Discover PHYSICS
for GCE ‘O’ Level Science
Unit 9: Thermal Properties of Matter
9.1 Temperature and Internal Energy
Learning Outcomes
In this section, you’ll be able to:
• describe a rise in temperature of a body in
terms of an increase in its internal energy
9.1 Temperature and Internal Energy
Recall
• Heat is the amount of thermal energy that
flows from a hotter region to a cooler region.
• Unit for thermal energy is the joule (J).
• Supplying thermal energy to an object leads
to a gain in the internal energy of the
object.
9.1 Temperature and Internal Energy
What is internal energy?
• Particles in a solid, held together by strong
interatomic or intermolecular bonds, vibrate
about fixed positions.
• The total energy of the particles is called
internal energy.
• Internal energy comprises two components:
• Kinetic energy
• Potential energy
9.1 Temperature and Internal Energy
What is internal energy?
• Kinetic component of internal energy is
due to the vibration of the particles.
• It is directly related to temperature i.e. the
higher the temperature, the more vigorous
the vibration.
• In liquids and gases, the kinetic energy is
due to their movement instead of vibrations.
9.1 Temperature and Internal Energy
What is internal energy?
• Potential component of internal energy is
due to the stretching and compressing of
the interatomic or intermolecular bonds as
particles vibrate.
• The amount of energy stored is dependent
on the force between the particles and the
distance between the particles.
9.1 Temperature and Internal Energy
What is internal energy?
• Hence, when the temperature of a
substance rises, it is due to an increase in
the average kinetic energy of its particles
only.
9.1 Temperature and Internal Energy
Key Ideas
•
•
Internal energy is made up of kinetic
energy and potential energy.
An increase in temperature leads to an
increase in kinetic energy component of the
internal energy.
9.1 Temperature and Internal Energy
Test yourself
1. Based on the kinetic model of matter, explain the
nature of the internal energy of an object in the
solid state.
Answer:
The internal energy of a solid object comprises of
kinetic energy and potential energy. The kinetic
component is related directly to the temperature and
is due to the vibrations of the particles. The potential
component is due to the molecular bonds between
the particles.
9.2 Melting and Solidification
Learning Outcomes
In this section, you’ll be able to:
• Describe the process of melting and
solidification as transfer of energy without a
change in temperature.
• Sketch and interpret a cooling curve.
9.2 Melting and Solidification
Melting
• The change of state from solid to liquid is
called melting.
• Melting occurs at a definite or constant
temperature.
• This temperature is called the melting point.
9.2 Melting and Solidification
Melting
Table 9.1 Melting points of some substances
9.2 Melting and Solidification
Possible Errors?
9.2 Melting and Solidification
Melting
9.2 Melting and Solidification
Melting
• Melting occurs at a constant temperature.
• During the change of state, there is no change in
temperature even though thermal energy is being
absorbed.
Figure 11.10 Melting in action
9.2 Melting and Solidification
How does a solid melt?
• Molecules in a solid are held by strong
intermolecular bonds.
• Thermal energy supplied to solid is used to
break these bonds during melting, hence
there is no increase in temperature (Section
QR of the curve in Figure 11.8)
• The thermal energy that is absorbed without a
change in temperature is called the latent
heat of fusion of a substance.
9.2 Melting and Solidification
Solidification and freezing point
• The reverse process of melting is called
solidification i.e. changing from liquid to
solid state.
• A pure substance will solidify or freeze at a
temperature equal to its melting point.
9.2 Melting and Solidification
9.2 Melting and Solidification
9.2 Melting and Solidification
Solidification and freezing point
• Freezing occurs at a constant temperature.
• During the change of state, there is no change in
temperature even though thermal energy is lost to
the surroundings.
• When liquids molecules come together and
solidify, intermolecular bonds are formed.
• Thermal energy is released during the formation of
bonds, hence there is no change in temperature.
9.2 Melting and Solidification
Key Ideas
1. Melting is the change of state from solid to liquid,
without a change in temperature.
2. During melting, the temperature remains
constant at the melting point.
3. Solidification is the change of state from liquid to
solid, without a change in temperature.
4. During solidification, the temperature remains
constant at the freezing point. Thermal energy is
released by the substance.
9.2 Melting and Solidification
Test yourself
1. In melting, there is no change in temperature
even though thermal energy is being absorbed.
Where has the thermal energy gone to?
Answer:
The thermal energy absorbed is used to break the
intermolecular bonds in order for the solid to melt.
9.2 Melting and Solidification
Test yourself
2. In freezing, there is also no change in temperature
even though thermal energy is being released.
Where does the thermal energy come from?
Answer:
When liquid molecules come together and solidify,
intermolecular bonds are formed. In the process,
thermal energy is released.
9.3 Boiling and Condensation
Learning Outcomes
In this section, you’ll be able to:
• Describe the process of boiling and
condensation as transfer of energy without a
change in temperature
9.3 Boiling and Condensation
Boiling
• When pure liquid is heated and changes to
vapour at fixed or constant temperature,
this change of state is called boiling.
• The temperature at which this occurs is
called the boiling point of the substance.
9.3 Boiling and Condensation
Boiling
Table 9.2 Boiling points of some substances
9.3 Boiling and Condensation
Condensation
• The reverse of boiling is condensation.
• It is a change of state from vapour to
liquid state at the same constant
temperature as in boiling.
• Thermal energy is given out during
condensation.
9.3 Boiling and Condensation
Figure 9.15 Thermal energy is still being absorbed in portion YZ of the graph
but there is no change in temperature. Why is this so?
9.3 Boiling and Condensation
How does a liquid boil?
• Thermal energy supplied to liquid but no rise in
temperature (YZ line in Figure 9.15).
• Thermal energy is used to do work to separate the
molecules and also to push back on the
surrounding atmosphere.
• Molecules become further apart with negligible
intermolecular forces of attraction.
• Change of state (boiling) from liquid to gas has
taken place.
• The thermal energy that is absorbed without a
change in temperature is called latent heat of
vaporisation of the substance.
9.3 Boiling and Condensation
Key Ideas
1. Boiling is the change of state from a liquid into
vapour, occurring at a constant temperature
called the boiling point.
2. Condensation is the process whereby vapour
changes into liquid at the same constant
temperature. Thermal energy is given out during
condensation.
9.3 Boiling and Condensation
Key Ideas
3. During boiling, the temperature remains constant
at its boiling point. Thermal energy is being
absorbed by the substance.
4. During condensation, the temperature remains
constant at the condensation point. Thermal
energy is released by the substance.
9.3 Boiling and Condensation
Test yourself
1. In boiling, there is no change in temperature even
though a large amount of heat is being absorbed.
Where has the thermal energy gone to?
Answer:
During boiling, thermal energy is needed to completely
break the intermolecular bonds and separate the
molecules, as well as to push back the atmosphere.
9.3 Boiling and Condensation
Test yourself
2. In condensation, there is also no change in
temperature even though heat is being released.
Where has the thermal energy gone to?
Answer:
During condensation, molecules come together and
intermolecular bonds are formed. In the process,
thermal energy are released.
9.4 Evaporation
Learning Outcomes
In this section, you’ll be able to:
• Explain the differences between boiling and
evaporation.
• Explain the effects of evaporation.
• State the factors affecting the rate of
evaporation.
9.4 Evaporation
What is evaporation?
• Evaporation, like boiling, is the change of
state from a liquid to vapour.
• Unlike boiling, evaporation can occur at any
temperature.
9.4 Evaporation
What is evaporation?
Table 9.3 Differences between boiling and evaporation.
9.4 Evaporation
Evaporation causes cooling
• Evaporation requires thermal energy from the
surroundings.
• If you step out of the swimming pool on a dry and
sunny day, your body will feel cold (especially if a
wind is blowing). Why is this so? This is because
the water on your body is evaporating.
9.4 Evaporation
9.4 Evaporation
How does evaporation occur?
• Evaporation causes cooling.
• This can be explained using the kinetic model of matter.
Figure 9.19 Evaporation of
ether takes place even at
room temperature. Energetic
molecules are able to
escape from the liquid
surface.
9.4 Evaporation
How does evaporation occur?
• Molecules of liquids are always moving randomly
at different speeds.
• These molecules can absorb thermal energy from
the surroundings.
• At the liquid surface, molecules are more energetic
and can overcome the downward attractive forces
of other molecules and escape into atmosphere.
• The slower-moving molecules left behind are
cooler as temperature is proportional to the
average kinetic energy of the molecules.
9.4 Evaporation
Applications of evaporation
• Cooling caused by spraying of perfume or
when perspiring.
• Drying of clothes or puddles of water.
• Sponging person with fever.
• Refrigeration.
9.4 Evaporation
Factors affecting the rate of evaporation
1. Temperature
 Raising the temperature of liquid will
increase the rate of evaporation.
 A warmer liquid means a greater number
of energetic molecules at the surface layer
 Hence more molecules are energetic
enough to escape from the surface.
9.4 Evaporation
Factors affecting the rate of evaporation
2. Humidity of the surrounding air
 Humidity is the measure of how much
water vapour there is in the air.
 The higher the amount of water vapour,
the higher the humidity.
 Evaporation decreases with increasing
humidity.
9.4 Evaporation
Factors affecting the rate of evaporation
3. Surface area of the liquid
 Rate of evaporation increases when there
is more exposed surface area of the liquid.
 This is because evaporation only takes
place at the exposed surface of a liquid.
 Larger exposed surface allows more
molecules to escape.
9.4 Evaporation
Factors affecting the rate of evaporation
4. Movement of air
 Moving air removes the molecules of liquid
as soon as they escape from the surface.
 Increasing air movement will increase the
rate of evaporation.
9.4 Evaporation
Factors affecting the rate of evaporation
5. Pressure
 Reducing atmospheric pressure increases
the rate of evaporation.
 For example, wet objects dry faster at
mountaintops or at high altitudes where
atmospheric pressure is lower than at sea
level.
9.4 Evaporation
Factors affecting the rate of evaporation
•
Boiling point of the liquid
 Liquids with lower boiling points will
evaporate faster.
 For example, ether will evaporate much
more rapidly than water under the same
physical condition.
 On the other hand, mercury hardly
evaporates at room temperature because it
has high boiling point.
9.4 Evaporation
Key Ideas
1. Evaporation is a cooling process.
2. Evaporation of liquid is due to molecules at
the surface with energy greater than the
average kinetic energy escaping from the
rest of the liquid.
9.4 Evaporation
Test yourself
1. State the differences between boiling and
evaporation.
Answer:
9.4 Evaporation
Test yourself
2. Explain why putting a layer of perfume on the skin
produces a cooling effect.
Answer:
Perfume is very volatile and evaporate rapidly. To
evaporate, the perfume absorbed the thermal energy
from the skin. Hence the skin now feels a cooling
effect.
9.4 Evaporation
Test yourself
3. Explain why the rate of evaporation of a liquid
increases with temperature.
Answer:
• Raising the temperature of liquid will increase
the rate of evaporation.
• A warmer liquid means that a greater number
of energetic molecules at the surface layer.
• Hence more molecules are energetic enough
to escape from the surface.