Warm-up

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Warm-up
The temperature of boiling water does not
increase even though energy is supplied to it
continually.
?
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Warm-up
Do these require energy supply?
?
?
3
Warm-up
less
Ice molecules moves ________ (more/ less)
freely than water molecules
less
so they have ________ (more / less) KE than water
molecules.
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Introduction
Change of state
Matter exists in 3 states
solid,
liquid,
gas
e.g. water
vaporization (takes place at boiling point)
Fusion (at melting point)
water
steam
ice
solidification
condensation
at freezing point
at boiling point
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Cooling curve
When a hot liquid is cooled down,
its temperature drops.
Graph of temperature vs time?
??
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Experiment 3a
Cooling curve of octadecan-1-ol
Record the temperature of the melted
octadecan-1-ol as it cools down.
Video
Video
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Cooling curve

• When solid is heated, it melts to a liquid
• On cooling, the variation of temperature with
  time from A to B
• The temperature remains constant from B to C, the
  energy evolved is called the latent heat
• The latent heat is used to change from liquid to
  solid at constant temperature, which is called
  the melting point

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Why temperature remains constant?

• From liquid to solid, energy is released to the
  surroundings due to the decrease in the kinetic
  energy of the particles
• From solid to liquid, energy is absorbed from the
  surroundings due to the increase in the kinetic
  energy of the particles

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Specific latent heat of fusion and vaporization

• Definition
• Latent heat is required is proportional to the
  mass of the substance that undergoes state change
• i.e. Latent heat (H) ? mass (m)
• H L m
• L is a constant called the specific latent
  heat
• Unit J kg-1

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Cooling curve
Here is a cooling curve of octadecan-1-ol .

• AB drops steadily liquid cooling (temperature
  falling )

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Cooling curve

• BC is flat liquid solidifying
• (temperature unchanged)

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Cooling curve

• CD drops steadily solid cooling to room
  temperature (temperature falling)

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Cooling curve
melting point
read from the flat part BC

• melting point

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Cooling curve of water
temperature
liquid-solid mixture
Simulation
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Latent heat
The cooling curve shows
When a substance is solidifying,

• it loses energy continuously but...

• its temperature remains unchanged

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Latent heat
During change of state
The energy given out/absorbed is called latent
heat
means hidden
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Latent heat
Ice-water mixture stays at 0 oC until all the ice
is melted.

• temperature unchanged

• energy is absorbed

from air to change the ice to water

• This energy is called
• latent heat of fusion of ice.

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Latent heat
Energy supplied continuously to keep water boils

• temperature unchanged

• energy is absorbed

to change the water to stream

• This energy is called
• latent heat of vaporization of water.

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steam
condensation
vaporization
releases latent heat of vaporization
water
solidification
fusion
ice
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steam
vaporization
condensation
absorbs latent heat of vaporization
water
solidification
fusion
ice
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steam
condensation
vaporization
releases latent heat of fusion
water
solidification
fusion
ice
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steam
vaporization
condensation
water
absorbs latent heat of fusion
solidification
fusion
ice
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State Change
solid
liquid
gas
liquid
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2 Latent heat and particle motion
molecule

• Regular arrangement breaks up

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2 Latent heat and particle motion
Energy has to be supplied to oppose the
attractive force of the particles.
PE related to the forces of attraction between
the particles
solid ? liquid or liquid ? gas
average potential energy ?
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2 Latent heat and particle motion
The transfer of energy does not change the KE.
Temperature does not change.
latent heat change in PE during change of state
Simulation
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Specific latent heat
Specific for 1 kg of a substance
e.g.
energy E
without temperature change
1 kg solid X
1 kg liquid X
E latent heat for 1 kg of X specific latent
heat of X
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Specific latent heat
Energy transferred to change the state of 1 kg of
the substance without a change in temperature.
symbol l
or E ml
unit J kg-1
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Specific latent heat of fusion of ice (lf )
lf energy needed to change 1 kg of ice to water
(without temperature change)
Find (1) mass melted m and (2) energy transferred
E ? lf E/m
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Experiment 3b
Measuring the specific latent heat of fusion of
ice
Video
Simulation
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Experiment 3b
Ice also melts at room temperature, so a control
is needed.
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Experiment 3b
Measuring the specific latent heat of fusion of
ice
For ice, lf 3.34 ? 105 J kg-1
ice (0 ?C)
water (0 ?C)
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b Specific latent heat of vaporization of water
(lv)
lv energy needed to change 1 kg of water to
steam (without change of temperature)
Find (1) mass boiled away m and (2) energy
transferred E ? lv E/m
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Experiment 3c
Video
Simulation
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Experiment 3c
For water, lv 2.26 ? 106 J kg-1
steam (0 ?C)
water (0 ?C)
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Summary change of state
vaporization
energy in 2260 kJ
steam (1 kg)
water (1 kg)
energy in 334 kJ
ice (1 kg)
fusion
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Summary from ice to steam
stream
100
0
energy / kJ
(420)
Energy involved in heating 1 kg of water
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Consider a cup of water...
Consider a cup of water (mass m) being heated
from 0 C until it starts to boil at 100 C.
Since E mc?T and E mlv ? mc?T mlv ? lv c
?T 4200 100 420 kJ kg-1
Is the student correct? (Yes/No)
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When vapour condenses...
When vapour condenses, is the surrounding air
warmed or cooled?
(warmed/cooled)
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Jimmy melts...
Jimmy melts three materials X, Y and Z of equal
mass at the same time and place.
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...
Which material(s) has/have the greatest melting
point? ( X / Y / Z )
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Which material(s) has/have the largest value of
specific latent heat of fusion? ( X / Y / Z )
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Which material(s) release(s) largest amount of
energy (per kg) when they freeze? ( X / Y / Z )
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Fusion and Boiling

• Boiling of water and fusion of ice
• when water boils, the temperature is found to
  remained at 100oC. The energy supplied is only
  used to change from water to steam without any
  change in temperature
• When the steam condenses, the temperature remains
  at 100oC and energy is given out

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Fusion and Boiling

• Fusion of ice
• when ice melts, the temperature is found to
  remained at 0oC. The energy supplied is only used
  to change from ice to water without any change in
  temperature
• When the water freezes, the temperature remains
  at 0oC and energy is given out

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Latent Heat of Fusion and Vaporization

• Energies used in fusion and vaporization are
  called the latent heat of fusion and
  vaporization
• The constant temperatures are called the melting
  point and boiling point

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Example 1
Finding specific latent heat of fusion of ice
Result of melting experiment Mass of water in
experimental cup m1 0.050 kg in control cup
m2 0.014 kg Joulemeter reading initial
j1 15 000 J final j2 29 200 J
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Example 1
Finding specific latent heat of fusion of ice
(a) Find the specific latent heat of fusion of
ice.
Results m1 0.050 kg m2 0.014 kg j1 15
000 J j2 29 200 J
lf E /m (j2 j1) / (m2 m1)
(29 200 15 000) / (0.050 0.014)
3.94 ? 105 J kg1
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Example 1
Finding specific latent heat of fusion of ice
(b) Account for any difference of the value
obtained from the standard value, 3.34 ? 105 J
kg1.
Experimental value 3.94 ? 105 J kg1
There is a rather large error of 18. The
possible sources of error are

• Difficulty in keeping the water dripping down the
  two funnels at the same rate.

• Energy lost to the surroundings.

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Example 2
How much energy is required to melt 0.5 kg of ice
at 0 C temperature raised to 80 C?
Total energy required
latent heat (ice at 0 C ? water at 0 C)
energy (water 0 C ? 80 C)
mlf mc ?T 0.5 ? 3.34 ? 105 0.5 ? 4200 ?
80 3.35 ? 105 J
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Example 3
Result of boiling experiment Mass of water boiled
away 0.10 kg KW h meter calibration 600
turns/kW h Number of rotations counted 41
(a) Find the specific latent heat of vaporization
of water.
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Example 3
kW h meter calibration 600 turns/kW h
1 kW h 1 kW ? 1h 3.6 ? 106 J
Energy supplied per revolution of the disc 3.6
? 106/600 6000 J
Energy supplied to boil 0.10 kg of water 6000
? 41 246 000 J
Number of rotations 41
Specific latent heat of vaporization of water lv
E /m 246 000/0.10 2.46 ? 106 J kg1
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Example 3
(b) Account for any difference of the value
obtained from the standard value, 2.26 ? 106 J
kg1.
Experimental value 2.26 ? 106 J kg1
There is an error of about 9.
1 Steam condensing on the heater drips back into
the cup.
2 Some water bubbles out of the cup.
3 Energy lost to the surroundings.
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Example 4
How much energy is required to change 0.5 kg of
water at 0 C to stream at 100 C .
m 0.5 kg
?T 100 C 0 C 100 C
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Example 4
m 0.5 kg, ?T 100 C 0 C 100 C
The total energy required
energy (water 0C ? 100 C) latent heat
(water at 100 C ? steam at 100 C)
mc?T mlv
0.5 4200 100 0.5 2.26 106
2.1 105 11.3 105
1.34 106 J
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Example 5
An expresso coffee machine injects 0.025 kg of
steam at 100 C into a cup of cold coffee of mass
0.15 kg at 20 C .
specific heat capacity of the coffee 5800 J
kg1 C
Find the final temeperature of the expresso
coffee.
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Example 5
0.025 kg of steam at 100 oC 0.15 kg of coffee at
20 oC
Let T be the final temperature of the coffee.
Assuming no energy loss to the surroundings,
energy loss by steam
energy gained by coffee

0.025 ? 2.26 ? 106 0.025 ? 4200 ? (100 T )

0.15 ? 5800 ? (T 20)
Solving the equation,
The temperature T of the coffee is 86.6 C
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Effect of pressure on melting point of water

• melting point of ice at normal pressure is 0oC
• it becomes lower when the external pressure is
  increased (i.e. below 0oC )
• vice versa

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Effect of pressure on boiling point of water

• boiling point of ice at normal pressure is 100oC
• it becomes higher when the external pressure is
  increased (i.e. above 100oC )
• vice versa

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Effect of impurities on melting and boiling point
of water

• Impurities such as salt is added to ice to lower
  the melting point, causing the ice to melt
• Addition of impurities into water raises the
  boiling point of water above 100oC