Living Science Physics Class 7 ICSE Solutions Chapter 5 Heat

ICSE Living Science Physics for Class 7 Solutions Chapter 5 Heat

Check Your Progress (Page 62)

A. Write true or false. Correct the false statements.

Question 1.
There is no flow of heat energy between two objects at the same temperature.
Answer:
True.

Question 2.
Substances that can catch fire are called nonflammable substances.
Answer:
False.
Substances that can catch fire are called inflammable substances.

Question 3.
In the Celsius scale, the lower fixed point is 273 K and the upper fixed point is 373 K.
Answer:
False. In the Celsius scale, the lower fixed point is 0 °C and the upper fixed point is 100 °C.

Question 4.
In the Fahrenheit scale, the lower fixed point is 32 °F and the upper fixed point is 212 °F.
Answer:
True.

Question 5.
The temperature on the Kelvin scale can be obtained from the temperature on the Celsius scale by using the relationship K = C + 273.
Answer:
True.

B. Answer the following.

Question 1.
Under what condition does heat energy flow from one object to the other ?
Answer:
Heat energy always flows from a body at higher temperature to a body at lower temperature, when brought in contact.

Question 2.
Differentiate between inflammable and nonflammable substances.
Answer:
Substances that catch fire easily are called inflammable substances. On the other hand, substances that do no catch fire easily are called nonflammable substances.

Question 3.
Describe the Fahrenheit scale.
Answer:
In the Fahrenheit scale, the lower fixed point is 32 °F, the upper fixed point is 212 °F, and the length of the thermometer between the lower fixed point and the upper fixed point is divided into 180 equal parts.

Question 4.
Electricity gives us heat in many ways. Explain with the help of examples.
Answer:
Electricity gives off heat in many ways. For example, an induction plate produces heat to cook food, a tea or coffee maker provides heat to boil water, a room heater or convector produces heat to warm up the air in a room and so on.

Question 5.
At what temperature, is the reading on the Celsius scale twice that on the Fahrenheit scale?
Answer:
We know, F = \(\frac {9 x C}{5}\) + 32. When Fahrenheit reading is double than Celsius reading,
F = 2C
2C = \(\frac {9 x C}{5}\) + 32
2C – 32 = \(\frac {9C}{5}\)
5 x (2C – 32) = 9C
10C – 160 = 9C
10C – 9C = 160
C = 160 °C
Therefore, when the reading on the Celsius scale is 160 °C, the reading on the Fahrenheit scale will be 320 °F.

Check Your Progress (Page 65)

A. Fill in the blanks.

Question 1.
The expansion produced in matter due to the absorption of heat is known as ……………. expansion.
Answer:
The expansion produced in matter due to the absorption of heat is known as thermal expansion.

Question 2.
Liquids ………….. more than solids on heating.
Answer:
Liquids expand more than solids on heating.

Question 3.
Water changes into steam at ________ on heating.
Answer:
Water changes into steam at 100 °C on heating.

Question 4.
For a given substance, the boiling point of its liquid state and the …………. point of its gaseous state are same.
Answer:
For a given substance, the boiling point of its liquid state and the condensation point of its gaseous state are same.

Question 5.
The reverse process of evaporation is called –
Answer:
The reverse process of evaporation is called condensation.

B. Look at the given figure and answer the questions.

Question 1.
Identify the processes A and B.
Answer:
Process A : Melting
Process B : Boiling

Question 2.
Name the temperatures at which processes A and B take place.
Answer:
Process A takes place at 0 °C while process B takes place at 100 °C.

Question 3.
The reverse of process A is known by which term?
Answer:
Freezing.

Question 4.
What is the reverse of the process B called?
Answer:
Condensation.

Question 5.
Name the process during which steam (gas) changes into water (liquid).
Answer:
Condensation.

Think And Answer (Page 69)

Is convection possible in solids? Give reason.
Answer:
As molecules in solids are tightly packed, actual movement of molecules is not possible in solids. Hence, convection does not take place in solids. So, heat transfer by convection takes place only in fluids, that is, in liquids and gases.

A. Tick the most appropriate answer.

Question 1.
Which of the following substances is inflammable?
a. asbestos
b. brick
c. water
d. LPG
Answer:
d. LPG

Question 2.
The SI unit of temperature is –
a. degree Celsius
b. degree Fahrenheit
c. calorie
d. kelvin
Answer:
d. kelvin

Question 3.
The temperature at which a substance in its vapour state changes into its liquid state is called –
a. sublimation point
b. vaporization point
c. freezing point
d. condensation point
Answer:
d. condensation point

Question 4.
An example of a substance that sublimes is –
a. fluorine.
b. bromine.
c. chlorine.
d. iodine.
Answer:
d. iodine.

Question 5.
Heat transfer in solids takes place through –
a. convection.
b. radiation.
c. conduction.
d. vaporization
Answer:
c. conduction.

Question 6.
The mode of transfer of heat that requires no material medium is –
a. convection.
b. radiation.
c. conduction.
d. sublimation
Answer:
b. radiation.

Question 7.
A thermos flask minimizes the flow of heat due to
a. convection.
b. radiation.
c. conduction.
d. all of these
Answer:
d. all of these

B. Fill in the blanks.

Question 1.
Substances that catch fire easily are called …………. substances.
Answer:
Substances that catch fire easily are called inflammable substances.

Question 2.
………….. is the measurement of the degree of hotness or coldness of a body.
Answer:
Temperature is the measurement of the degree of hotness or coldness of a body.

Question 3.
Transfer of heat between two molecules without their actual movement is called ………….
Answer:
Transfer of heat between two molecules without their actual movement is called conduction.

Question 4.
The process during which a liquid changes to its solid state is known as ………….. or …………
Answer:
The process during which a liquid changes to its solid state is known as freezing or solidification.

Question 5.
Black objects are good of heat.
Answer:
Black objects are good absorbers of heat.

Question 6.
Heat transfer by convection is possible in only.
Answer:
Heat transfer by convection is possible in fluids only.

Question 7.
An appliance used to keep hot liquids hot and cold liquids cold is called a ………….
Answer:
An appliance used to keep hot liquids hot and cold liquids cold is called a thermos flask.

C. Match the columns.

Solid to liquid	a. vaporization
2. Liquid to vapour	b. condensation
3. Solid to vapour	c. fusion
4. Vapour to liquid	d. solidification
5. Liquid to solid	e. sublimation

Answer:

1.Solid to liquid	c. fusion
2. Liquid to vapour	a. vaporization
3. Solid to vapour	e. sublimation
4. Vapour to liquid	b. condensation
5. Liquid to solid	d. solidification

D. Define the following terms.

Question 1.
Melting
Answer:
The process during which a solid changes to a liquid state at a constant temperature by absorption of heat.

Question 2.
Boiling
Answer:
The process during which a liquid changes to vapour state at a constant temperature by absorption of heat.

Question 3.
Condensation
Answer:
The process during which a substance in its vapour or gaseous state changes into its liquid state at a constant temperature by release of heat.

Question 4.
Solidification
Answer:
The process during which a liquid changes to solid state at a constant temperature by release of heat energy.

Question 5.
Sublimation.
Answer:
The process by which certain substances which are solid directly changes into vapours, without changing into liquid state.

Question 6.
Evaporation
Answer:
The change of a substance from its liquid state to its gaseous state at any temperature below its boiling point is called evaporation.

Question 7.
Conduction
Answer:
Conduction is the process of transfer of heat energy in solids, from one particle to another, without the actual movement of the particles.

Question 8.
Convection
Answer:
Convection is the process of transfer of heat in fluids (liquids and gases) wherein the molecules actually move from one part of the fluid to the other.

E. Answer the following in short.

Question 1.
Write any two precautions that we need to take while handling fire.
Answer:
Some precautions that need to be taken while handling fire are as follows:
(a) Any inflammable object such as plastics, wood, clothes and paper should not be left close to fire/electrical switches.
(b) LPG gas pipes/cylinders/regulators should be checked regularly for any leakage.

Question 2.
State any three effects of heat.
Answer:
Effects of heat are :

• Change in temperature.
• Change in dimensions.
• Change in state.

Question 3.
Name the different modes of transfer of heat. Which one is the fastest?
Answer:
Heat can be transmitted in three ways :

• Conduction.
• Convection.
• Radiation.
  Fastest mode is Radiation.

Question 4.
Write two differences between conduction and .convection.
Answer:
Differences between conduction and convection are :
Conduction :

• It is the process of transfer of heat from one particle to the other, without the actual movement of the particles.
• Conduction takes place in solids only.

Convection:

• It is the process of transfer of heat wherein the particles actually move from one part to the other.
• Convection takes place in fluids, i.e., liquids and gases.

Question 5.
Write any two applications of convection currents in air.
Answer:
Applications of convection currents in air are as follows :
(a) In nature, sea breeze and land breeze are examples of convection currents set up in air in coastal areas. .
(b) Chimneys are fitted in factories to remove undesirable smoke and fumes. Being hot, and thus lighter, these gases rise up and escape through the chimneys.

F. Answer the following in detail.

Question 1.
Describe the different temperature scales.
Answer:
There are three different scales to measure temperature. These are the Celsius scale, the Fahrenheit scale and the Kelvin I scale.

Celsius scale : In the Celsius scale, the lower fixed point is j 0 °C, the upper fixed point is 100 °C, and the length of the thermometer between the lower fixed point and the upper j fixed point is divided into 100 equal parts.

Fahrenheit scale : In the Fahrenheit scale, the lower fixed point is 32 °F, the upper fixed point is 212 °F, and the length of the thermometer between the lower fixed point and the 1 upper fixed point is divided into 180 equal parts.

Kelvin scale : In the Kelvin scale, the lower fixed point is 273 K, the upper fixed point is 373 K, and the length of the thermometer between the lower fixed point and the upper fixed point is divided into 100 equal parts.

Question 2.
Explain the Gravesand’s ring-and-ball experiment to describe thermal expansion in solids.
Answer:
Gravesand’s ring-and-ball experiment: Take an iror*ball and an iron ring such that ball can just pass through the ring. Take a metal stand with a hook and a burner. Suspend ball by chain from hook. Heat the ball and try passing through the ring. Now, cool it and try again.

Observation : When ball is heated, it doesn’t pass through the ring due to thermal expansion. When it is cooled, it regains its original size and easily passes through it. This shows solids expand on heating.

Question 3.
Differentiate between thermal conductors and insulators.
Answer:
Substances that allow heat energy to flow through them easily are called good conductors of heat or thermal conductors. Some good conductors of heat are copper, aluminium, gold and iron. On the other hand, substances that do not allow heat energy to flow through them easily are called bad conductors of heat or thermal insulators. Glass, mica, cotton, cork, wood, asbestos, plastics and ebonite are some of the bad conductors of heat.

Question 4.
Water is a bad conductor of heat but it becomes hot when kept over a flame. Name the phenomenon that takes place in this case. Explain it with the help of an experiment.
Answer:

• Half fill a test-tube with cold water.
• Wrap a piece of ice in wire gauze and drop it in test tube. It will sink to the bottom.
• Now, heat the top end of test tube. The water soon begins to boil at the top. But ice below it still not melted.

This shows water is bad conductor of heat. It does not easily conduct heat from top to bottom of test tube. But water becomes hot when it is kept over a flame because of convection.

Activity :
Aim : To show convection currents in water.
Materials Required: A large potassium permanganate crystal, a beaker, a burner and some water.
Procedure :
(a) Drop the potassium permanganate crystal into the beaker containing a little water. The crystal starts dissolving in , water and colouring it.
(b) Now pour some more water slowly into the beaker without disturbing the crystal.
(c) Heat the water using a very small flame just below the crystal.

Observation : The coloured water rises from the place where the heat is supplied. After moving up some distance, it spreads out and comes down along the sides of the : beaker.

Question 5.
Describe the construction of a thermos flask with the help of a neat diagram and label it.
Answer:
A thermos flask consists of a double-walled glass or metal bottle. The narrow region between the inner and the outer walls is evacuated and sealed together at their necks or at the bottom. The walls are silvered. The vessel is put in a metallic or plastic case and is separated from it by a cork or a plastic piece to prevent it from breaking. The mouth of the vessel is closed by a stopper made from an insulating material.

The vacuum between the glass walls of the thermos flask considerably reduces the flow of heat by conduction and convection. This is because both conduction and convection need a material medium for transfer of heat. The silvered surfaces reflect the heat back, thus reducing the heat exchange due to radiation. This causes the liquid in a thermos flask to stay hot or cold for a longer period of time.

G. Give reasons for the following.

Question 1.
A metal spoon is heated at one end and the heat is felt at the other end after a while.
Answer:
It is due to conduction. This happens because heat energy is transmitted from the heated end of the spoon to the other end without actual movement of the particles of the spoon.

Question 2.
The bottom of a cooking utensil is blackened while its inner part is made shining.
Answer:
The base of cooking utensils is made black to increase the intake of heat from the flame. The inside of cooking utensils is generally kept shiny to minimize the loss of heat by radiation.

Question 3.
Two thin blankets layered together are warmer than one thick blanket.
Answer:
Two thin blankets trap air in between our body and themselves. The trapped air, being a bad conductor of heat does not allow heat from our body to escape to the surroundings and thus we stay warm.

Question 4.
Ice wrapped in a wire gauze placed in water does not ‘ melt even when the water boils.
Answer:
It is because water is a bad conductor of heat. It does not easily conduct heat from the top to the bottom of the test tube.

Question 5.
Outer surface of the inner wall and inner surface of the outer wall of a thermos flask are silvered. ‘
Answer:
The silvered surfaces reflect the heat back, thus reducing the heat exchange due to radiation and liquid in the thermos flask stays hot or cold for a longer duration.

H. Solve the following numerical problems.
1. Convert
a. 65°F into °C
b. 37°C into °F
c. 273 K into °C
d. 96°F into K
Answer:
(a) C = \(\frac {5}{9}\)(F – 32)
= \(\frac {5}{9}\)(65 – 32)
= \(\frac {5}{9}\) x 33
= 18.33 °C

(b) F = \(\frac {9}{5}\) x C + 32
= \(\frac {9}{5}\) x 37 + 32
= 66.6 + 32 = 98.6 °F

(c) K = C + 273
C = K – 273 = 273 – 273 = 0°C

(d) First convert 96 °F into °C
C = \(\frac {5}{9}\) (F – 32)
= \(\frac {5}{9}\)(96 – 32) = \(\frac {5}{9}\) x 64
= 35.56 °C
We know,
K = C + 273 = 35.56 + 273 = 308.56 K

Question 2.
The temperature of milk in a glass is 50 °C. Express this temperature in the Fahrenheit and the Kelvin scales.
Answer:
F = \(\frac {9}{5}\) x C + 32
= \(\frac {9}{5}\) x 50 + 32 = 90 + 32 = 122 °F
We also know that
K = C + 273 = 50 + 273 = 323 K

Question 3.
The temperature of water in a bucket is 40 °C. Find its , value in the SI unit.
Answer:
SI unit of temperature is kelvin (K).
K = C + 273 = 40 + 273 = 313 K

Question 4.
The average body temperature of a normal human body is 98.6 °F. A patient checks his body temperature, which reads 38 °C. Help this patient to convert his body temperature in °F. Does he have fever?
Answer:
F = \(\frac {9}{5}\) x C + 32
= \(\frac {9}{5}\) x 38 + 32
= 68.4 + 32 = 100.4 °F
Yes, the patient has fever as the normal body temperature of humans is 98.6 °F.

Question 5.
At what temperature will the reading on the Fahrenheit scale be double of that on the Celsius scale ?
Answer:
Let reading on the Celsius scale = x
Let reading on the Fahrenheit scale = 2x
Using the formula,
C = \(\frac {5}{9}\) (F – 32)
Substitute values of C and F by x and 2x
x = \(\frac {5}{9}\) (2x – 32)
x = \(\frac {10}{9}\)x – \(\frac {160}{9}\)
x – \(\frac {10}{9}\)x = \(\frac {-160}{9}\)
\(\frac {9x – 10x}{9}\) = \(\frac {-160}{9}\)
– \(\frac {1x}{9}\) = \(\frac {-160}{9}\) x \(\frac {-9}{1}\) = 160
At 160 °C, reading on the Fahrenheit scale will be double of that on the Celsius scale.

Think And Answer

Question 1.
Why is asbestos used in fire-proof buildings?
Answer;
Asbestos is used in fire-proofing buildings because it is a non-flammable material and does not catch fire.

Question 2.
Why is water used as one of the main substances for extinguishing fire?
Answer:
Water is easily available everywhere and helps in extinguishing fire as it cuts off oxygen supply to the fire and stops combustion.

Question 3.
Why are wood, kerosene oil and petrol used as common fuels?
Answer:
Wood, kerosene oil and petrol are used as fuels because they are inflammable substances that bum easily and release a lot of heat and energy.

Living Science Physics Class 7 ICSE Solutions