NCERT Exemplar Solution Class 8 Science Chapter 6

NCERT Exemplar Solution Class 8 Science Chapter 6

NCERT Exemplar Solution Class 8 Science Chapter 6 Combustion and flame all Questions Answer Solution. NCERT Exemplar Class 8 Science Chapter 6 Solution PDF.

NCERT Exemplar Solution Class 8 Science Chapter 6: Overview

NCERT Exemplar Solution Class 8 Science Chapter 6

Board

NCERT

Topic

Exemplar Problem Solution
Class

8

Subject

Science
Chapter

6

Chapter Name

Combustion and flame

NCERT Exemplar Solution Class 8 Science Chapter 6 Combustion and flame

Multiple Choice Questions

1) A substance which reacts with oxygen giving heat is called a combustible substance. Which , one of the following is a combustible substance?

a) Iron nail

b) Glass

c) Stone piece

d) Wood

Answer:-  d) Wood

Wood is combustible substance while the other does not catch fire these are non- combustible.

2) Which of the following has the highest calorific value?

a) Kerosene

b) Biogas

c) LPG

d) Petrol

Answer:- c) LPG

Calorific value is the burning efficiency of the fuel. Among the above mentioned, LPG has the highest calorific value. Than the petrol, kerosene and biogas.

 3) Magnesium ribbon on burning in air produces

a)Magnesium oxide, water and light

b)Magnesium oxide and heat

c)Magnesium oxide, heat and light

d)Magnesium oxide, water and heat

Answer:- c) Magnesium oxide, heat and light

On burning magnesium ribbon it will produce, magnesium oxide after reaction, and heat and light as well.

4) Which of the following is not a combustible substance?

a) Camphor

b) Glass

c) Straw

d) Alcohol

Answer:- b) Glass

Glass is Non-combustible substance it does not catch fire. Other given options camphor, alcohol and straw can catch fire. These are combustible.

5) The substance that does not burn with flame is

a) LPG

b) Camphor

c) Dry grass

d) Charcoal

Answer:- d) Charcoal

Charcoal does not burn with a flame. It just produce extreme heat and glow. Other burn with a flame.

6) On placing an inverted tumbler over a burning candle, the flame extinguishes after some time. This is because of non- availability of

a) Oxygen

b) Water vapours

c) Carbon dioxide

d) Wax

Answer:- a) Oxygen

Oxygen gas is the supporter of the fire. If there will not be oxygen fire will be extinguished after some time. Flame needs continuously supply of oxygen for burning.

7) If a person’s clothes catches fire, the best way to extinguish the fire is to :

a) Throw water on the clothes.

b) Use fire extinguisher.

c) Cover the person with a woollen blanket.

d) Cover the person with a polythene sheet.

Answer:- c) Cover the person with a woollen blanket.

Covered by Woollen blankets will cut the contact of the fire and the oxygen and thus fire will be extinguished.

8) The substance expected to have the highest ignition temperature out of the following is

a) Kerosene

b) Petrol

c) Coal

d) Alcohol

Answer:- c) Coal

Coal has the highest ignition temperature. Therefore it takes more time to catch fire Kerosene, petrol and alcohol has less ignition temperature.

9) Choose the correct statement about inflammable substances from the following.

They have :

a) Low ignition temperature and cannot catch fire easily.

b) High ignition temperature and can catch fire easily.

c) Low ignition temperature and can catch fire easily.

d) High ignition temperature and cannot catch fire easily.

Answer:-  c) Low ignition temperature and can catch fire easily.

Inflammable substances has the low ignition temperature. They catch fire easily when almost small amount of heat is also supplied to it.

10) Choose the incorrect statement from the following. Forest fires are usually due to :

a) Carelessness of humans

b) Heat of sun

c) Cutting of trees

d) Lighting strike

Answer:-  c) Cutting of trees

Forests does not catch fire due to cutting trees. It catches fire due to heat of sun, carelessness of humans, lightning strike etc. These actions provide heat to catch fire. Cutting wood does not provide heat.

11) The calorific value of fuel of fuel is expressed in a unit called

a) Kilojoule per litre

b) Kilogram per millilitre

c) Kilojoule per gram

d) Kilojoule per kilogram

Answer:- d) kilojoule per kilogram

The measuring unit of the calorific value is kilojoule per kg. (KJ/kg)

12) In village, people use wood as fuel because:

a) It is considered to be an ideal fuel.

b) Of its easy availability and low cost.

c) It is environment friendly.

d) It catches fire easily.

Answer:- b) of its easy availability and low cost.

Villages has lots of trees planted in their area. And it is easily available to them in very low cost paying for it. Therefore these people use wood for fire.

 13) which among the following is considered as the cleanest fuel?

a) Cow dung cake

b) Petrol

c) Kerosene

d) Hydrogen gas

Answer:- d) Hydrogen gas

Hydrogen gas has the highest calorific value among all the fuels. It does not produce any air pollutants during burning therefore it is the cleanest fuel.

14) Choose the incorrect statement from the following. 

A good fuel is one which:

a) Is readily available.

b) Produces a large amount of heat.

c) Leaves behind many undesirable substances.

d) Burns easily in air at a moderate rate.

Answer:-  c) Leaves behind many undesirable substances.

A good fuel is the one which does not leaves any undesirable substances after burning.

15) Shyam was cooking potato curry on a chulha. To his surprise he observed that the copper vessel was getting blackened from outside. It may be due to :

a) Proper combustion of fuel

b)Improper cooking of potato curry.

c) Improper combustion of the fuel.

d) Burning of copper vessel.

Answer:- c) Improper combustion of the fuel.

The flame of the burning fuel is directly in contact with the vessels, due to improper combustion of the fuel it gets blackened.

Very Short Answer Questions

16) Fill in the blanks in the following sentences.

 a) A ___________process in which a substance reacts with ________ to give off heat is called combustion.

Answer:- A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

b) When the clothes of a person catch _________, the person is covered with a ___________ to extinguish fire.

Answer:- when the clothes of a person catch fire , the person is covered with a blanket to extinguish fire.

c) The _________ temperature at which a substance catches fire is called it’s ________ temperature.

Answer:-  The lowest  temperature at which a substance catches fire is called it’s ignition temperature.

d) The substances which have very _____________ ignition temperature and can easily catch fire with a flame are called _____________ substances.

Answer:- the substances which have very lowest ignition temperature and can easily catch fire with a flame are called inflammable substances.

e) The substances which vaporiseduring ___________, give flame.

Answer:- the substance which vaporise during burning, give flame.

 17) Some words (underlined) in the following sentences are jumbled up. Write them in their correct form.

a) Seldie is a combustible substance.

Answer:- Diesel is a combustible substance.

b) Slags is a non- combustible material.

Answer:- Glass is a non- combustible material.

c) Chittsmack does not burn by itself.

Answer:- Matchstick does not burn by itself.

d) Some substances on combustion produce thea and mafel.

Answer:-  some substances on combustion produce heat and flame.

e) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ficalroic value.

Answer:-  The amount of heat energy produced on complete combustion of 1 kg of a fuel is called it’s calorific value.

18) Two glass jars A and B are filled with carbon dioxide and oxygen gases, respectively. In each jar a lighted candle is placed simultaneously. In which jar will the candle remain lighted for a longer time and why?

Answer:- Candle in the jar B will be lighted for the longer time, because jar B contains oxygen gas, which is responsible for combustion. But Carbon dioxide does not support combustion , therefore flame in jar A will be lighted for longer time.

19) Anu wants to boil water quickly in a test tube. On observing the different zones of the flame, she is not able to decide which zone of the flame will be best for boiling water quickly. Help her in this activity.

 Answer:- The outermost zone of the flame will be best for the boiling water quickly. As the outermost zone of the flame is the hottest part of the flame. If test tube will be kept on the outermost zone then water will boiled quickly.

20) Why is the use of diesel and petrol as fuels in automobiles being replaced by compressed natural gas (CNG) in big cities?

Answer:-  In the big cities, automobiles fuel like petrol and diesel is replaced by the CNG because, Compressed natural gas produce the harmful products in small amount. And it is a cleaner fuel. Therefore it is being replaced.

Short Answer Questions

21) Boojho wants to separate the following materials as combustible and non- combustible . Can you help him?

 Charcoal, chalk, stone, iron rod , copper coin, straw, cardboard, glass, paper, candle, wood.

Answer:-

Combustible :- charcoal, straw, card board, paper , candle , wood.

Non- combustible :- chalk, stone, iron rod, copper coin, glass.

22) Indicate whether the following statements are true or false. Also write the false statements in their correct form.

a) Air is necessary for combustion.

Answer:- True

b)Magnesium is a non- combustible metal.

Answer:- False

Magnesium is not a non- combustible metal. It is a combustible metal.

c) Carbon dioxide is an excellent fire extinguisher.

Answer:- True

d) Calorific value of wood is higher than that of coal.

Answer:- False .

Calorific value of wood is not higher than that of wood, but calorific value of coal is higher than the wood.

23) Match the items of Column A with the items of Column B.

Column A Column B
a)          Oxides of sulphur and nitrogen i)                          Fire extinguisher
b)         CNG ii)                      Incomplete combustion of coal
c)           Oxygen iii)                 Very low ignition temperature
d)         Inflammable substance iv)                  Acid rain
e)          Carbon dioxide v)                      Necessary for combustion
f)             Carbon monoxide vi)                  Fuel for automobiles.

 

Answer:-

a)Oxides of sulphur

And nitrogen          

 

 

→ iv) Acid rain
b) CNG                       vi) Fuel for                                         Automobiles
c) Oxygen                   v) Necessary for                                  Combustion

 

d) Inflammable

     Substance            

iii) very low ignition

                                                Temperature

e)  Carbon dioxide    i) fire extinguisher
f) carbon monoxide  ii) Incomplete Combustion of coal

 

 

24) Match the following for the flame of a candle.

Column A Column B

(Zone)

Column C

(Colour)

a)hottest part i) innermost zone of unburnt wax vapours x) blue
b) moderately hot ii) middle zone of partial combustion y) black
c) least hot

 

iii) outer zone of complete combustion z)  yellow

Answer:-

a) Hottest part →(iii) outer zone of complete combustion → (x) blue

b) Moderately hot → (ii) middle zone of partial combustion → (z) yellow

c) Least hot →(i) innermost zone of unburnt wax vapours → (y) black

25) If you hold a piece of iron wire with a pair of tongs inside a candle flame or a Bunsen burner flame, what will you observe? Will it produce a flame ?

Answer:- No. It will not produce a flame. As the iron nail is non- combustible material. After holding iron nail on the flame it will become red hot and soften after some time. It will start glowing. But it will not produce flame.

26) Fill in the blanks using the words given in the box.

Ignition, petrol, combustion, calorific value, combustible, inflammable.

a) A chemical process in which a substance reacts with oxygen to give off heat is called_________.

Answer:-  A chemical process in which a substance reacts with oxygen to give off heat is called combustion.

b) Wood, paper, CNG are ___________ substances.

Answer:- wood , paper, CNG are combustible substance.

c) The lowest temperature at which a substance catches fire is called its ____________ temperature.

Answer:-  The lowest temperature at which a substance catches fire is called it’s ignition temperature.

d) Ignition temperature of ____________ is lower than that of wood.

Answer:- Ignition temperature of petrol is lower than that of wood.

e) The substances which have very low __________ temperature and can easily catch fire with a flame are called __________ substances.

Answer:-  The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.

f) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ____________.

Answer:- The amount of heat energy produced on complete combustion of 1 kg of a fuel is called it’s calorific value.

27) People usually keep Angethi/ burning coal in the closed rooms during winter season. Why is it advised to keep the door open?

Answer:- burning coal in the closed room will produce Carbon monoxide due to insufficient oxygen present over there. And carbon monoxide is dangerous for human health, the inhalation of carbon monoxide can cause death. Therefore it is advised to keep doors open of that room.

28) Write True/ False against the following statements and also correct the false statement.

a) A physical process in which a substance reacts with oxygen to give off heat is called combustion.

Answer:-  False

A process in which a substance reacts with oxygen to give of heat is called Combustion is the chemical process.

b) Water is the best extinguisher for fires involving electrical equipment.

Answer:- False

Water is not the best extinguisher for fires involving electrical equipment but the carbon dioxide is the best extinguisher.

c) Alcohol, CNG and LPG are inflammable substances.

Answer:-  True

d) Increased concentration of nitrogen in air is believed to cause global warming.

Answer:- False.

Global warming is caused by the increased concentration of carbondioxide in air.

e) Greater the calorific value, better is the fuel.

Answer:- True

f) Middle zone is the hottest zone of a flame.

Answer:-  False.

Middle zone is not the hottest zone of a flame. Outer zone is the hottest zone of a flame.

g) The substances which vaporise during burning, give flame.

Answer:- True

29) Cracker on ignition produces sound. Why?

Answer:- Cracker when ignited produces sound, these is because the sudden chemical reaction is takes place in it. These reaction takes place with the evolution of heat, sound and light. When the reaction is liberated large amount of gas is formed. These reaction is known as explosion. Explosion of the cracker can also be done if large amount of pressure is applied over it.

30) What do you understand by fuel efficiency?

Answer:-  the different type fuel produces different amount of energy. That produced energy is called as its Calorific value. Calorific value is known as the amount of heat energy produced when there is complete combustion of 1 kg fuel is done. It is measured in the kilojoule per kg (KJ/kg). Calorific value of the fuel is its fuel efficiency.

Long Answer Questions

31) You are provided with three watch glasses containing milk, petrol and mustard oil, respectively. Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why?

Answer:- The substances which have very low ignition temperature, these substances easily catch fire and produces flame. These substances are known as inflammable substances.   Petrol is highly inflammable substance. Thus it will catch fire instantly.

32) Manu was heating oil to fry potato chips. The cooking  oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable. If yes, why? If not, why not ? In such a condition what should manu have done ?

Answer:- No. The action done by the manu was not correct. Pouring water is not good for the burning oil. It will again catch fire with high flame with that action. He should first switch off the flame of the burner. After that he should put a lid on the cooking pan so that the contact between the oil and the oxygen will be cut off. Oxygen gas is supporter of the fire. These action will be correct in that condition.

33) What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire.

Answer:-

To produce fire there are three essential requirements which are fuel, heat to produce fire, and air. By these three things fire can be easily ignite.

The job of the fire extinguisher is –

To cut the supply of the air and fire.

To decrease the temperature of fuel

If these both the activities are done then the fire can be easily controlled.

34) Give two examples each for a solid, liquid and gaseous fuel along with some important uses.

Answer:-

There are three types of fuels which are as follows –

A)Solid fuels

E.g. Coal, wood,

These are mainly used for cooking.

B) Liquid fuels

E.g. petrol, diesel, kerosene etc.

Petrol and diesel are used for the vehicle and automobiles as a fuel.

Kerosene is used in the lamps, stoves and as fuel in the aircrafts.

C) Gaseous fuel

E.g. LPG, CNG etc.

LPG is used in the fuel industry

LPG is also used in the cooking food in homes.

CNG is used as the fuel in the vehicles.

35) The calorific values of petrol and CNG are 45000 kj/ kg and 50,000 kj/ kg, respectively. If you have vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?

Answer:-

I will prefer to use CNG vehicle. as the calorific value of the CNG is higher than that of the petrol, CNG will be more economical. Also the petrol and diesel Vehicles produce more air pollutants than the CNG. These will be the one more benefit of using CNG vehicle.

36) Although wood has a very high calorific value , we still discourage its use as a fuel. Explain.

Answer:- Wood is obtained after cutting trees. Since trees are very important for us. Trees gives oxygen and stops soil Pollution also keeps earth surface cool. Therefore we need trees. We cannot cut down trees for firewood. As wood has high calorific value but then also we cannot use these as main fuel. Wood also produces so much air Pollution. Therefore it is not good to use wood as a fuel.

37) Forest fire produces a lot of air Pollution. Write in brief about the reasons of forest fires.

Answer:- Sometimes, forest catches extreme fire. These may be due to some plants are dead and dried due to sun, grass is dried due to sun and these are ready to catch fire at higher temperature. Sometimes in rainy season due to lightning some trees can catch fire and it can be spread. One of the reason is human negligence and camps fire. Sometimes near the forest we ignite fire and keep it so, these fire due to air can be spread easily. As forest has lots of tress and wood has the highest calorific value, it catches fire immediately and spread. These Emits lots of dangerous gases and release it into the atmosphere. It causes air Pollution.

38) Complete the crossword fig. 6.1 with the help of the clues.

Fig. 6.1

Across-

1)Non- metal which catches fire if exposed to air(10)

3) The lowest temperature at which a substance catches fire is called it’s ___________ temperature (8).

5) the most common fire extinguisher.(5)

Down-

2) A chemical process in which a substance reacts with oxygen to give off heat. (10)

4) petrol is used as a in automobiles (4)

6) it is a hard as stone and black in colour.(4)

Answer:-

Updated: April 23, 2021 — 11:47 am

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