a result, the molecules can move around (by sliding past each other) allowing
the liquid to flow
In a gas:
• The molecules are now moving around randomly at high speeds
• The molecules have broken the bonds between them: They are widely separated
with no long-range forces binding them together
• As a result the molecules are able to move freely and so the gas can flow freely
• Because of the large spaces between the molecules (along with the absence of
long-range forces) the gas can easily be compressed and is also able to expand

30. LEARNING
OBJECTIVES
2.1.1 States of matter
Core • State the distinguishing properties
of solids, liquids and gases
2.1.2 Molecular model
Core • Describe qualitatively the molecular
structure of solids, liquids and gases in
terms of the arrangement, separation and
motion of the molecules • Interpret the
temperature of a gas in terms of the
motion of its molecules
• Describe qualitatively the pressure of a
gas in terms of the motion of its molecules
• 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
Supplement • Relate the properties of
solids, liquids and gases to the forces and
distances between molecules and to the
motion of the molecules
Explain pressure in terms of the change of
momentum of the particles striking the
walls creating a force • Show an
appreciation that massive particles may be
moved by light, fast- moving molecules

31. • The molecules in a gas move around randomly at high speeds
• The temperature of a gas is related to the average speed of the
molecules: The hotter the gas, the faster the molecules move
Motion of Molecules in a Gas
• As the molecules move around, they
collide with the surface of nearby
walls
• Each collision applies a force across
the surface area of the walls
• Pressure is the force per unit area:
and hence a pressure will be
exerted on those walls

32. Molecule Momentum
• When molecules collide against a wall, they bounce off, changing their momentum
• When molecules (in a gas) collide against a wall, they undergo a change in momentum
• There are many such collisions every second, resulting in a large change in momentum
each second
• This change in momentum each second results in a force being exerted against the
wall:
• The pressure exerted by the gas is equal to this force divided by the area of the
wall

33. Pressure changes – and temperature
(at a constant volume)
Increasing the temperature of a gas increases the
pressure.
If a gas is heated, the particles
move faster and have more kinetic
energy. As the KE increases, the
particles hit the container walls
harder and more often, resulting
in more pressure.
Pressure is directly proportional
to absolute temperature (in K).
Doubling the temperature
produces a doubling in pressure.

34. Pressure changes – and temperature
(at a constant volume)
0
2000
4000
6000
8000
10000
12000
14000
16000
18000
20000
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000
PRESSURE
(KPA)
TEMPERATURE (K)
Pressure (kPa)

35. Pressure changes – and temperature
(at a constant volume)
Increasing the temperature of a gas increases the
pressure.
If a gas is heated, the particles
move faster and have more kinetic
energy. As the KE increases, the
particles hit the container walls
harder and more often, resulting
in more pressure.
Pressure is directly proportional
to absolute temperature (in K).
Doubling the temperature
produces a doubling in pressure.
In a sealed container (constant
volume:
pressure = constant
temperature
P = constant
T
P1 = P2
T1 T2

36. Pressure changes – and temperature
(at a constant volume)
Increasing the temperature of a gas increases the
pressure.
If a gas is heated, the particles
move faster and have more kinetic
energy. As the KE increases, the
particles hit the container walls
harder and more often, resulting
in more pressure.
Pressure is directly proportional
to absolute temperature (in K).
Doubling the temperature
produces a doubling in pressure.
Eg. A sealed container has a volume
of 25 litres. The gas inside is at a
pressure of 100 kPa and a
temperature of 300K. What will the
pressure be if the temperature is
increased to 325K?
Answer: P1 = P2
T1 T2
1 / 300 = P2 / 325
P2 = (325 x 100)/300 = 108.3 kPa

37. Thermal Expansion and gases
The pressure law
For a fixed mass of gas at constant
volume, the pressure is directly
proportional to the Kelvin temperature.

38. If a gas has a pressure of 1.2 x 105 Pascals at 27 °C. The piston is heated
slowly until the pressure on the gauge reads 2.0 x 105 Pascals. What is the
final temperature of gas?
If a gas has a pressure of 1.5 x 105 Pascals the temperature of 27 °C, what
will the pressure change to if the container is heated to 427 °C. What is this
pressure in Pa?
Questions
𝑷𝟏
𝑻𝟏
=
𝑷𝟐
𝑻𝟐
1.2 x 105
300
=
2.0 x 105
𝑇2
𝑻𝟐 =
𝟑𝟎𝟎 𝒙 2.0 x 105
1.2 x 105
𝑻𝟐 = 𝟓𝟎𝟎 𝑲
𝑷𝟏
𝑻𝟏
=
𝑷𝟐
𝑻𝟐
1.5 x 105
300
=
𝑃2
700
𝑷𝟐 =
700 x 1.5 x 105
300
𝑃2 = 3.5 x 105

39. BROWNIAN motion
microscope
Glass cover
Glass cell
smoke
lamp
Microscope view
Zig-zag paths of
smoke particles
http://physics.bu.edu/~duffy/HTML5/brownian_motion.html

40. Brownian Motion
• When small particles (such as pollen or smoke particles) are suspended in a liquid or gas,
the particles can be observed through a microscope moving around in a random, erratic
fashion. This movement is called Brownian Motion
• Brownian Motion: the erratic motion of small particles when observed through a
microscope
• This motion is caused by molecules in the gas (or liquid) colliding at high speeds with the
small particles
• These collisions give the particles a little nudge, causing them to change their speed and
directions randomly, each time they are struck by a molecule
• This effect provides important evidence concerning the behaviour of molecules in gases

41. • When small particles (such as pollen or smoke particles) are suspended in a liquid or gas,
the particles can be observed through a microscope moving around in a random, erratic
fashion
• The small particles observed in Brownian motion are significantly bigger than the
molecules that cause the motion
• The molecules are able to affect the particles in this way because they are travelling at
very high speeds (much higher than the particles) and so have a lot of momentum,
which they transfer to the particles when they collide
Brownian Motion

42. Gases and Pressure
Kinetic theory tells us that gases
consist of very small particles that
are constantly moving in completely
random directions.
The particles have mass, so
whenever they collide with
something they exert a force on it.
In sealed containers, gas particles
will smash against the walls of the
container – creating an outward
pressure.
If the same amount of gas is put
into a bigger container, there will be
fewer collisions with the walls of
the container, so the pressure will
decrease.
If a smaller container is used then
there will be more collisions with
the walls as the particles are being
squashed closer together. The
pressure will increase.

43. Gases and Pressure

44. Gases and Pressure

45. Pressure changes – and volume
(at a constant temperature)
Decreasing the volume of a gas increases the
pressure.
The particles have mass, so
whenever they collide with
something they exert a force on
it. In sealed containers, gas
particles will smash against the
walls of the container – creating an
outward pressure.
So long as the temperature is kept
constant, if the container is made bigger
(with the same amount of gas) the
pressure will decrease as there are fewer
collisions between the particles and the
walls of the container (and vice versa).
Volume is inversely proportional
to pressure. Halving the volume
produces a doubling in pressure.

46. 0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
0 2 4 6 8 10 12 14 16
Pressure
(kPa)
Volume (m3)
Pressure changes – and volume
(at a constant temperature)

47. Pressure changes – and volume
(at a constant temperature)
Decreasing the volume of a gas increases the
pressure.
So long as the temperature is
kept constant, if the container is
made bigger (with the same
amount of gas) the pressure will
decrease as there are fewer
collisions between the particles
and the walls of the container
(and vice versa).
Volume is inversely proportional
to pressure. Halving the volume
produces a doubling in pressure.
At constant temperature,
pressure x volume = constant
P x V = constant
P1 x V1 = P2 x V2

48. Pressure changes – and volume
(at a constant temperature)
Decreasing the volume of a gas increases the
pressure.
So long as the temperature is
kept constant, if the container is
made bigger (with the same
amount of gas) the pressure will
decrease as there are fewer
collisions between the particles
and the walls of the container
(and vice versa).
Volume is inversely proportional
to pressure. Halving the volume
produces a doubling in pressure.
Eg. A gas a constant temperature in a
100 ml container has a pressure of 1.2
atmosphere (atm). What is the new
pressure if the container volume is
reduced to 60ml?
Answer: P1 x V1 = P2 x V2
1.2 x 100 = P2 x 60
1.2 x 100 = P2 = 2.0atm
60

49. Solid
Liquid
Gas
{
melting
{
Boiling
(evaporating)
} condensing
} freezing
Changes of State

50. Evaporation
Previously: ………
What do we know so
far about the
properties of liquids
and evaporation?

51. Evaporation
Evaporation is a change in state of a liquid to a gas that can happen at any temperature
from the surface of a liquid
• The molecules in a liquid have a range of different energies:
Some have lots of energy, others have very little
• Evaporation occurs when more energetic molecules near the surface of
the liquid have enough energy to escape

52. Evaporation
When this happens energy is lost from the liquid:
• The average energy of the remaining molecules decreases
• This means that the temperature of the remaining liquid will also decrease

53. Factors Affecting Evaporation
•A number of factors affect the rate of evaporation:
• The temperature of the liquid – At higher temperatures, more molecules
have enough energy to escape
• The surface area of the liquid – If the liquid has a greater surface area
there will be more area from which the molecules can escape
• The movement of air across the surface of the liquid – The presence of a
draft can help to remove less energetic molecules (which might not have
quite enough energy to escape) from the liquid
•The process of evaporation can be used to cool things down:
• If an object is in contact with an evaporating liquid, as the liquid cools the
solid will cool as well
• This process is used in refrigerators and air conditioning units
Evaporation

54. Evaporation
When a liquid evaporates, faster
particles escape from its surface to
form a gas. However, unless the gas is
removed, some of the particles will
return to the liquid.
GAS
LIQUID

55. Evaporation
How can we
increase the
rate of
evaporation?
1. Increase the temperature.
Wet clothes will dry faster on
a hot day because more of the
water molecules have sufficient
energy to escape from the
surface of the liquid.
HEAT

56. Evaporation
How can we
increase the
rate of
evaporation?
2. Increase the surface area.
If the surface area is
increased (eg. pour a hot drink
from a cup into the saucer) then
more of the molecules are closer
to the surface of the liquid..

57. Evaporation
How can we
increase the
rate of
evaporation?
3. Blow air across the surface
Wet clothes will dry faster
on a windy day because the
moving air carries escaping water
molecules away before many of
them can return to the liquid.

58. Evaporation
So what’s the
difference between
evaporation and
boiling?
http://www.fphoto.com/
Boiling is a very rapid form of
evaporation
Vapour bubbles form in the
liquid and as they rise and
burst at the surface they
release large amounts of
vapour.

59. Evaporation and Cooling
If your hands are
wet then water
will start to
evaporate from
the surface.
H2O
H2O
H2O
Your skin feels
cooler as the
evaporating water
takes thermal
energy away.
Kinetic theory:
- Faster
particles are
escaping, so the
ones left behind
are slower and
so have less
energy – liquid
temperature is
therefore less
than before.

60. Condensation
• Gas changing back into a liquid.
• Cold air can hold less water
vapour than warm air, so some
water vapour may condense if
humid air is suddenly cooled.
• These tiny water droplets in
the air may be seen as fog,
mist or clouds.
• We might also see
condensation on a mirror in a
bathroom, or other window
surface.