# WBBSE Class 8 Science Solutions Chapter 1.3 Heat

Detailed explanations in West Bengal Board Class 8 Science Book Solutions Chapter 1.3 Heat offer valuable context and analysis.

## WBBSE Class 8 Science Chapter 1.3 Question Answer – Heat

Multiple Choice Questions & Answers : (1 mark for each question)

Question 1.
The quantity of heat gained by a body depends on-
i) mass of the substance
ii)-nature of the substance
iii) rise in temperature of the substance
iv) on all the above three factors
iv) on all the above three factors

Question 2.
SI unit of heat is –
i) a) Calorie
ii) Joule
iii) Kelvin
iv) Calorie/joule
ii) Joule

Question 3.
The form of energy that produces feeling of hotness is called as –
i) work
ii) energy
iii) heat
iv) none of the above
iii) theat

Question a.
Heat necessary to increase 1° C temperature of 10 gm water-
i) 1 Calorie
ii) 1 Joule
iii) 10 Calorie
iv) 10 Joule
iii) 10 Calorie

Question 5.
Which of the following has least specific heat capaticy-
i) water
ii), iron
iii) milk
iv) copper
iv) copper Question 6.
Which of the following relations is correct?
i) Q = $$\frac{r}{mt}$$
ii) r = $$\frac{q}{mt}$$
iii) t = $$\frac{mq}{r}$$
iv) m = $${mq}{r}$$
I Where Q is the quantity of heat absorbed or given out, m is the mass of the body, r is specific heat capacity, t is change in temperature]
ii) r = $$\frac{q}{mt}$$

Question 7.
Two objects of iron and copper of same mass and same amount is given same heat, which object will became heated up quickly.
i) both at same time
ii) iron will heat up quickly
iii) copper willegeat up quickly
iv) we cannot tell definitely
iii) copper will heat up quickly

Question 8.
In which season evaporation rate is high?
i) rainy season
ii) summer season
iii) winter season
iv) in all seasons
ii) summer season

Question 9.
If impurities are mixed with pure substance then-
i) its boiling point would remain same
ii) its boiling point will decrease
iii) its boiling point will increase
iv) its boiling point may some time increase or decrease
ii) its boiling point will decrease Question 10.
If area of upper surface of any liquid increase-
i) evaporation increase
ii) evaporation decrease
iii) boiling point increase
iv) boiling point decrease
i) evaporation increase

Question 11.
Which of the following is fastest?
i) evaporation
ii) boiling
iii) none of the above
iv) both are same
ii) boiling

Question 12.
Boiling occures-
i) at any temperature and any pressure
ii) at fixed pressure specific temperature
iii) at any pressure fixed temperature
iv) none of the above
ii) at fixed pressure specific temperature Question 13.
If at 0° C temperature, 80 calorie heat is given to 1 g ice then temperature would be-
i) 0° C
ii) more than 0° C
iii) Less than 0° C
iv) 10° C
i) 0° C

Question 14.
Transformation of liquid from solid is called-
i) melting
ii) boiling
iii) condensation
iv) freezing
i) melting

Question 15.
Which of the following has highest heat capacity-
i) soil
ii) air
iii) water
iv) none of the above
iii) water Question 16.
– is neither created nor destroyed, it can only changed one form to another.
i) mass of body
ii) heat
iii) work
iv) energy
iv) energy

State whether True or False : (1 mark for each question)

1) Glass, fat, wax have fixed melting or freczing points.
2) Melting and freezing point of ice is 0° C.
3) Melting point of mercury is 0° C.
4) If ice melts into water its volume increase.
5) A liquid gives up heat when it freexes into a solid.
6) If pressure increase melting point of a substance increase.
7) Transformation of liquid from vapour is called condensation.
8) Boiling point of a liquid does not depend on the area of upper surface.
9) Graphite is bad conductor of heat.
10) In industrial belt fog is mostly seen at the time of winter.
11) Density of hot air is less.
12) melting point of wax is fixed.
13) At 0° C temperature if 80 calorie heat is given to 1 g ice temperature remains uncharged.
14) Volume increases during melting of cast iron.
15) Pressure of vapour in a closed container increases with the increase in the amount of vapour inside.
16) Condensation of water vapour in air produces cloud.
17) Builfing materials should be good conductor of heat.
18) Glass is a bad conductor of heat.
1) false
2) true
3) false
4) false
5) true
6) false
7) true
8) true
9) false
10) true
11) true
12) false
13) true
14) false
15) true
16) true
17) false
18) true Fill in the blanks : (1 mark for each question)

1) Heat is measured with __________.
2) SI unit of heat __________.
3) Heat is a kind of __________.
4) In solid heat is transfered by __________ process.
5) ________ has highest specific heat capacity.
6) Heat _________ the volume of any object__________.
7) Heat comes from sun to earth by __________ process.
8) Dew is the example of natural __________.
9) Specific heat of water __________.
10) The amount of Heat required to increase 1° C temperature of 1 g water is __________.
11) Heat lost by the hot body = Heat __________ by the cold body.
12) If __________ increase boiling capacity increases..
13) Heat capacity = mass of the body x __________.
14) In conduction heat is transferred by __________.
15) The density of the solid ice is less than the __________ water.
16) The process in which a liquid changes very rapidly into its __________ is called boiling.
17) A __________ is necessary for the floe of heat through conduction and conversation.
18) in __________ hot liquid remains hot and cold liquid renains cold for a long time.
19) The houses made up by hay and mud are __________ in winter and __________ in summer.
1) Caloriemeter
2) Joule
3) energy
4) conduction
5) water
6) increases
8) condensation
9) 1 cal/g° C
10) 1 calorie
11) gained
12) pressure
13) specific heat
14) molecules
15) density
16) vapour
17) medium
19) warm, cool Match the following : (1 mark for each question)

a) Match the column A with column B.

 Column A Column B a) Heat is a kind of i) to produce a freezing mixture. b) Condensation of water vapour ii) is 1083°C c) Sprecific heat of Lead is iii) different boiling points. d) Melting point of wax iv) less than water. e) In winter wet clothes dry up v) bad conductor of heat. f) Ice and salt are mixed in a fixed ratio vi) in air produces cloud, dew, fog etc. g) Freezing point of copper vii) dyne h) Different liquids have viii) is not fixed. i) Boiling point of water is ix) quicker than rainy season. j) Glasswool is x) 100°C

a) Heat is a kind of energy.
b) Condensation of water vapour in air produces cloud, dew, fog etc.
c) Specific heat of Lead is less than water.
d) Melting point of wax is not fixed.
e) In winter wet clothes dry up quicker than rainy season.
f) Ice and salt are mixed in a fixed ratio to produce a freezing mixture.
g) Freezing point of copper is 1083° C
h) Different liquids have different boiling points.
i) Boiling point of water is 100° C
j) Glasswool is bad conductor of heat. b) Match the column A with column B.

 Column A Column B a) Boiling point of salty water is more i) the pull of gravity. b) Spirit is a ii) a thin piece of paper. c) Formation of dew is an example of iii) than pure water. d) Heat flow is prevented fully iv) through radiation. e) Inside a solid substance v) volatile substance. f) Heat comes from sun towards carth vi) is greater, it evaporates quickly. g) Heavier air comes downward due to vii) in a thermoflask. h) Heat can easily flow through a viii) natural condensation. i) When the exposed area of a liquid ix) heat is transmitted by conduction. Column A Column B

a) Boiling point of salty water is more than pure water.
b) Spirit is a volatile substance.
c) Formation of dew is an example of natural condensation.
d) Heat flow is prevented fully in a thermoflask.
e) Inside a solid substance heat is transmitted by conduction.
f) Heat comes from sun towards earth through radiation.
g) Heavier air comes downward due to the pull of gravity.
h) Heat can easily flow through a a thin piece of paper.
i) When the exposed area of a liquid is greater, it evaporates quickly.

Very Short Questions & Answers : (1 mark for each question)

Question 1.
What is heat?
Heat is a kind of energy.

Question 2.
Which instrument is used to measure heat?
Heat is measured with caloriemeter. Question 3.
What is SI unit of heat?
SI unit of heat is joule.

Question 4.
What is CGS unit of heat?
CGS unit of Heat is calorie.

Question 5.
What is the process of solid, changing into liquid called?
Melting or fusion.

Question 6.
What is the process of liquid substance changing into solid state called?
Freezing or solidification.

Question 7.
Write the name of a substance whose melting point is not fixed.
Wax. Question 8.
Write the name of a substance which increase in volume after melting.
Gold increase in volume after melting.

Question 9.
Write a name of a substance which decrease in volume after melting.
Ice decrease in volume after melting.

Question 10.
If pressure increase boiling point of liquid increase- using this principle an instrument is made. Write its name.
Pressure Cooker.

Question 11.
Name a volatile substance.
Ether is a volatile substance.

Question 12.
What is the melting point of mercury?
Melting point of mercury is -39.5° C. Question 13.
Give an example of natural process of condensation.
A natural process of condensation is formation of dew.

Question 14.
By which process does heat come to earth from sun?
Heat comes to earth from sun by radiation.

Question 15.
How transfer of heat occurs in liquid?
through convection.

Question 16.
Why are Igloos made of ice?
Ice is bad conductor of heat. inside of Igloo is warmer then the outer environment.

Question 17.
Write the name of two substances which are bad conductor?
Diamond and glass are two bad conductor.

Question 18.
British scientist Sir James Dewar invented thermoflask.

Question 19.
What is the boiling point of liquid hydrogen?
The boiling point of liquid hydrogen is -253° C.

Question 20.
If pressure increases does it help or hinder the boiling process?
The process of boiling is hindered by excess pressure. Question 21.
When a liquid evaporates quickly?
When the exposed area of a liquid is greater, it evaporates quickly.

Question 22.
What is conduction?
The process by which heat flows through a solid substance is called conduction.

Question 23.
What is good conductor of heat?
The substances through which heat can flow easily are called good conductors of heat.

Question 24.
What is bad conductor of heat?
The substances through which heat cannot flow easily are called bad conductors of heat.

Question 25.
In which layer of water do the aquatic animals feel more comfortable during summer?
In lower layer of water acquatic animals feel more comfortable during summer. Short Questions & Answers : (2 marks for each question)

Question 1.
What is the specific heat capacity of a substance?
The specific heat capacity of a substance is defined as the amount of heat required to change the temperature of unit mass of the substance by one unit.

Question 2.
What is the CGS and SI unit of specific heat capacity?
In CGS system, specific heat capacity is measured in joules per gram per degree and in SI system. specific heat capacity is measured in joules per kilogram per degree. Question 3.
Why evaporation of water is faster when poured in a plate than a glass?
More exposed the area of a liquid is more quickly does it evaporate. Surface area of plate is greater than glass. So evaporation of water is faster when poured in a plate than a glass, as plate has greater surface area.

Question 3.
Why it takes much time to dry cloth in rainy season?
Humidity is greater in rainy season so relative humidity is also greater. So rate of evaporation is slow. As rate of evaporation decreases water of wet cloths does not evaporate quickly so it takes much time to dry cloth in rainy season.

Question 4.
What is latent heat?
Latent Heat of a substance is defined as the amount of heat required to change the state of the substance without any change in temperature. Question 5.
What is freezing or solidification? Is the melting point and the freezing point of a substance same?
The freezing point or the solidification point is the fixed temperature at which a liquid starts changing to its solid state.
For a pure substance, the melting point and the freezing point are the same.

Question 6.
Latent heat of fusion of ice is 80 calorie/g, what does this signify.
This signifies under standard pressure 80 calorie heat has to be given to ice of mass 1 g and temperature 0° C to convert it into water of mass g and temperature 0° C.

Question 7.
The latent heat of stcam is 537 calorie/g, what does this signify.
It signifies that under the standard pressure at 100° C, 537 calorie heat has to be rendered to 1 g of water to convert it into steam of 1 g.

Question 8.
Why are burns caused by steam more painful than those caused by hot water at same temperature?
1 g of steam at 100° C releases 537 calorie of heat. For this reason that burns caused by steam are more severe then those caused by boiling water.

Question 9.
What is boiling? How is boiling affected by pressure?
Boiling is a process in which a liquid changes into its vapour state at a specific temperature by supplting external heat energy to the liquid.
If pressure increase boiling also increase and if pressure decrease boiling also decrease. Question 10.
Why is water used as a coolant in car radiators?
Since water has a high specific heat capacity. it is used as a coolant in car radiators because it can absorb large ameunts of heat from the car engine without any substantial increase in its temperature.

Question 11.
Why we feel cool when spirit is poured on our hand?
Spirit is a volatile substance. It starts to change in gaseous state as soon as it is poured in our hand. It takes the necessary latent heat to change into gascous state from our hand so we feel cool.

Question 12.
Why it takes mu:h time to cook food in mountain region?
Hitly areas are of higher altitude than sea level. The atmospheric pressure decrease, due to increase n in inint. So the boiling point of water also decrease. That is why it takes much time to cook food in mountain region.

Question 13.
In summer it is comfortable to stand below a fan when perspiring heavily. Why?
When we stand below fan after perspiring heavily, evaporation starts from our body. The latent heat necessary for evaporation of sweat is absorbed from the body, so the body cools down, and there is a feeling of comfort.

Question 14.
What is dew?
After sunset the earth begins to radiate its heat and gradually becomes cold, when the earth surface becomes sufficiently cold, then it is saturated with water vapour. When some water vapour of the air condenses to water and separates out from the air. dew is formed. Question 15.
What is conduction?
The mode of transfer of heat energy from one molecule to another molecule in the direction of heat flow, from higher temperature to lower temperature, without the actual movement of molecules from their mean position is called conduction.

Question 16.
What is convection?
The mode of transfer of heat energy through a fluid from a place of higher temperature to a place of lower temperature by the actual movement of the molecules is called convection.

Question 17.
Radiation is the process of heat transfer in which heat directly passes from one body to the other body without affecting the medium.

Long Questions & Answers : (3 marks for each question)

Question 1.
What is fog?
If there is little or no wind in a considerable space near the earth surface at night, then the air in that atomsphere cools down gradually. As a result, that air is saturated by water vapour. At that time the water vapour present in the atmosphere condenses as small droplets around the suspended dust particles, smoke particles. coal dust etc and continues to float in air. This is called fog.

Question 2.
On What factors the quantity of heat gained by a body depends?
The quantity of heat gained by a body depends on three factors.
a) Mass of the substance : amount of heat absorbed by a substance is directly proportional to its mass.
b) Nature of the substance : amount of heat absorbed by a substance depends on the nature of the substance.
c) Rise in temperature of the substance : the amount of heat absorbed by a substance is proportional to change in temperature. Question 3.
What is melting or fusion? How melting point of a substance is affected by pressure?
The melting point or the fusion point is the fixed temperature at which a solid starts changing to its liquid state.
Melting point of a substance is affected by pressure in two ways-
i) The substances which decrease in volume after melting, if pressure is increased on them the melting point of that substance decrease. For example ice. cast iron, antimoni bismath.
ii) Again the substances which increase in volume after melting. if pressure is increased on them melting point increase. For example gold. silver, iron. copper.

Question 4.
What is fuse wire? Why melting point of a fuse wire is low?
There is a possibility to catch fire if excess amount of current passes through the conducting wire. So to prevent this danger a wire of low melting point is connected with the circuit. This wire is called fuse wire. A fuse wire is made of a mixture of lead and tin. Since melting point of fuse wire is very low, so it melts in low temperature and it does not allow flow of current through it.

We know that if impurities are mixed with pure substance its melting point becomes low. The melting point of a mixture obtained by mixing point of both lead and tin. That is why, melting point of a fuse wire is low. Question 5.
What is specific latent heat? What is the SI and CGS unit of specific latent heat?
Specific latent heat of a substance is defined as the amount of heat required to change the state of unit mass of the substance without any change in temperature. The SI unit of specific latent heat is joules per kilogram and the CGS unit of specific latent heat is joules per gram.

Question 6.
Discuss the features of conduction.
Features of conduction-

1. This mode of transfer predominantly takes place in solids.
2. The molecules of the medium do not leave their mean positions, but they traisfer heat as a result of vibration
4. The transfer of heat can be in any direction along any path.
5. The process is slow. Question 7.
Discuss the features of convection.
Features of convection-

1. This mode of transfer predominantly takes place in liquids and gases.
2. The molecules of the medium absorb heat, they rise and other molecules of medium come to take their place. Thus there is mass movement of medium.
3. The transfer of heat takes place only in vertically upword direction.
4. The process is faster than the conduction.

Question 8.

1. No material medium is required for the transfer of heat.
2. The medium does not play any role.
3. The transfer of heat is in all dircetions along a straight line path in the form of electro magnetic waves.

Question 9.
State the differences between evaporation and boiling.

 Evaporation Boiling i) Evaporation takes place at all temperatures i) It takes place at the boiling point of the liquid ii) Evaporation is slow and silent process ii) Evaporation is a fact and violent process iii) It takes place at the surface of the liquid iii) It takes place over entire mass of the liquid iv) It causes cooling iv) It does not cause cooling

Question 10.
On what factors the boiling point of a liquid depend?
Factors which influence the boiling point of a liquid are-

1. Nature of liquid: Different liquids has different boiling point.
2. Presence of it? purities: Generally boiling point increases if some impurities are dissolved in a liquid.
3. Pressure : If pressure increase boiling point of liquid also increases. Mathematical Problems : (3 marks for each question)

Question 1.
Specific heat capacity of a substance is 0.12 cal^gm gm° C, mass is 10 gm. If there is 20° C increase in temperature then how much heat is required?
Heat required = mass × specific heat × tcmperature
= 10 × 0.12 × 20 calorie = 240 caloric.

Question 2.
To rise temperature of 30 gm water from 20° C to 50° C how much heat is required?
Required Heat (H) = m s(t2-t1) Here m = mass = 50 gm
s = specific heat capacity = 1 calorie / gm
t1 = initial temperature = 20° C
t2 = final temperature =50° C
∴ H = 30 × 1 × (50-20) calorie = 30 × 30 calorie = 900 calorie.

Question 3.
How much heat is required to melt 5 gm ice?
Latent heat of fusion of ice is 80 calorie / g.
∴ To melt 1 g ice 80 calorie heat is required
∴ To melt 5 g ice = 80 × 5 calori = 400 calorie heat is required

Question 4.
In a tumbler there is 100 gm ice. 6400 calorie heat is given to it. What will be the result?
Latent heat of fusion of ice is 80 calorie / g
∴ In 80 calorie heat melts ice 1 g
∴ In 1 calorie heat melts ice g $$\frac{1}{80}$$
∴ In 6400 calorie heat melts $$\frac{6400}{80}$$ g=80 g
∴ Result would be 80 g water and 20 g ice. Question 5.
How much heat is required for 40 g water to boil and turn into vapour?
The latent heat of steam is 537 calorie / g
To turn Ig water into steam needed 537 calorie
∴ To turn 40 g water into steam needed 537 × 40 calorie = 21480 calorie heat.

Question 6.
Mass of an object is 200 gm and specific heat 0.09 calorie / gm 1°C. To rise temperature of that object from 20° C to 90° C how much heat is required?
We know required heat h = m s(t1-t2)

Here
m = mass = 200 gm
s = specific heat = 0.09 unt
t1 = 20° C and t2 = 90° C
H = 200 × 0.09 × (90-20) calorie
= 200 × 0.09 × 70 calorie = 1260 calorie.

Question 7.
Calculate the thermal capacity 200 g of aluminium. Specific heat capacity of aluminium 0° 21 calorie g-1 °C-1.
Thermal capacity = ms
= 200 × 0.21 calorie / °C = 42 calorie / °C Question 8.
How much heat is required to rise temperature from -8°C to 50°C of 10 g ice. Specific heat of ice 0.5 caloric gm-1° C-1.
Answer: Heat would be released in three steps.
First from -8° C ice would become 0°C then from 0° C ice would take latent heat and transform in 0°C water and lastly 0° C water would become 50° C water.
From -8° C to 0° C heat required
= (0+8) 0.5 × 10 calorie = 4 × 10 calorie = 40 calorie
From 0° C ice to 0° C water heat required = 80 × 10 calorie = 800 calorie .
From 0° C-50° C water heat required = 10 × 1 × (50-0) calorie = 10 × 50 calorie = 500 calorie
Total heat required = (40+800+500) = 1340 calorie.