Textbook: Heat | Science and Technology Class 10 (Maharashtra SSC Board) PDF Download

 Page 1


62
5. Heat
Ø	 Latent heat               Ø 	Regelation  
Ø	 Anomalous behaviour of water   Ø 	Dew point and humidity
Ø	 Specific heat capacity 
1. What is the difference between heat and temperature?
2. What are the different ways of heat transfer?
In the previous standard, we have learnt about heat and different types of heat transfer. 
We have also performed few experiments related to expansion and contraction of solids, 
liquids and gases. We have learnt about the difference between heat and temperature. We 
have also seen how temperature is measured using  a thermometer.
 Concepts like latent heat of phase transformation, anomalous behaviour of water, 
dew point, humidity, specific heat capacity etc. are related to certain phenomena experi-
enced by us in our day-to-day life. Let us learn more about these concepts. 
1. Take a few pieces of ice in a glass beaker. 
As shown in figure 5.1. 
2. Insert the bulb of a thermometer in ice and 
measure its temperature.
3. Put the beaker on a stand and heat the ice 
using a burner.
4. Record the temperature using the 
thermometer after every minute.
5. As the ice is heated, it starts melting. Stir 
the mixture of ice and water.
6. Continue the heating even after ice starts 
melting.
7. Draw a graph of temperature versus time.
You will observe that the temperature of the mixture remains 0 
0
C till the ice melts 
completely. If we continue heating, even after conversion of all the ice into water, the 
temperature of water starts rising and reaches 100 
0
C. At this temperature water starts 
converting into steam. The temperature of water remains constant at 100 
0
C till all water 
converts into steam. The graph of temperature versus time is shown in figure 5.2.
In this graph, line AB represents conversion of ice into water at constant temperature. 
When ice is heated it melts at 0 
0
C and converts into water at this constant temperature. 
The ice absorbs heat energy during this transition and the absorption of energy continues 
till all the ice converts into water. 
Latent heat
5.1  Latent heat 
Can you recall?
Try this
Thermometer
Ice cubes/pieces
Stand
Burner
Page 2


62
5. Heat
Ø	 Latent heat               Ø 	Regelation  
Ø	 Anomalous behaviour of water   Ø 	Dew point and humidity
Ø	 Specific heat capacity 
1. What is the difference between heat and temperature?
2. What are the different ways of heat transfer?
In the previous standard, we have learnt about heat and different types of heat transfer. 
We have also performed few experiments related to expansion and contraction of solids, 
liquids and gases. We have learnt about the difference between heat and temperature. We 
have also seen how temperature is measured using  a thermometer.
 Concepts like latent heat of phase transformation, anomalous behaviour of water, 
dew point, humidity, specific heat capacity etc. are related to certain phenomena experi-
enced by us in our day-to-day life. Let us learn more about these concepts. 
1. Take a few pieces of ice in a glass beaker. 
As shown in figure 5.1. 
2. Insert the bulb of a thermometer in ice and 
measure its temperature.
3. Put the beaker on a stand and heat the ice 
using a burner.
4. Record the temperature using the 
thermometer after every minute.
5. As the ice is heated, it starts melting. Stir 
the mixture of ice and water.
6. Continue the heating even after ice starts 
melting.
7. Draw a graph of temperature versus time.
You will observe that the temperature of the mixture remains 0 
0
C till the ice melts 
completely. If we continue heating, even after conversion of all the ice into water, the 
temperature of water starts rising and reaches 100 
0
C. At this temperature water starts 
converting into steam. The temperature of water remains constant at 100 
0
C till all water 
converts into steam. The graph of temperature versus time is shown in figure 5.2.
In this graph, line AB represents conversion of ice into water at constant temperature. 
When ice is heated it melts at 0 
0
C and converts into water at this constant temperature. 
The ice absorbs heat energy during this transition and the absorption of energy continues 
till all the ice converts into water. 
Latent heat
5.1  Latent heat 
Can you recall?
Try this
Thermometer
Ice cubes/pieces
Stand
Burner
63
 The temperature remains constant 
during this transition. This constant 
temperature, at which the ice converts into 
water is called the melting point of ice.
Thus, during transition of solid phase to 
liquid, the object absorbs heat energy, but its 
temperature does not increase. This heat 
energy is utilised for weakening the bonds 
between the atoms or molecules in the solid 
and transform it into liquid phase. The heat 
energy absorbed at constant temperature 
during transformation of solid into liquid is 
called the latent heat of fusion.
The amount of heat energy absorbed at constant temperature by unit mass of  a 
solid to convert into liquid phase is called the specific latent heat of fusion. 
Once all the ice is transformed into water, the temperature of water starts rising. It 
increases up to 100
 0
C. Line BC in the graph represents rise in temperature of water from   
0 
0
C to 100 
0
C. Thereafter, even though heat energy is supplied to water, its temperature 
does not rise. The heat energy is absorbed by water at this temperature and used to break 
the bonds between molecules of the liquid and convert the liquid into gaseous state. Thus, 
during transformation from liquid phase to gas phase, heat energy is absorbed by the 
liquid, but its temperature does not change. The constant temperature at which the liquid 
transforms into gaseous state is called the boiling point of the liquid. The heat energy 
absorbed at constant temperature during transformation of liquid into gas is called the 
latent heat of vapourization.
The amount of heat energy absorbed at constant temperature by unit mass of a 
liquid to convert into gaseous phase is called the specific latent heat of vapourization. 
Different substances have different melting points and boiling points. The values of 
melting point, boiling point and latent heat depend on atmospheric pressure.
1. Is the concept of latent heat applicable during trans-
formation of gaseous phase to liquid phase and from 
liquid phase to solid phase?
2. Where does the latent heat go during these 
transformations?
5.2 Temperature vs Time Graph
Substance
Melting 
point 
0
C
Boiling 
point 
0
C
Specific latent heat of 
fusion
Specific latent heat of 
vaporization
kJ/kg cal/g kJ/kg cal/g
Water/ Ice 0 100 333 80 2256 540
Copper 1083 2562 134 49 5060 1212
Ethyl alcohol -117 78 104 26 8540 200
Gold 1063 2700 144 15.3 1580 392
Silver 962 2162 88.2 25 2330 564
Lead 327.5 1749 26.2 5.9 859 207
Boiling water + Vapour 
Time (Minutes)
Water
Liquid 
0
C
Ice+ Water
(Solid+liquid)
Use your brain power !
Liquid - Gaseous state 
Page 3


62
5. Heat
Ø	 Latent heat               Ø 	Regelation  
Ø	 Anomalous behaviour of water   Ø 	Dew point and humidity
Ø	 Specific heat capacity 
1. What is the difference between heat and temperature?
2. What are the different ways of heat transfer?
In the previous standard, we have learnt about heat and different types of heat transfer. 
We have also performed few experiments related to expansion and contraction of solids, 
liquids and gases. We have learnt about the difference between heat and temperature. We 
have also seen how temperature is measured using  a thermometer.
 Concepts like latent heat of phase transformation, anomalous behaviour of water, 
dew point, humidity, specific heat capacity etc. are related to certain phenomena experi-
enced by us in our day-to-day life. Let us learn more about these concepts. 
1. Take a few pieces of ice in a glass beaker. 
As shown in figure 5.1. 
2. Insert the bulb of a thermometer in ice and 
measure its temperature.
3. Put the beaker on a stand and heat the ice 
using a burner.
4. Record the temperature using the 
thermometer after every minute.
5. As the ice is heated, it starts melting. Stir 
the mixture of ice and water.
6. Continue the heating even after ice starts 
melting.
7. Draw a graph of temperature versus time.
You will observe that the temperature of the mixture remains 0 
0
C till the ice melts 
completely. If we continue heating, even after conversion of all the ice into water, the 
temperature of water starts rising and reaches 100 
0
C. At this temperature water starts 
converting into steam. The temperature of water remains constant at 100 
0
C till all water 
converts into steam. The graph of temperature versus time is shown in figure 5.2.
In this graph, line AB represents conversion of ice into water at constant temperature. 
When ice is heated it melts at 0 
0
C and converts into water at this constant temperature. 
The ice absorbs heat energy during this transition and the absorption of energy continues 
till all the ice converts into water. 
Latent heat
5.1  Latent heat 
Can you recall?
Try this
Thermometer
Ice cubes/pieces
Stand
Burner
63
 The temperature remains constant 
during this transition. This constant 
temperature, at which the ice converts into 
water is called the melting point of ice.
Thus, during transition of solid phase to 
liquid, the object absorbs heat energy, but its 
temperature does not increase. This heat 
energy is utilised for weakening the bonds 
between the atoms or molecules in the solid 
and transform it into liquid phase. The heat 
energy absorbed at constant temperature 
during transformation of solid into liquid is 
called the latent heat of fusion.
The amount of heat energy absorbed at constant temperature by unit mass of  a 
solid to convert into liquid phase is called the specific latent heat of fusion. 
Once all the ice is transformed into water, the temperature of water starts rising. It 
increases up to 100
 0
C. Line BC in the graph represents rise in temperature of water from   
0 
0
C to 100 
0
C. Thereafter, even though heat energy is supplied to water, its temperature 
does not rise. The heat energy is absorbed by water at this temperature and used to break 
the bonds between molecules of the liquid and convert the liquid into gaseous state. Thus, 
during transformation from liquid phase to gas phase, heat energy is absorbed by the 
liquid, but its temperature does not change. The constant temperature at which the liquid 
transforms into gaseous state is called the boiling point of the liquid. The heat energy 
absorbed at constant temperature during transformation of liquid into gas is called the 
latent heat of vapourization.
The amount of heat energy absorbed at constant temperature by unit mass of a 
liquid to convert into gaseous phase is called the specific latent heat of vapourization. 
Different substances have different melting points and boiling points. The values of 
melting point, boiling point and latent heat depend on atmospheric pressure.
1. Is the concept of latent heat applicable during trans-
formation of gaseous phase to liquid phase and from 
liquid phase to solid phase?
2. Where does the latent heat go during these 
transformations?
5.2 Temperature vs Time Graph
Substance
Melting 
point 
0
C
Boiling 
point 
0
C
Specific latent heat of 
fusion
Specific latent heat of 
vaporization
kJ/kg cal/g kJ/kg cal/g
Water/ Ice 0 100 333 80 2256 540
Copper 1083 2562 134 49 5060 1212
Ethyl alcohol -117 78 104 26 8540 200
Gold 1063 2700 144 15.3 1580 392
Silver 962 2162 88.2 25 2330 564
Lead 327.5 1749 26.2 5.9 859 207
Boiling water + Vapour 
Time (Minutes)
Water
Liquid 
0
C
Ice+ Water
(Solid+liquid)
Use your brain power !
Liquid - Gaseous state 
64
Activity:
1. Put a slab of ice on a stand as shown in Figure 5.3.
2. Hang two  equal weights to the two ends of a metal 
wire and put the wire on the slab as shown in the 
figure.
 What do you observe?
 It is observed that the wire gradually penetrates the ice 
slab. After some time, the wire comes out of the lower 
surface of the ice slab. However, the ice slab does not 
break. The phenomenon in which the ice converts to 
liquid due to applied pressure and then re-converts to 
ice once the pressure is removed is called regelation.
  The melting point of ice becomes lower than 0 
0
C 
 due to pressure. This means that at 0 
0
C, the ice gets 
converted into water. As soon as the pressure is 
removed, the melting point is restored to 0 
0
C and 
water gets converted into ice again. 
2.  Is there any relationship of latent heat with the regelation?
3. You know that as we go higher than the sea level, the boiling point of water decreases
What would be effect on the melting point of solid?
5.3 Regelation
Weight
Ice
Regelation
 You may have seen the preparation of  an ice-ball. First, an ice slab is shredded and 
then the shredded ice is pressurised around the tip of a stick to prepare the ice-ball. How 
does the shredded ice convert into solid ice ball? If two small pieces of ice are taken and 
pressed against each other for a while, they stick to each other. Why does this happen?
 Take a small slab of ice, a thin wire, two identical weights. 
Try this
Use your brain power !
1. In the above experiment, the wire moves through 
the ice slab. However, the ice slab does not break. 
Why?
Can you tell?
Anomalous behaviour of water 
In general, when a liquid is heated up to a certain temperature, it expands, and when 
cooled it contracts. Water, however, shows a special and exceptional behaviour.  If we 
heat water from 0 
0
C  up to 4 
0
C, it contracts instead of expanding. At 4 
0
C its volume is 
minimum (due to contraction). If the water is heated further, it expands and its volume 
increases. The behaviour of water between its temperature from  0 
0
C to 4 
0
C is called 
anomalous behaviour of water. 
If 1 kg of water is heated from 0 
0
C and its volume is plotted as a function of temperature, 
we get the graph, shown in fig 5.4. At 4 
0
C, the volume of water is minimum. It means that 
the density of water is maximum at 4 
0
C.
We feel that some objects are cold, and some are hot. Is this 
feeling related in some way to our body temperature?
Page 4


62
5. Heat
Ø	 Latent heat               Ø 	Regelation  
Ø	 Anomalous behaviour of water   Ø 	Dew point and humidity
Ø	 Specific heat capacity 
1. What is the difference between heat and temperature?
2. What are the different ways of heat transfer?
In the previous standard, we have learnt about heat and different types of heat transfer. 
We have also performed few experiments related to expansion and contraction of solids, 
liquids and gases. We have learnt about the difference between heat and temperature. We 
have also seen how temperature is measured using  a thermometer.
 Concepts like latent heat of phase transformation, anomalous behaviour of water, 
dew point, humidity, specific heat capacity etc. are related to certain phenomena experi-
enced by us in our day-to-day life. Let us learn more about these concepts. 
1. Take a few pieces of ice in a glass beaker. 
As shown in figure 5.1. 
2. Insert the bulb of a thermometer in ice and 
measure its temperature.
3. Put the beaker on a stand and heat the ice 
using a burner.
4. Record the temperature using the 
thermometer after every minute.
5. As the ice is heated, it starts melting. Stir 
the mixture of ice and water.
6. Continue the heating even after ice starts 
melting.
7. Draw a graph of temperature versus time.
You will observe that the temperature of the mixture remains 0 
0
C till the ice melts 
completely. If we continue heating, even after conversion of all the ice into water, the 
temperature of water starts rising and reaches 100 
0
C. At this temperature water starts 
converting into steam. The temperature of water remains constant at 100 
0
C till all water 
converts into steam. The graph of temperature versus time is shown in figure 5.2.
In this graph, line AB represents conversion of ice into water at constant temperature. 
When ice is heated it melts at 0 
0
C and converts into water at this constant temperature. 
The ice absorbs heat energy during this transition and the absorption of energy continues 
till all the ice converts into water. 
Latent heat
5.1  Latent heat 
Can you recall?
Try this
Thermometer
Ice cubes/pieces
Stand
Burner
63
 The temperature remains constant 
during this transition. This constant 
temperature, at which the ice converts into 
water is called the melting point of ice.
Thus, during transition of solid phase to 
liquid, the object absorbs heat energy, but its 
temperature does not increase. This heat 
energy is utilised for weakening the bonds 
between the atoms or molecules in the solid 
and transform it into liquid phase. The heat 
energy absorbed at constant temperature 
during transformation of solid into liquid is 
called the latent heat of fusion.
The amount of heat energy absorbed at constant temperature by unit mass of  a 
solid to convert into liquid phase is called the specific latent heat of fusion. 
Once all the ice is transformed into water, the temperature of water starts rising. It 
increases up to 100
 0
C. Line BC in the graph represents rise in temperature of water from   
0 
0
C to 100 
0
C. Thereafter, even though heat energy is supplied to water, its temperature 
does not rise. The heat energy is absorbed by water at this temperature and used to break 
the bonds between molecules of the liquid and convert the liquid into gaseous state. Thus, 
during transformation from liquid phase to gas phase, heat energy is absorbed by the 
liquid, but its temperature does not change. The constant temperature at which the liquid 
transforms into gaseous state is called the boiling point of the liquid. The heat energy 
absorbed at constant temperature during transformation of liquid into gas is called the 
latent heat of vapourization.
The amount of heat energy absorbed at constant temperature by unit mass of a 
liquid to convert into gaseous phase is called the specific latent heat of vapourization. 
Different substances have different melting points and boiling points. The values of 
melting point, boiling point and latent heat depend on atmospheric pressure.
1. Is the concept of latent heat applicable during trans-
formation of gaseous phase to liquid phase and from 
liquid phase to solid phase?
2. Where does the latent heat go during these 
transformations?
5.2 Temperature vs Time Graph
Substance
Melting 
point 
0
C
Boiling 
point 
0
C
Specific latent heat of 
fusion
Specific latent heat of 
vaporization
kJ/kg cal/g kJ/kg cal/g
Water/ Ice 0 100 333 80 2256 540
Copper 1083 2562 134 49 5060 1212
Ethyl alcohol -117 78 104 26 8540 200
Gold 1063 2700 144 15.3 1580 392
Silver 962 2162 88.2 25 2330 564
Lead 327.5 1749 26.2 5.9 859 207
Boiling water + Vapour 
Time (Minutes)
Water
Liquid 
0
C
Ice+ Water
(Solid+liquid)
Use your brain power !
Liquid - Gaseous state 
64
Activity:
1. Put a slab of ice on a stand as shown in Figure 5.3.
2. Hang two  equal weights to the two ends of a metal 
wire and put the wire on the slab as shown in the 
figure.
 What do you observe?
 It is observed that the wire gradually penetrates the ice 
slab. After some time, the wire comes out of the lower 
surface of the ice slab. However, the ice slab does not 
break. The phenomenon in which the ice converts to 
liquid due to applied pressure and then re-converts to 
ice once the pressure is removed is called regelation.
  The melting point of ice becomes lower than 0 
0
C 
 due to pressure. This means that at 0 
0
C, the ice gets 
converted into water. As soon as the pressure is 
removed, the melting point is restored to 0 
0
C and 
water gets converted into ice again. 
2.  Is there any relationship of latent heat with the regelation?
3. You know that as we go higher than the sea level, the boiling point of water decreases
What would be effect on the melting point of solid?
5.3 Regelation
Weight
Ice
Regelation
 You may have seen the preparation of  an ice-ball. First, an ice slab is shredded and 
then the shredded ice is pressurised around the tip of a stick to prepare the ice-ball. How 
does the shredded ice convert into solid ice ball? If two small pieces of ice are taken and 
pressed against each other for a while, they stick to each other. Why does this happen?
 Take a small slab of ice, a thin wire, two identical weights. 
Try this
Use your brain power !
1. In the above experiment, the wire moves through 
the ice slab. However, the ice slab does not break. 
Why?
Can you tell?
Anomalous behaviour of water 
In general, when a liquid is heated up to a certain temperature, it expands, and when 
cooled it contracts. Water, however, shows a special and exceptional behaviour.  If we 
heat water from 0 
0
C  up to 4 
0
C, it contracts instead of expanding. At 4 
0
C its volume is 
minimum (due to contraction). If the water is heated further, it expands and its volume 
increases. The behaviour of water between its temperature from  0 
0
C to 4 
0
C is called 
anomalous behaviour of water. 
If 1 kg of water is heated from 0 
0
C and its volume is plotted as a function of temperature, 
we get the graph, shown in fig 5.4. At 4 
0
C, the volume of water is minimum. It means that 
the density of water is maximum at 4 
0
C.
We feel that some objects are cold, and some are hot. Is this 
feeling related in some way to our body temperature?
65
Specific Volume (cm
3
/g)
Temperature 
o
C
5.4 Graph between the volume and temperature of water 
Study of anomalous behaviour 
of water using Hope’s apparatus.
The anomalous behaviour of 
water can be studied with Hope’s 
apparatus. In Hope’s apparatus, a flat 
bowl is attached to a cylindrical 
container as shown in figure 5.5. There 
is provision to attach thermometers 
above (to measure temperature T
2
) 
and below (to measure temperature 
T
1
) the flat bowl on the cylindrical 
container. Water is filled in the 
cylindrical container and a mixture of 
ice and salt (freezing mixture) is put in 
the flat bowl. 
During the study of anomalous behaviour of water using Hope’s apparatus, temperature 
T
1
 and T
2
 are recorded after every 30 seconds. 
The temperatures are plotted on the Y-axis and the time in minutes on the X-axis. The 
graph is shown in figure 5.6. The graph shows that initially, both the temperatures T
1
 and T
2 
are identical. However, as time passes, temperature T
1
 of water in the lower part of the 
cylinder decreases fast, while, temperature T
2
 of water in the upper part of the cylinder 
decreases comparatively slowly. 
However, once the temperature T
1
 of the lower part reaches 4
 0
C, it remains almost stable 
at that temperature. T
2
 decreases slowly to 4 
0
C. Thereafter, since T
2
 starts changing rapidly, 
it records 0 
0
C first and after that the lower thermometer T
1 
records 0 
0
C temperature. The 
point of intersection of the two curves shows the temperature of maximum density.
 How can we explain these 
observations? Initially, the temperature of 
water in the middle of cylinder lowers due 
to freezing mixture in the outer bowl. Since 
the temperature of water there decreases, 
its volume decreases, and its density 
increases. The water with higher density 
moves downwards. Therefore, the lower 
thermometer T
1
 shows rapid fall in 
temperature and this continues till the 
temperature of water becomes equal to 4 
0
C. When the temperature of water starts 
decreasing below 4 
0
C, its volume increases, 
and density decreases. It, therefore, moves 
in the upward direction. The temperature 
of water in upper part (T
2
), therefore, 
decreases rapidly to 0 
0
C. The temperature 
of water in the lower part (T
1
), however, 
remains at 4 
0
C for some time and then 
decreases slowly to 0
 0
C.
5.5 Hope’s Apparatus
Freezing 
mixture
Thermometer  
T
2
Thermometer 
T
1
4 
0
C  
0
 0
C
Freezing 
mixture
Page 5


62
5. Heat
Ø	 Latent heat               Ø 	Regelation  
Ø	 Anomalous behaviour of water   Ø 	Dew point and humidity
Ø	 Specific heat capacity 
1. What is the difference between heat and temperature?
2. What are the different ways of heat transfer?
In the previous standard, we have learnt about heat and different types of heat transfer. 
We have also performed few experiments related to expansion and contraction of solids, 
liquids and gases. We have learnt about the difference between heat and temperature. We 
have also seen how temperature is measured using  a thermometer.
 Concepts like latent heat of phase transformation, anomalous behaviour of water, 
dew point, humidity, specific heat capacity etc. are related to certain phenomena experi-
enced by us in our day-to-day life. Let us learn more about these concepts. 
1. Take a few pieces of ice in a glass beaker. 
As shown in figure 5.1. 
2. Insert the bulb of a thermometer in ice and 
measure its temperature.
3. Put the beaker on a stand and heat the ice 
using a burner.
4. Record the temperature using the 
thermometer after every minute.
5. As the ice is heated, it starts melting. Stir 
the mixture of ice and water.
6. Continue the heating even after ice starts 
melting.
7. Draw a graph of temperature versus time.
You will observe that the temperature of the mixture remains 0 
0
C till the ice melts 
completely. If we continue heating, even after conversion of all the ice into water, the 
temperature of water starts rising and reaches 100 
0
C. At this temperature water starts 
converting into steam. The temperature of water remains constant at 100 
0
C till all water 
converts into steam. The graph of temperature versus time is shown in figure 5.2.
In this graph, line AB represents conversion of ice into water at constant temperature. 
When ice is heated it melts at 0 
0
C and converts into water at this constant temperature. 
The ice absorbs heat energy during this transition and the absorption of energy continues 
till all the ice converts into water. 
Latent heat
5.1  Latent heat 
Can you recall?
Try this
Thermometer
Ice cubes/pieces
Stand
Burner
63
 The temperature remains constant 
during this transition. This constant 
temperature, at which the ice converts into 
water is called the melting point of ice.
Thus, during transition of solid phase to 
liquid, the object absorbs heat energy, but its 
temperature does not increase. This heat 
energy is utilised for weakening the bonds 
between the atoms or molecules in the solid 
and transform it into liquid phase. The heat 
energy absorbed at constant temperature 
during transformation of solid into liquid is 
called the latent heat of fusion.
The amount of heat energy absorbed at constant temperature by unit mass of  a 
solid to convert into liquid phase is called the specific latent heat of fusion. 
Once all the ice is transformed into water, the temperature of water starts rising. It 
increases up to 100
 0
C. Line BC in the graph represents rise in temperature of water from   
0 
0
C to 100 
0
C. Thereafter, even though heat energy is supplied to water, its temperature 
does not rise. The heat energy is absorbed by water at this temperature and used to break 
the bonds between molecules of the liquid and convert the liquid into gaseous state. Thus, 
during transformation from liquid phase to gas phase, heat energy is absorbed by the 
liquid, but its temperature does not change. The constant temperature at which the liquid 
transforms into gaseous state is called the boiling point of the liquid. The heat energy 
absorbed at constant temperature during transformation of liquid into gas is called the 
latent heat of vapourization.
The amount of heat energy absorbed at constant temperature by unit mass of a 
liquid to convert into gaseous phase is called the specific latent heat of vapourization. 
Different substances have different melting points and boiling points. The values of 
melting point, boiling point and latent heat depend on atmospheric pressure.
1. Is the concept of latent heat applicable during trans-
formation of gaseous phase to liquid phase and from 
liquid phase to solid phase?
2. Where does the latent heat go during these 
transformations?
5.2 Temperature vs Time Graph
Substance
Melting 
point 
0
C
Boiling 
point 
0
C
Specific latent heat of 
fusion
Specific latent heat of 
vaporization
kJ/kg cal/g kJ/kg cal/g
Water/ Ice 0 100 333 80 2256 540
Copper 1083 2562 134 49 5060 1212
Ethyl alcohol -117 78 104 26 8540 200
Gold 1063 2700 144 15.3 1580 392
Silver 962 2162 88.2 25 2330 564
Lead 327.5 1749 26.2 5.9 859 207
Boiling water + Vapour 
Time (Minutes)
Water
Liquid 
0
C
Ice+ Water
(Solid+liquid)
Use your brain power !
Liquid - Gaseous state 
64
Activity:
1. Put a slab of ice on a stand as shown in Figure 5.3.
2. Hang two  equal weights to the two ends of a metal 
wire and put the wire on the slab as shown in the 
figure.
 What do you observe?
 It is observed that the wire gradually penetrates the ice 
slab. After some time, the wire comes out of the lower 
surface of the ice slab. However, the ice slab does not 
break. The phenomenon in which the ice converts to 
liquid due to applied pressure and then re-converts to 
ice once the pressure is removed is called regelation.
  The melting point of ice becomes lower than 0 
0
C 
 due to pressure. This means that at 0 
0
C, the ice gets 
converted into water. As soon as the pressure is 
removed, the melting point is restored to 0 
0
C and 
water gets converted into ice again. 
2.  Is there any relationship of latent heat with the regelation?
3. You know that as we go higher than the sea level, the boiling point of water decreases
What would be effect on the melting point of solid?
5.3 Regelation
Weight
Ice
Regelation
 You may have seen the preparation of  an ice-ball. First, an ice slab is shredded and 
then the shredded ice is pressurised around the tip of a stick to prepare the ice-ball. How 
does the shredded ice convert into solid ice ball? If two small pieces of ice are taken and 
pressed against each other for a while, they stick to each other. Why does this happen?
 Take a small slab of ice, a thin wire, two identical weights. 
Try this
Use your brain power !
1. In the above experiment, the wire moves through 
the ice slab. However, the ice slab does not break. 
Why?
Can you tell?
Anomalous behaviour of water 
In general, when a liquid is heated up to a certain temperature, it expands, and when 
cooled it contracts. Water, however, shows a special and exceptional behaviour.  If we 
heat water from 0 
0
C  up to 4 
0
C, it contracts instead of expanding. At 4 
0
C its volume is 
minimum (due to contraction). If the water is heated further, it expands and its volume 
increases. The behaviour of water between its temperature from  0 
0
C to 4 
0
C is called 
anomalous behaviour of water. 
If 1 kg of water is heated from 0 
0
C and its volume is plotted as a function of temperature, 
we get the graph, shown in fig 5.4. At 4 
0
C, the volume of water is minimum. It means that 
the density of water is maximum at 4 
0
C.
We feel that some objects are cold, and some are hot. Is this 
feeling related in some way to our body temperature?
65
Specific Volume (cm
3
/g)
Temperature 
o
C
5.4 Graph between the volume and temperature of water 
Study of anomalous behaviour 
of water using Hope’s apparatus.
The anomalous behaviour of 
water can be studied with Hope’s 
apparatus. In Hope’s apparatus, a flat 
bowl is attached to a cylindrical 
container as shown in figure 5.5. There 
is provision to attach thermometers 
above (to measure temperature T
2
) 
and below (to measure temperature 
T
1
) the flat bowl on the cylindrical 
container. Water is filled in the 
cylindrical container and a mixture of 
ice and salt (freezing mixture) is put in 
the flat bowl. 
During the study of anomalous behaviour of water using Hope’s apparatus, temperature 
T
1
 and T
2
 are recorded after every 30 seconds. 
The temperatures are plotted on the Y-axis and the time in minutes on the X-axis. The 
graph is shown in figure 5.6. The graph shows that initially, both the temperatures T
1
 and T
2 
are identical. However, as time passes, temperature T
1
 of water in the lower part of the 
cylinder decreases fast, while, temperature T
2
 of water in the upper part of the cylinder 
decreases comparatively slowly. 
However, once the temperature T
1
 of the lower part reaches 4
 0
C, it remains almost stable 
at that temperature. T
2
 decreases slowly to 4 
0
C. Thereafter, since T
2
 starts changing rapidly, 
it records 0 
0
C first and after that the lower thermometer T
1 
records 0 
0
C temperature. The 
point of intersection of the two curves shows the temperature of maximum density.
 How can we explain these 
observations? Initially, the temperature of 
water in the middle of cylinder lowers due 
to freezing mixture in the outer bowl. Since 
the temperature of water there decreases, 
its volume decreases, and its density 
increases. The water with higher density 
moves downwards. Therefore, the lower 
thermometer T
1
 shows rapid fall in 
temperature and this continues till the 
temperature of water becomes equal to 4 
0
C. When the temperature of water starts 
decreasing below 4 
0
C, its volume increases, 
and density decreases. It, therefore, moves 
in the upward direction. The temperature 
of water in upper part (T
2
), therefore, 
decreases rapidly to 0 
0
C. The temperature 
of water in the lower part (T
1
), however, 
remains at 4 
0
C for some time and then 
decreases slowly to 0
 0
C.
5.5 Hope’s Apparatus
Freezing 
mixture
Thermometer  
T
2
Thermometer 
T
1
4 
0
C  
0
 0
C
Freezing 
mixture
66
5.6  Time - Temperature Graph
How will you explain following statements with 
the help of the anomalous behaviour of water?
1.  In regions with cold climate, the aquatic plants and animals can survive even when the  
atmospheric temperature goes below 0 
0
C (See figure 5.7).
2.  In cold regions in winter the pipes for water supply break and even rocks crack. 
Dew point and Humidity
About 71% surface of the Earth is covered with water. Due to constant evaporation of 
water, water vapour is always present in the atmosphere. The amount of water vapour in 
the atmosphere helps us to understand the state of daily weather.  The presence of water 
vapour in the air makes it moist. The moisture in the atmosphere is called humidity.
For a given volume of air, at a specific temperature, there is a limit on how much 
water vapour the air can contain. If the amount exceeds this limit, the excess vapour 
converts into water droplets. When the air contains maximum possible water vapour, it is 
said to be saturated with vapour at that temperature. The amount of vapour needed to 
saturate the air depends on temperature of the air. If air temperature is low, it will need less 
vapour to saturate the air. For example, if temperature of air is 40 
0
C, it can contain 49 
grams of water vapour per kilogram of dry air without condensation. If the amount of 
vapour exceeds this limit, the additional vapour will condense. However, if the temperature 
of air is 20 
0
C, it can contain only 14.7 grams of water vapour per kilogram of dry air 
without condensing. If the vapour contained in air is less that the maximum limit, then the 
air is said to be unsaturated. 
Suppose unsaturated air at a certain temperature is taken and its temperature is 
decreased, a temperature is reached at which the air becomes saturated with vapour. 
This temperature is called the dew point temperature.    
 The vapour content in the air is measured using a physical quantity called  absolute 
humidity. The mass of vapour present in a unit volume of air is called absolute humidity. 
Generally absolute humidity is measured in kg/m
3
.
The feeling of humid or dry nature of air not only depends on the amount of vapour in 
the air, but it also depends on how close that amount is for making the air saturated with 
vapour. It means that it depends on temperature of the air also. 
The ratio of actual mass of vapour content in the air for a given volume and temperature 
to that required to make the air saturated with vapour at that temperature is called the 
relative humidity.
5.7   Aquatic animals in cold regions 
       5    10     15     20    25    30    35   40    45     
Time (Minutes) 
Temperature 
0
C 
Use your brain power !
actual mass of water vapour content in the air in a given volume
mass of vapour needed to make the air saturated in that volume
% Relative humidity =
x 100
T
2
T
1
Ice Layer