4.3 Specific latent heat

MODUL PDP SPM
PASCA PKP
NEGERI PERAK
2020

PHYSICS Form 4

4.3:
Specific Latent Heat

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4.3 Specific Latent Heat

Learning Standard:

Pupils are able to:

• explain latent heat.

• define

(i) specific latent heat, ℓ

ℓ =

(ii) specific latent heat of fusion, ℓ

(iii) specific latent heat of evaporization, ℓ

• experiment to determine,
(i) specific latent heat, ℓ of fusion of ice
(ii) specific latent heat of evaporation, ℓ of water

• communicate to explain the applications of specific latent heat in daily life.

• solve problems involving latent heat.

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4.3 notE

1. Latent Heat:
Heat that is absorbed during melting and boiling without change in temperature.

2. The arrangement and movement of molecules among the three stages of matter are
shown as below:
(i) Solid

• Strongest molecular bond
• Molecules cannot move freely, only

vibrate in place

(ii) Liquid

• Moderate molecular bond
• Molecules move about

(iii) Gas

• Weakest molecular bond
• Molecules move freely at random

3. Diagram 1 shows the changes in phases of matter from solid to liquid.

Heat is being supplied
continually, but no

increase in temperature
during melting

Diagram 1: Changes in phases of matter Heat is being released
continually, but no decrease in

temperature during freezing

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4. Diagram 2 shows the changes in phases of matter from liquid to gas.

Heat is being Heat is being released continually,
supplied but no decrease in temperature
continually, during condensation
but no
increase in
temperature
during boiling

Diagram 2: Changes in phases of matter
Diagram 2: Changes in phases of matter

5. Specific Latent Heat,

The quantity of heat, that is absorbed/ released during a change of phase of 1 kg

of the substance without any change in temperature.

= ℎ ( / ) =
= ( ) ( )
= ℎ = : ( )

S. I. unit for = −1

6. Specific latent heat of fusion,

Quantity of heat, that is absorbed during melting/ quantity of heat released during

freezing of 1 kg of the substance without any change in temperature.

7. Specific latent heat of vaporisation,

Quantity of heat, that is absorbed during boiling/ quantity of heat released during
condensation of 1 kg of the substance without any change in temperature.

8. Kinetic Theory of Matter: the higher the average kinetic energy of a molecule, the
higher the temperature of the object.

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9. During melting and boiling, latent heat absorbed does not increase the average
kinetic energy of the molecule, therefore melting and boiling occur at constant
temperature.

10. Diagram 3 shows the heating curve when an object changes its state from solid to gas.

Diagram 3: Heating curve

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11. Diagram 4 shows the cooling curve when an object changes its state from gas to solid.

Diagram 4: Cooling curve
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12. Diagram 5 shows the changes of phase of water when latent heat is absorbed and
released.

Melting Boiling Condensation

Absorption of latent heat: Release of latent heat:
To give the effect of cooling For the purpose of heating

Diagram 5: Changes in phase of water

13. Diagram 6 shows the change of phase from liquid to gas by using alcohol.

2. Alcohol evaporates to become gas.

1. Specific latent heat of
vaporisation is absorbed
to break the bond
between the alcohol

3. The water outside the
beaker becomes cold
because heat is absorbed.

Diagram 6: Change of phase from liquid to gas

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14. Applications of specific latent heat in daily life:
(i) Cooling system in refrigerator as shown in Diagram 7.

Diagram 7: Cooling system in a refrigerator
 Use the cooling effect from evaporation.
 During circulation of the cooling agent, heat is absorbed from inside the

refrigerator.
 Then, the heat absorbed is released to the surroundings.
(ii) Evaporation of sweat as shown in Diagram 8.

Diagram 8: Evaporation of sweat
 We sweat on hot days/ while doing heavy work, sweat evaporates,

heat is absorbed from the body.
 Cooling effect occurs.
 The rate of evaporation will increase when there is air circulation.

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15. Problem solving:
(i) Diagram 9 shows a 480 W immersion heater used to melt ice in a container.
In 120 s, the reading of the electronic balance decreases by 0.172 kg.

Diagram 9

(a) What is the mass of ice that has melted during the heating period?

Mass of ice melted = decrease in reading of the electronic balance

= .

Make assumptions:

• Ice is melted by heat from the immersion heater only.

• All water from the melting ice flows out of the container.

(b) Calculate the specific latent heat of fusion of ice,

Immersion heater of 480 W: = 480
Time 120 s: = 120
Decrease in reading of the electronic balance of 0.172 kg: = 0.172

=
=
480 × 120 = 0.172 ×

= 480 × 120
0.172
= . × −

Make assumptions:
• All heat supplied by the immersion heater is absorbed by the
melting ice.
• No transfer of heat from the surrounding into the apparatus.

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(ii) What is the amount of heat supplied by a water heater to change 0.75 of
water at 30℃ to steam at 100℃?
State the assumptions you make in your calculations.
[Specific heat capacity of water, = 4.20 × 103 −1 ℃−1
Specific latent heat of vaporization of water, = 2.26 × 106 −1]

Amount of heat supplied, = 1 + 2
= 1 + 2

= ∆ +
= [0.75 × (4. .20 × 103) × (100 − 30)] + [0.75 × (2.26 × 106)]
= 1.92 × 106
Make assumptions:

• All heat supplied by the heater is absorbed by the water.
• No loss of heat to the surrounding during heating and change of

phase.
• There are two stages of change:

(i) increase in water temperature from 30℃ to its boiling point
of 100℃.

(ii) change of phase from water to steam without change in
temperature.

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4.3 MIND MAP

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4.3 FAQ

No Question And Answer
1Q What is the difference between specific latent heat and specific heat
capacity?
A
Specific heat capacity is the amount of heat required to raise the temperature
2Q of 1 kg substance by 1oC. Specific Latent heat is the amount of heat released
A or absorbed during a change of state without change in temperature, i.e. the
boiling of water or the melting of ice.
3Q
A What is meant by latent heat in simple words?

4Q The quantity of heat absorbed or released by a substance undergoing a
A change of state, such as ice changing to liquid water or liquid water changing
to ice, at constant temperature and pressure.
5Q
A What are the two types of latent heat?

6Q Two common forms of latent heat are latent heat of fusion (melting) and
A latent heat of vaporization (boiling). These names describe the direction of
energy flow when changing from one phase to the next; from solid to liquid,
and liquid to gas.

What are factors does latent heat depends on?

There is no temperature change until a phase change is complete. Latent heat
is an intensive property measured in units of Jkg-1. Both latent heat of fusion
(Lf) and latent heat of vaporization (Lv) depend on the substance, particularly
on the strength of its molecular forces as noted earlier. Lf and
Lv are collectively called latent heat coefficients.

What are the application of latent heat?

The specific latent heat of vaporisation for water is large. Plates filled with
food are able to absorb heat from the hot steam. The condensation of steam
at the base of the plate releases a large quantity of heat and thus enables food
such as cakes, fish, eggs and others to be steamed.

What is an example of latent heat?

When we boil the water and the temperature remains at 100°C until the last
drop evaporates, due to the latent heat of vaporization. The added heat in the
water is absorbed and carried away by releasing vapor molecules.

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4.3 EXERCISE

SECTION A
1. Diagram 1 shows a graph of the heating curve of a substance.

Diagram 1
At what stage is the substance in the liquid and gas phase at the same time?

[MENGETAHUI]
A PQ
B QR
C RS
D ST

2 Diagram 2 shows a freezing process from water to ice.

Diagram 2 [MENGETAHUI]
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The heat release during the process is called ...

A specific heat capacity of vapour
B specific heat capacity of water
C latent heat of vapourization
D latent heat of fusion

3. Diagram 3 shows the graph heating curve of water.

Diagram 3
What is the effect of the heat absorbed by the water between point S and point T?

[MEMAHAMI]
A It weakens the forces between the water molecules.
B It breaks the bonds between the water molecules.
C It increases the kinetic energy of the water molecules.
D it strengthens the bonds between the water molecules.

4 How much heat is needed to be supplied to change 500 g of ice block at 0°C to

water at 0°C?
[Specific latent heat of fusion = 3.36 × 105 J kg–1]

[MENGAPLIKASI KUANTITATIF]

A 1.68 × 105 J
B 6.72 × 102 J
C 6.72 × 105 J
D 1.68 × 108 J

5. How much heat is needed to be supplied to change 1 kg ice block from 0 °C to

water at 100 °C.
[ Specific latent heat of fusion of ice = 3.36 x 105 J kg-1 ]
[Specific heat capacity of water = 4.2 x 103 J kg-1 ⁰C-1 ]

[MENGAPLIKASI KUANTITATIF]
A 3.40 x 107 J
B 3.40 x 105 J
C 4.20 x 105 J
D 7.56 x 105 J

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SECTION B

6. Diagram 6 shows two cups of coffee of equal mass being cooled by adding a few pieces of
ice cubes.

Cup A Cup B

Diagram 6

Initially, each cup contains 0.2 kg of coffee at 75.5 oC. It is observed that both cups of
coffee reach a final temperature of 25 oC at the same time.

(a) What is meant by specific latent heat of fusion?

[MENGETAHUI]

…………………………………………………………………………………………
[1 mark]

(b) From the diagram 6, [MEMAHAMI]
(i) Which cup of coffee requires more ice?

………………………………………………………………………………
[1 mark]

(ii) Explain your answer in (b)(i).

…………………………………………………………………………………..
[1 mark]

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(c) Calculate the heat released when the coffee is cooled down to 25 oC.

[Specific heat capacity of coffee = 4500 J kg–1 oC–1]
[MENGAPLIKASI KUANTITATIF]

[2 marks]

(d) (i) Calculate the mass of ice needed to cool the coffee in cup A to 25 oC.
[Specific heat capacity of water = 4200 J kg–1 oC–1,
[specific latent heat of fusion of ice = 3.36 x 105 J kg–1]

[MENGAPLIKASI KUANTITATIF]

[2 marks]

(ii) State one assumption which you made in your calculation in (d)(i).
[MEMAHAMI]

..............................................................................................................
[1 mark]

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QR CODE QR CODE

4.3 Specific Latent Heat
Title link
4.3 SPECIFIC LATENT https://youtu.be/dxSeWCB31xk
HEAT

4.3 SPECIFIC LATENT https://youtu.be/FLeZNBmuuGQ
HEAT 1

Measuring the https://youtu.be/D5T7GZAuffc
specific latent heat
of fusion of Ice

Measuring the https://youtu.be/LbZkFuJn_S4
specific latent heat
of vaporization of
water

REINFORCEMENT https://drive.google.com/file/d/15DHHxRcnsFd

TEST DRRpldrGHwlcWH_brIEQB/view?usp=sharing

ANSWER https://drive.google.com/file/d/1dkAL0CXHYK

REINFORCEMENT k1KyN26d7oMmdvUMPEOocA/view?usp=shar

TEST ing

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4.3 ANSWER

Section A

1D
2D
3B
4A
5D

Section B

6 (a) Quantity of heat required to change 1 kg of a substance from solid to liquid
without any change in temperature

(b) (i) Cup B

(ii) The ice is exposed to the surroundings.
The ice absorb heat from the surroundings

(c) Q = mc(Ɵ1- Ɵ2)
Heat released = 0.2 x 4500 x (75.5 - 25)
= 45450 J @ 4.55 kJ

(d) (i) mL + mc Ɵ = 45450
mi x 3.36 x 105 + mi x 4200 (25 – 0)
mi x 3.36 x 105 + mi x 4200 (25 – 0) = 45450
= 0.103 kg

(ii) No heat loss to surroundings

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