This presentation is prepared for the students of grades 11 and 12 concluding the chapter thermodynamics. Proper notes with diagrams, facts, and figures are presented. Numericals are solved too.

2. Unit 2: Heat And Thermodynamics
Heat and Temperature
Thermal Expansion
Quantity of Heat : i) Calorimetry
ii) Change of States
Thermal Properties of Matter : i) Gas Laws
ii) Kinetic Theory of Gases
Hygrometry
Transfer of Heat
First Law of Thermodynamics
Second Law of Thermodynamics

3. Chapter 1 Heat and Temperature
Heat
Heat is a form of energy which gives us the sensation of warmth. The energy which is transferred
from one body to another without any mechanical work involved is known as HEAT. Heat energy is
due to the molecular vibration in the body. On microscopic level heat energy possessed by a body is
the sum of kinetic energy of all molecules that makes up the body.
Every type of motion (translational, vibrational and rotational) provides kinetic energy to the
molecules of the body. The energy associated with configuration and random motion of the
molecules of the body is called internal energy. Heat energy is a part of internal energy which is
transferred form one body to another on account of temperature difference between the two
bodies. Thus, the heat energy is the internal energy of a body in transit.
Heat is a form of energy which is transferred from one body at higher temperature to another body
at lower temperature when they are placed in contact with each other. It is not necessarily a flow
from a body with more heat energy to another with less energy. As heat energy is a total kinetic
energy of all molecules, heat contained in a body depend on number of molecules i.e mass of the
body. The measurement of heat energy gained or lost by a body is carried out in a device called
Calorimeter.
Its S.I unit is Joule and CGS unit is Calorie. 1 cal = 4.2 J. It is also measured in British thermal unit
(B.T.U). 1 B.T.U = 252 calorie.

4. .
Temperature
Temperature is the measurement of degree of hotness or coldness of a body i.e it represents the
amount of heat contained in a body. Temperature of a body is the measure of average kinetic
energy of the molecules contained in the body. As the temperature increase, molecular motion
increase and therefore kinetic energy of molecules increase.
Temperature determines the direction of flow of heat when two bodies with different thermal state
are kept in contact. The bodies having different temperatures have the molecules with different
kinetic energy. So they have different tendency to provide heat energy to other body or to receive
heat energy from other body. In fact, temperature is the measure of ability of a body to transfer
heat to another body. The higher the temperature of a body, the greater is the tendency of that
body to transfer heat.
It is measured with a device called thermometer. Its S.I unit is Kelvin(K). For convenient
measurement, Celsius, Fahrenheit unit are used.

5. Differences between Heat and Temperature
Heat Temperature
1. Heat is a form of energy which gives the sensation of
warmth.
1. Temperature is the degree of hotness or coldness of
a body.
2. It is a measure of total kinetic energy of all
molecules of a body.
2. It is a measure of average kinetic energy of all
molecules of a body.
3. Two bodies in thermal equilibrium may or may not
have the same amount of heat.
3. Two bodies in thermal equilibrium must have same
temperature.
4. Heat is exchangeable. It can flow from one body to
another body
4. Temperature is not exchangeable. Only heat can be
exchanged.
5. Heat flow between two bodies is independent on
the amount of heat contained in the bodies
5. Heat flow between two bodies depends on their
temperature.
6. Heat is not the fundamental property of matter. 6. Temperature is the fundamental property of
matter.
7. Heat transfer is the reason behind the temperature
change
7. Temperature variation can be result of gain or loss
of heat.
8. Heat exchange is measured by calorimeter. It is
measured in Joule in S.I and calorie in CGS system
8. Temperature is measured by thermometer. It is
measured in Kelvin in S.I and ℃ in CGS system.

6. Thermal Equilibrium:
When two bodies at different temperature are brought in thermal contact, heat flow from body at
higher temperature to body at lower temperature and the flow of heat continues till the
temperature of two bodies becomes equal. This condition of equal temperature achieved by two
bodies kept in contact is called thermal equilibrium. Net transfer of heat is zero when two bodies in
contact are in thermal equilibrium.
The principle of thermal equilibrium is applied in the measurement of temperature. For instance, by
placing a thermometer in contact with a body and waiting until the column of liquid in the
thermometer stops rising or falling, we can find the temperature of the body. The reason is that the
thermometer is in thermal equilibrium with the body when they comes in contact with each other.

7. Zeroth Law of Thermodynamics
Statement: Zeroth law states that “If two systems are in thermal equilibrium with a third system,
then they are also in thermal equilibrium with each other”.
Now, if the insulating wall between systems A and B is replaced by conducting wall, there will be no
further change in temperature takes place in systems A and B indicating that they are also in thermal
equilibrium with each other. If TA, TB, and TC be the temperature of systems A, B and C respectively,
then, TA = TC and TB = TC
Hence, TA = TB This experiment shows that if two systems (A and B) are in thermal equilibrium with
the third system C then they are also in thermal equilibrium with each other.
To explain Zeroth Law of thermodynamics, let us consider two
systems A and B initially separated from each other by an
insulating wall but each system is in contact with a third system C
through a conducting wall as shown in fig. The whole system is
surrounded by the insulating wall. If they are left as such for
some time, the system A will be in thermal equilibrium with
system C and similarly system B will be in thermal equilibrium
with system C as they have conducting wall.

8. Concept of Temperature
The concept of thermal equilibrium or Zeroth law gives us the concept of temperature. All bodies
which are in thermal equilibrium have a common property whose value is the same for all the
bodies. We call this property as temperature. Thus, temperature of a system can be defined as the
property that determines whether the system is in thermal equilibrium with the neighbouring
systems or not. So, if a number of systems are in thermal equilibrium, then a common property of
the system can be represented by a single numerical value called Temperature. It means if two
systems are not in thermal equilibrium then they are at different temperatures.
Absolute Zero temperature
The lowest possible temperature of a body at which the entire molecular motion cease is called
absolute zero temperature. At this temperature, kinetic energy of molecules becomes zero but the
intermolecular potential energy is not zero. Since the total energy of a body is sum of kinetic energy
and potential energy of its molecules, absolute zero is not a zero energy temperature. It is
represented as 0 kelvin or -273.15℃.
A graph may be plotted to show how the pressure of a fixed mass of gas varies as the temperature is
changed. It is found that when graph were extrapolated, pressure comes out to be zero at −273.15
℃ and this is the lowest temperature. Lord kelvin suggested that−273.15 ℃ should be regarded as
the zero of the temperature scale.

9. .
Such scale of temperature is absolute scale of
temperature and −273.15 ℃ as absolute zero of this
new scale and is denoted as 0Kelvin.
Thermometry : The branch of physics which deals with the measurement of temperature is
called thermometry. Instrument designed to measure temperature is called thermometer.
Construction of thermometers generally requires a measurable property of a substance which is
sensitive to temperature variation. For example in mercury thermometer, the mercury expand with
increasing temperature.
A thermometer is usually a glass tube essentially consisting of a bulb and capillary tube extending
out from the bulb. The bulb is filled with the thermometric substance which expand over the
capillary tube with the rise in temperature. The rate of expansion being proportional to the rise in
temperature. To measure the temperature, thermometer should be marked with appropriate scale
known as calibration of thermometer.

10. .
For this, two fixed points; ice point and boiling point of water are marked near two end of the
thermometer and the distance between these two point is divided into equal interval.
Advantages of using mercury as Thermometric Substance
Mercury is usually used as thermometric substance in glass thermometers because of the following
reasons:
i. Its boiling point is 357°C and freezing point is – 39°C . So it can measure wide range of
temperature.
ii. It is good conductor of heat and attains the temperature of the body quickly.
iii. It does not wet the wall of the glass tube. This is an important point for the construction of a
thermometer.

11. .
iv. Mercury is shinny and silvery white in colour. Hence the reading can be easily seen.
v. It has a uniform coefficient of expansion and contraction over a wide range of temperature.
vi. It has a low specific heat capacity and hence more sensitive to heat transfer.
vii. Its specific gravity is high. Therefore, even a small bulb can contain sufficient quantity of
mercury.
viii. Due to its cohesive nature, it is generally found in pure form in nature.
Disadvantages of using mercury as Thermometric Substance:
i. Since, freezing point of mercury is only – 39°C. So, it is not useful to measure the temperature
of very cold places like Arctic and Antarctic region. In such place, we use Alcohol Thermometer.
ii. Its expansion is less than alcohol.
Advantages of using Alcohol as Thermometric Substance:
i. Its freezing point is very low i.e – 117°C. So it is suitable in very cold places.
ii. Its expansivity is very large. So it is more sensitive than mercury thermometer.
Disdvantages of using Alcohol as Thermometric Substance:
i. It cannot be used to measure the temperature above 78 °C as its b.p is 78 °C .

12. .
ii. It is colourless, so difficult to observe the reading.
iii. It sticks to the wall of glass tube.
iv. Its expansion is not uniform.
v. The specific gravity of alcohol is smaller than that of mercury.
Fixed point of Thermometer
During calibration of thermometer, two fixed points are marked near two ends in the thermometer.
i) Lower fixed point : It is a temperature of melting point of pure ice at standard atmospheric
pressure.
ii) Upper fixed point : It is a temperature of steam produced from pure boiling water at standard
atmospheric pressure.

13. Temperature Scales
To define temperature scale completely, we require temperatures of two fixed points called Upper
fixed point and lower fixed point.
1. Centigrade(or Celsius) scale: In this scale lower fixed point is marked as 0°C and upper fixed
point marked at 100 °C. The interval between the two fixed points is divided into 100 equal
parts. Each part or division represents 1oC.
2. Fahrenheit scale: In this scale lower fixed point is marked as 32 °F and upper fixed point
marked at 212 °F. The interval between the two fixed points is divided into 180 equal parts.
Each part or division represents 1o F.
3. Reaumur scale: In this scale lower fixed point is marked as 0°R and upper fixed point marked at
80°R. The interval between the two fixed points is divided into 80 equal parts. Each part or
division represents 1oR.
4. Kelvin scale: In this scale lower fixed point is marked as 273K and upper fixed point marked at
373K. The interval between the two fixed points is divided into 100 equal parts. Each part or
division represents 1K.
5. Rankin scale: In this scale lower fixed point is marked as 492°RA and upper fixed point marked
at 672°RA. The interval between the two fixed points is divided into 180 equal parts. Each part
or division represents 1oRA.

14. Temperature Scales: UFP LFP
1. Centigrade Scale 100℃ 0℃
2. Fahrenheit Scale 212℉ 32℉
3. Reaumer Scale 80°R 0°R
4. Kelvin Scale 373K 273K
5. Rankin Scale 672°RA 492°RA

15. If C,F,R,K and RA be the temperature of a body in Celsius, Fahrenheit, Reaumer, Kelvin and Rankin
scale respectively, then we have conversion relation
Temperature −LFP
UFP−LFP
=
C−0
100−0
=
F−32
212−32
=
R−0
80−0
=
K−273
373−273
=
RA −492
672−492
∴
C
100
=
F − 32
180
=
R
80
=
K − 273
100
=
RA − 492
180
Relation between temperature scales
In general, the conversion of one scale into
another is given by the relation,
R1 – L1
U1 – L1
=
R2 – L2
U2 – L2
= 𝒔𝒂𝒎𝒆 𝒇𝒐𝒓 𝒂𝒍𝒍 𝒕𝒆𝒎𝒑 𝒔𝒄𝒂𝒍𝒆𝒔
R1 = temp. reading of 1st temp. scale
L1 = lower fixed point of 1st temp. scale
U1 = upper fixed point of 1st temp. scale
R2 = temp. reading of 2nd temp. scale
L2 = lower fixed point of 2nd temp. scale
U2 = upper fixed point of 2nd temp. scale

16. .
1. At what point of thermometric scale does Kelvin reading coincide with
Fahrenheit scale reading?
Solution:
Let, F and K be the Fahrenheit scale reading and Kelvin scale reading
respectively. Then according to the question, we have, F=K = x (say)
we have,
F−32
180
=
K−273
100
Or,
X−32
180
=
X−273
100
or, 18(x – 273) = 10(x – 32)
or, x = 574.25
Thus, Kelvin scale reading coincides with Fahrenheit scale reading at 574.25 K
or 574.25F.

17. 2. At what temperature will the Kelvin scale reading double the Fahrenheit
reading?
Solution:
Let, the Kelvin scale reading be 2x and Fahrenheit reading be x.
i.e. F = x K = 2x
As we know,
or,
K−273
100
=
F−32
180
or,
2x−273
100
=
x−32
180
or, 9(2x – 273)= 5(x – 32)
or, 18x – 2457 = 5x – 160
or, 13x = 2297
 x= 176.7 and 2x = 353.4
Hence the required temperature is 353.4 K or 176.7F.

18. 3. If a Fahrenheit thermometer reads 100 o when standard centigrade scale thermometer reads 37o
, what is the error in the Fahrenheit thermometer?
Solution:
Correct reading in Celsius (C) = 37o Fault reading in Fahrenheit (F) = 100 o
Correct reading in Fahrenheit (F) = ? error = ?
Now, from conversion relation,
C
100
=
F−32
180
37
100
=
F−32
180
 F = 98.6o
Fault reading in Fahrenheit (F) = 100 o
& Correct reading in Fahrenheit (F) = 98.6o
 Error = Fault reading – Correct reading = F – F = 100 – 98.6 = + 1.4o (Ans)
Similarly, Correction in thermometer =  error =  1.40

19. .
Relation between Difference in Temperature of different Temperature
Scales
∆C
100
=
∆F
180
=
∆K
100
=
∆R
80
=
∆RA
180
Where ∆C, ∆F, ∆K, ∆R and ∆RA represents change in temperature in Celsius, Fahrenheit, Kelvin,
Reaumer and Rankine temperature scales, respectively.
1. What is the temperature change in centigrade degree when the temperature change
in Fahrenheit scale is 72°
Solution: Here, change in temperature in Centigrade scale (C) = ?
change in temperature in Fahrenheit (F) = 72oF
∆C
100
=
∆F
180
Or,
∆C
100
=
72
180
 C = 40C

20. 2. A centigrade and a Fahrenheit thermometer are placed in hot water. The water is then cooled.
What fall of temperature will the Fahrenheit thermometer register, when the centigrade
thermometer records the fall in temperature as 45°.
change in temperature in Celsius (C) = 45°C
change in temperature in Fahrenheit (F) = ?
If C1 and F1 be the initial temperature reading recorded by the centigrade and Fahrenheit thermometers
respectively then we can write
C1
100
=
F1−32
180
Or,
C1
5
=
F1−32
9
……………………………….(i)
Similarly, If C2 and F2 be the final temperature readings recorded by the centigrade and Fahrenheit
thermometers respectively then we can write
C2
100
=
F2−32
180
Or,
C2
5
=
F2−32
9
……………………………….(ii)
Subtracting equation(ii) from equation(i), we get
C1−C2
5
=
F1−F2
9
or,
𝐂
5
=
𝐅
9
∴ F =
𝟒𝟓
5
× 9 = 81°F Hence Fahrenheit thermometer register 81°F fall in temperature.

21. .
Faulty thermometer: A thermometer having wrong calibration is known as faulty thermometer.
Numerical Problems
1. A faulty thermometer has its fixed points marked at –2 and 98. What is the correct temperature
on the Celsius scale when the thermometer reads 20C?
Solution: For faulty thermometer For correct thermometer
Reading R1= 20C Reading R2 = ?
Lower fixed point L1= –2C Lower fixed point L2 = 0C
Upper fixed point U1= 98C Upper Fixed point U2 = 100C
Now, using the relation,
R1 – L1
U1 – L1
=
R2 – L2
U2 – L2
Faulty temperature scale Correct temperature scale
Lower fixed point = L1 Lower fixed point = L2
Upper fixed point = U1 Upper fixed point = U2
Temperature reading = R1 Temperature reading = R2
The conversion relation is
R1 – L1
U1 – L1
=
R2 – L2
U2 – L2

22. .
or,
20 –(–2)
98 –(–2)
=
R2 – 0
100– 0
Or,
22
100
=
R2
100
∴ The correct temperature R2 = 22 C (Ans)
2. A thermometer has wrong calibration. It reads the melting point of ice -12C. It reads 72 C in
place of 60 C. Calculate the temperature of boiling point of water on this scale.
Solution:
For faulty thermometer For correct thermometer
Reading R1= 72C Reading R2 = 60 C
Lower fixed point(m.p of ice) L1= –12C Lower fixed point(m.p of ice) L2 = 0C
Upper fixed point(b.p of water) U1= ? Upper Fixed point(b.p of water) U2 = 100C
Now, using the relation,
R1 – L1
U1 – L1
=
R2 – L2
U2 – L2
72 –(–12)
U1–(–12)
=
60 – 0
100– 0
or,
84
U1+12
=
60
100
∴ U1 = 128 C
The temperature of boiling point of water in faulty thermometer is 128 C (Ans)

23. 3. A centigrade thermometer reads 1 C at melting point of ice and 99 C at the boiling point of
water at normal pressure. What is the correct temperature when it reads 25 C and at what
temperature is its reading exactly correct?
Solution:
For faulty thermometer For correct thermometer
Reading R1= 25C Reading R2 = ?
Lower fixed point L1= 1C Lower fixed point L2 = 0C
Upper fixed point U1= 99C Upper Fixed point U2 = 100C
Now, using the relation,
R1 – L1
U1 – L1
=
R2 – L2
U2 – L2
or,
25−1
99−1
=
R2 – 0
100 – 0
or,
24
98
=
R2
100
∴ R2 =
24
98
× 100 = 24.5 C
Let θ C be the temperature at which reading is correct. Then,
Reading R1= θC Reading R2 = θC
Lower fixed point L1= 1C Lower fixed point L2 = 0C
Upper fixed point U1= 99C Upper Fixed point U2 = 100C
Now, using the relation,
R1 – L1
U1 – L1
=
R2 – L2
U2 – L2
or,
θ−1
99−1
=
θ – 0
100 – 0

24. .
Or
θ−1
98
=
θ
100
or, 100 θ – 100 = 98 θ
Or, 2 θ = 100 ∴ θ = 50 C (Ans)
4. The distance between the upper and lower fixed point is 80cm. Find the temperature on the
Celsius scale if the mercury level rise to a height 10.4cm above the lower fixed point.
Solution: let θ be the required temperature on Celsius scale.
height of Hg level at LFP, H0 =0 cm Temperature Reading θ = ?
height of Hg level at UFP, H100= 80cm Lower fixed point L2 = 0C
height of Hg level above LFP, Hθ =10.4cm Upper Fixed point U2 = 100C
Now using the relation,
Hθ –H0
H100 –H0
=
θ – L2
U2 – L2
or,
10.4−0
80−0
=
θ−0
100−0
Or,
10.4
80
=
θ
100
∴ θ = 13C (Ans)

25. 5. The pressure of air in a constant volume gas thermometer is 80cm and 109.3cm of Hg at 0C and
100C respectively. Find the temperature of hot water bath when bulb is inserted in it and record
the pressure as 100 cm of Hg
Solution: let θ be the required temperature on Celsius scale.
Pressure reading at LFP (P0) = 80cm Temperature Reading θ = ?
Pressure reading at UFP (P100) = 109.3 cm Lower fixed point L2 = 0C
Pressure reading at  (P) = 100 cm Upper Fixed point U2 = 100C
Now using the relation,
Pθ –P0
P100 –P0
=
θ – L2
U2 – L2
or,
100−80
109.3−80
=
θ−0
100−0
or,
20
29.3
=
θ
100
∴ θ = 68.25C (Ans)

26. Types of Thermometers
There are various thermometers which depends on some measurable property of a thermometric
substance which are highly sensitive with temperature variation. For example, liquid thermometer
uses the property of expansion on heating, gas thermometer is based on change in pressure with
temperature at constant volume, variation of electrical resistance with temperature is applied in gas
thermometer etc.
1. Liquid Thermometer: Liquid thermometers are based on the principle of change in volume of
liquid with the change in temperature. The thermometric liquid used in these thermometers
must be highly sensitive to heat so that expansion can be observed in very small rise in
temperature. The commonly used thermometric liquid are mercury, alcohol, ethanol, kerosene
etc. These thermometers can measure the temperature range from -200oC to 600oC. For
example, Mercury thermometer, Alcohol thermometer, ethanol filled thermometer etc.
2. Gas Thermometer: Gas thermometer are based on the principle of variation of pressure or
volume with change in temperature. These thermometers are very sensitive as it has large
expansion. In such thermometers gas like H2, N2, and He are used. Gas thermometers can be
used for wide range of temperature measurement from -270oC to 1600oC. For example, Constant
volume H2 thermometer, Callendar’s constant pressure thermometer etc.

27. .
3. Resistance Thermometer: Resistance thermometer depends on the principle that, the
resistance of a substance changes with temperature when current is flowing through it. In such
thermometer material like Platinum is used whose resistance changes by current flowing
through the platinum wire inside. It can measure the temperature ranging from -250oC to
1200oC. For example, Platinum resistance thermometer.
4. Thermoelectric Thermometer: Thermoelectric thermometer is based on principle of
thermoelectricity i.e production of thermo emf in a thermocouple when its two junctions are
kept at different temperature. This is very sensitive thermometer that provide accurate
temperature result between -250oC to 1600oC. For example, copper constantan thermometer.
5. Radiation Thermometer: Radiation thermometer is based on the quantity of heat radiation
(Infrared) emitted by a body. These are also called Pyrometers used to measure high
temperature or radiation from sun beyond 1250K to 5000oC. It is also used to measure very
high or low surface temperature without any contact with the surface.
6. Vapour Pressure Thermometer: Vapour Pressure thermometer is based on the principle of
change in vapour pressure with change in temperature. It is used to measure low temperature
ranging from -272.5oC to -150oC. For example, Helium vapor pressure thermometer.

28. Short Answer Questions
1. What are the effects of heat?
Ans: when heat is supplied to a body , it can bring about the following effects due to change in
temperature.
i. Rise in Temperature
ii. Change in Volume
iii. Change of State of Matter
iv. Electric Effect
v. Change in Electric Properties
vi. Change in Physical and Chemical Properties
2. Why heat flows from the body at higher temperature to the body at lower temperature?
Ans: The average kinetic energy of the molecules of the body at higher temperature is greater than
that of the body at lower temperature. When this body is brought in thermal contact with the body
at lower temperature, these molecules give up some of their Kinetic energy to less energetic
molecules of the body at lower temperature. This ultimately starts to increase the kinetic energy
and hence the temperature of the body at lower temperature. This phenomenon stops when their
average kinetic energy becomes equal i.e. when both the body reaches to final common
temperature.

29. .
3. Absolute zero temperature is not the temperature of zero energy. Explain.
Ans: The total energy of a body is equal to the sum of total K.E and P.E of its molecules. At absolute
zero temperature (T = 0K = -273.15℃), only the K.E of the molecules is zero but the intermolecular
P.E of the body is not zero and this remaining energy is called zero-point energy. Thus, absolute zero
temperature is not the temperature of zero energy.
4. How can the sensitivity of a thermometer be increased?
Ans : The sensitivity of thermometer can be increased by using following steps:
1. The bulb of thermometer is constructed to have large surface area .
2. The capillary tube is made very thin.
3. By using a liquid of high expansivity.
5. Among two thermometers, one with spherical bulb and another with cylindrical bulb, which
will respond quickly to temperature change?
Ans: The gain or loss of heat by a body is directly proportional to its surface area. ( i.e. Q ∝ A ) . As
the surface area of cylindrical bulb is greater than that of a spherical bulb, for a given volume,
cylindrical bulb will respond quickly to temperature change.
6. what is the temperature of a vacuum?
Ans: According to molecular theory, the temperature of a body is the measure of average kinetic
energy of the molecules. As the vacuum consists of no molecules, the average K.E. of the molecules
is zero. So, the temperature of vacuum is zero. However, no space in the universe is completely free
from radiation. From this view vacuum must have certain temperature which depends on the
energy of radiation.

30. .
7. A thermometer is laid out in direct sun light. Does this thermometer measure the temperature
of the air, of sun or what?
Ans:When a thermometer is laid out in direct sun light, it will not be in thermal equilibrium
with the air around it and sun. Thus, it exchanges heat with its surrounding and tries to be in
thermal equilibrium which makes the thermometric substance to change the reading which
depends on its own temperature. So it does not measure the temperature of air and the sun;
instead it measures the temperature of thermometric substance used in the thermometer.
8. A student claimed that thermometers are useless because a thermometer always registers its
own temperature. How would you respond?
Ans: when a thermometer is used to measure temperature of a body, there will be exchange of
heat between the body and mercury inside the bulb of thermometer until they attain a common
temperature (i.e. thermal equilibrium). It is true that the thermometer registers its own
temperature. But during the measurement, the temperature recorded by thermometer is also the
temperature of the body in thermal equilibrium. Hence student’s claim was wrong that
thermometer are useless.
9. Why a clinical thermometer should not be sterilized by boiling?
Ans: The range of clinical thermometer is usually from 95℉ to 110 ℉ and the boiling point of water
is 212 ℉ (100℃). So on sterilizing by boiling, the capillary of thermometer will burst due to thermal
expansion of mercury in the capillary.

31. .
10.Why water is not used as thermometric substance?
Ans: some reasons for not using water as thermometric substance are:
i. it has high value of specific heat capacity so it is less sensitive to heat.
ii. it does not have uniform expansion as it shows the phenomenon of anomalous behaviour
within temperature range of 0 to 4oC.
iii. it sticks on glass and does not give correct reading.
iv. it is transparent & cannot be seen easily
v. its freezing point is 0oC & boiling point 100oC. So, it can measure the small range of temperature.
11. Gas thermometer is more sensitive than liquid (mercury) thermometer. Why?
Ans: the coefficient of thermal expansion of gas is much larger than that of coefficient of thermal
expansivity of mercury. So there will be much larger change in volume of gas as compared to
mercury for same rise in temperature.
12. Mercury boils at 357oC. How can then a mercury thermometer be used to measure temperature
up to 500oC?
Ans: The boiling point of liquid rise with increase in pressure. If the space above the mercury
column inside the thermometer is filled with nitrogen gas at high pressure, the boiling point of
mercury rise above 500oC. In this way it is possible to measure the temperature up to 500oC using
mercury thermometer.

32. .
13. Can the temperature of a body be negative on the Kelvin scale?
Ans: NO. This is because the absolute zero on the kelvin scale is the minimum possible temperature.
If the temperature falls from absolute zero, volume of gas will be negative which is impossible.
14. Explain the significance of Kelvin temperature scale.
Ans: Kelvin temperature is the absolute temperature scale which is independent on the
thermometric properties of any particular substance. While the other temperature scales depends
on the specific property of the thermometric substance used.
15. The triple point of water is a standard fixed point in modern thermometry. Why?
Ans: The triple point of water is unique i.e it occurs at particular pressure and temperature (0.46 cm
and 273.15K). But the melting point of ice and boiling point of water changes due to presence of
impurities in water. Hence triple point of water chosen as standard fixed point in modern
thermometry.

33. Numerical Questions
1. Convert the following temperatures:
i. 98.4℉ into ℃
ii. - 80 ℃ into ℉
iii. 5000 ° RA into ℉
iv. -460 ℉ into K
v. 36 °R into ℃
(Ans : i. 36.9 ℃, ii. -112 ℉, iii. 4540 ℉, iv. 0K, v. 45 ℃)
2. At what temperature Celsius and Fahrenheit scale coincide? (Ans : - 40 °)
3. Calculate the temperature on absolute scale whose value is twice the reading on centigrade
scale. (Ans: 546K)
4. At what temperature will the reading on Fahrenheit thermometer be double than that of the
centigrade one? (Ans: 160 ℃ or 320 ℉)
5. A faulty Fahrenheit thermometer shows 206 ℉ where as a centigrade thermometer register 96
℃. Find the correction for the faulty thermometer. (Ans: -1.2 ℉)

34. .
6. What was the temperature change in Celsius degrees if it is changed from 44.0oF to – 56.0oF
Solution:
• Here, change in temperature in Fahrenheit (F) = [(– 56.0) – 44.0] oF = - 100 oF
change in temperature in Centigrade scale (C) = ?
We have,
∆C
100
=
∆F
180
Or,
∆C
100
=
−100
180
 C = 55.6oC (Ans)
7. Two beaker of water A and B initially are at same temperature. The temperature of the water in
beaker A is increased by 10 oF and the temperature of the water in beaker B is increased by 10K.
After the temperature changes, which beaker of water has the higher temperature? (Ans
beaker B)
Hint: F = 10 oF and K = 10K
Convert K = 10K into oF and the result is 18 oF
So temperature of beaker A increased by 10 oF and that of B increased by 18 oF.
Hence beaker B has higher temperature.

35. .
8. A faulty thermometer has its fixed points marked as 5 ℃ and 95 ℃. What is the correct
temperature in Celsius when this thermometer reads 95 ℃? (Ans: 100 ℃)
9. A faulty Celsius thermometer reads -4 ℃ when placed in melting ice and 98 ℃ when placed in
contact with steam at normal pressure. What is the correct temperature on Celsius scale when
this thermometer reads 32°? (Ans: 35.3 ℃)
10. A centigrade thermometer reads 5 ℃ at melting point of ice and 90 ℃ at boiling point of water
at normal pressure. What is the correct temperature when it reads 20 ℃, and at what
temperature is its reading exactly correct? ( Ans: 17.65 ℃, 33.33 ℃)
11. The temperature of melting ice and that of steam above water boiling at normal pressure are
marked 40 and 200 respectively on a certain thermometer. What is i) the temperature in ℃
when the reading on this thermometer is 80 ii) the thermometer reading when the
temperature is 60 ℃? ( Ans: i) 25 ℃ ii)136 )
12. A constant volume gas thermometer shows pressure reading of 50cm and 90 cm of mercury at
0 ℃ and 100 ℃ respectively. Find the temperature reading when the pressure reading is 60cm
of mercury. (Ans: 25 ℃)