Science Ncert Exemplar 2019 Solutions for Class 8 Science Chapter 6 Combustion And Flame are provided here with simple step-by-step explanations. These solutions for Combustion And Flame are extremely popular among class 8 students for Science Combustion And Flame Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Science Ncert Exemplar 2019 Book of class 8 Science Chapter 6 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Science Ncert Exemplar 2019 Solutions. All Science Ncert Exemplar 2019 Solutions for class 8 Science are prepared by experts and are 100% accurate.
Page No 30:
Question 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:
Wood is a combustible substance as it produces smoke during combustion.
Hence, the correct answer is option (d).
Page No 30:
Question 2:
Which one of the following has the highest calorific value?
(a) kerosene
(b) biogas
(c) LPG
(d) petrol
Answer:
The amount of heat energy produced on completely burning one kilogram of fuel (in O2​) is called the calorific value of a fuel. LPG has the highest calorific value, i.e. 55000 kJ/kg. It means that if 1 kg of LPG is burnt completely, then it will produce 55000 kilojoules of heat energy.
​Hence, the correct answer is option (c).
Page No 30:
Question 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:
Magnesium is a very reactive metal and magnesium ribbon on burning in air produces magnesium oxide, heat and light.
Magnesium + oxygen → Magnesium oxide + heat + light
(from air)
Hence, the correct answer is option (c).
Page No 30:
Question 4:
Which of the following is not a combustible substance?
(a) camphor
(b) glass
(c) straw
(d) alcohol
Answer:
Those substances which do not burn in the presence of air are called non-combustible substance. Some of the non-combustible substances are stone, glass, cement, soil, sand, iron nails, etc.
​Hence, the correct answer is option (b).
Page No 30:
Question 5:
The substance that does not burn with flame is
(a) LPG
(b) camphor
(c) dry grass
(d) charcoal
Answer:
Charcoal is a solid fuel which does not vaporise on heating. So, charcoal does not burn by producing a flame.
​Hence, the correct answer is option (d).
Page No 30:
Question 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:
The flame extinguishes because of non-availability of oxygen. Air or oxygen is necessary for combustion.
​Hence, the correct answer is option (a).
Page No 30:
Question 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 woolen blanket.
(d) cover the person with a polythene sheet.
Answer:
If a person’s clothes catches fire, the best way to extinguish the fire is to cover the person with a woollen blanket so that the supply of air to the burning clothes is cut off and hence, the burning (or fire) stops.
​Hence, the correct answer is option (c).
Page No 31:
Question 8:
The substance expected to have the highest ignition temperature out of the following is
(a) kerosene
(b) petrol
(c) coal
(d) alcohol
Answer:
The lowest temperature at which a substance catches fire and starts burning, is called its ignition temperature. Coal has the highest ignition temperature.
Hence, the correct answer is option (c).
Page No 31:
Question 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:
Inflammable substances have low ignition temperature and can catch fire easily.
Hence, the correct answer is option (c).
Page No 31:
Question 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) lightning strike
Answer:
Forest fires are caused due to carelessness of humans, heat of sun, lightning strike etc. It is not due to the cutting of trees.
Hence, the correct answer is option (c).
Page No 31:
Question 11:
The calorific value of a fuel is expressed in a unit called
(a) kilojoule per litre
(b) kilogram per mililitre
(c) kilojoule per gram
(d) kilojoule per kilogram
Answer:
The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).
Hence, the correct answer is option (d).
Page No 31:
Question 12:
In villages, 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:
In villages, people use wood as fuel because of its low cost and easy availability.
​Hence, the correct answer is option (b).
Page No 31:
Question 13:
Which among the following is considered as the cleanest fuel?
(a) cow dung cake
(b) petrol
(c) kerosene
(d) hydrogen gas
Answer:
Hydrogen gas is considered as cleanest fuel as hydrogen produces no harmful by-products on combustion. Only energy and clean water are produced.
Hence, the correct answer is option (d).
Page No 31:
Question 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:
A good fuel should be readily available, produce large amount of heat and burn easily in air at moderate rate. It should not leave behind any harmful gases or residue after burning.
Hence, the correct answer is option (c).
Page No 31:
Question 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:
Due to incomplete combustion of fuel, copper vessel was getting blackened from outside.
​Hence, the correct answer is option (c).
Page No 32:
Question 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.
(b) When the clothes of a person catch __________, the person is covered with a __________ to extinguish fire.
(c) The __________ temperature at which a substance catches fire is called its __________ temperature.
(d) The substances which have very ________________ ignition temperature and can easily catch fire with a flame are called __________ substances.
(e) The substances which vapourise during __________, give flame.
Answer:
(a) 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 fire, the person is covered with a blanket to extinguish fire.
(c) The lowest temperature at which a substance catches fire is called its ignition temperature.
(d) The substances which have very low ignition temperature and can easily catch fire with a flame are called inflammable substances.
(e) The substances which vapourise during burning, give flame.
Page No 32:
Question 17:
Some words (underlined) in the following sentences are jumbled up. Write them in their correct form.
(a) Seldie is a combustible substance.
(b) Slags is a non-combustible material.
(c) Chittsmack does not burn by itself.
(d) Some substances on combustion produce thea and mafel.
(e) The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its ficalroic value.
Answer:
(a) Diesel
(b) Glass
(c) Matchstick
(d) Heat, flame
(e) Calorific
Page No 32:
Question 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:
The candle remains lighted for a longer time in jar B because oxygen is a supporter of combustion. Therefore, we can say that oxygen (air) is necessary for combustion to take place.
Page No 32:
Question 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:
Anu should keep her test tube in the outermost zone or non-luminous zone of the flame because it is the hottest zone of a flame and has higher temperature. In the outer zone of the flame, complete combustion of the fuel takes place because there is plenty of air around it and hence, water in a test tube will boil quickly in this zone.
Page No 32:
Question 20:
Why is the use of diesel and petrol as fuels in automobiles being replaced by Compressed Natural Gas (CNG) in big cities?
Answer:
Use of diesel and petrol as fuels in automobiles being replaced by Compressed Natural Gas (CNG) in big cities because CNG produces harmful products in very small amount. It is a clean fuel as it burns without producing any smoke. The use of CNG in automobiles has also reduced the air pollution in big cities.
Page No 33:
Question 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 materials: charcoal, straw, cardboard, paper, candle, wood.
Non-combustible materials: chalk, stone, iron rod, copper coin, glass.
Page No 33:
Question 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.
(b) Magnesium is a non-combustible metal.
(c) Carbon dioxide is an excellent fire extinguisher.
(d) Calorific value of wood is higher than that of coal.
Answer:
(a) True
(b) False, magnesium is a combustible metal.
(c) True
(d) False, calorific value of coal is higher than that of wood.
Page No 33:
Question 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 nitrogen |
| (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:
The correct match between column A and column B is given as
(a) - (iv), (b) - (vi), (c) - (v), (d) - (iii), (e) - (i), (f) - (ii)
Page No 33:
Question 24:
Match the following for the flame of a candle.
| Column A | Column B |
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:
The correct match is given as:
(a)-(iii), (x)
(b)-(ii), (z)
(c)-(i),(y)
Page No 34:
Question 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:
When a piece of iron wire is held with a pair of tongs inside a candle flame, it becomes red hot and glows. However, it does not produce any flame.
Page No 34:
Question 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 __________.
(b) Wood, paper, CNG are __________ substances.
(c) The lowest temperature at which a substance catches fire is called its __________ temperature.
(d) Ignition temperature of __________ 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.
(f) The amount of heat energy produced on complete combustion of 1kg of a fuel is called its __________.
Answer:
(a) A chemical process in which a substance reacts with oxygen to give off heat is called combustion.
(b) Wood, paper, CNG are combustible substances.
(c) The lowest temperature at which a substance catches fire is called its ignition temperature.
(d) Ignition temperature of petrol is lower than that of wood.
(e) 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 1kg of a fuel is called its calorific value.
Page No 34:
Question 27:
People usually keep Angethi/burning coal in their closed rooms during winter season. Why is it advised to keep the door open?
Answer:
When coal is burnt in air, carbon dioxide is produced which is non poisnous. But when, Angethi or coal is burnt in closed rooms, there is a limited supply of oxygen and hence, incomplete combustion of coal takes place and it produces carbon monoxide which is a highly poisonous gas. It causes suffocation in a closed room. The carbon monoxide gas produced can kill persons sleeping in that room. Therefore, door and windows should be open.
Page No 34:
Question 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.
(b) Water is the best extinguisher for fires involving electrical equipment.
(c) Alcohol, CNG and LPG are inflammable substances.
(d) Increased concentration of nitrogen in air is believed to cause global warming.
(e) Greater the calorific value, better is the fuel.
(f) Middle zone is the hottest zone of a flame.
(g) The substances which vapourise during burning, give flame.
Answer:
(a) False, a chemical process in which substance reacts with oxygen to give off heat is called combustion.
(b) False, carbon dioxide is the best extinguisher for fires involving electrical equipment.
(c) True
(d) False, increased concentration of carbon dioxide is believed to cause global warming.
(e) True
(f) False, outer zone is the hottest zone of the flame.
(g) True
Page No 34:
Question 29:
Cracker on ignition produces sound. Why?
Answer:
When a cracker is ignited, a sudden reaction takes place with the evolution of heat, light and sound and a large amount of gas is liberated. Such a reaction is called explosion. The gases produced are heated by the heat evolved in the reaction. The hot gases expand rapidly and cause an explosion.
Page No 34:
Question 30:
What do you understand by fuel efficiency?
Answer:
Fuel efficiency can be determined by its calorific value which is the amount of heat energy produced on complete combustion of 1 kg of a fuel. The calorific value of a fuel is expressed in kJ/kg. A good fuel should have high calorific value i.e., it should produce large amount of heat per unit mass.
Page No 35:
Question 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:
Petrol has a low ignition temperature and an inflammable substance, therefore, catches fire instantly.
Page No 35:
Question 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:
Pouring water to extinguish the fire due to oil was not a suitable action. It is because oil is lighter than water. The oil floats on water and continues to burn. In such condition, sand or soil should be used. They cut off the supply of air to the fire and hence, fire can be controlled. But sand or soil will not be available at that place. So, Manu should have switched off the flame of the burner and put a lid on the frying pan. By doing this, the contact between fuel and oxygen is cut off and the fire will go off.
Page No 35:
Question 33:
What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire.
Answer:
The three essential requirements to produce fire are:
(i) A combustible substance.
(ii) Air (or oxygen)
(iii) Heat to attain the ignition temperature.
The fire extinguisher is used to cut off the supply of air or bring down the temperature of fuel or both. The most common fire extinguisher is water. But water works only when things like wood and paper are on fire. Water extinguishes fire by cooling the burning substance. For fires involving electrical equipment and inflammable materials like petrol, then carbon dioxide (CO2) is the best extinguisher. Carbon dioxide being heavier than oxygen, covers the fire like a blanket. Since, the contact between the fuel and oxygen is cut off, the fire can be controlled.
Page No 35:
Question 34:
Give two examples each for a solid, liquid and gaseous fuel along with some important uses.
Answer:
Examples of solid fuel are coal and wood.
They are used for cooking of food. Coal is also an important fuel in industries.
Examples of a liquid fuel are kerosene and petrol.
Kerosene is used in stoves to cook food and petrol is used as a fuel in automobiles.
Examples of a gaseous fuel are natural gas and petroleum gas.
They are used in industries and CNG is also used to run automobiles.
Page No 35:
Question 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:
Calorific value is the amount of heat produced in the complete combustion of 1 kg of a fuel. It is measured in kJ/kg. Higher is the calorific value, better is the fuel. As CNG has a higher calorific value than petrol, it will be prefered. CNG is economical and produces large amount of energy with least air pollutants.
Page No 35:
Question 36:
Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.
Answer:
Burning of wood has various disadvantages such as:
(i) Burning of wood produces a lot of smoke which causes respiratory diseases.
(ii) The cutting down of trees to obtain as a wood fuel leads to deforestation which is very harmful to the environment.
​(iii) Trees provide us many-useful substances. To obtain fuel wood, when trees are cut down, then all useful substances which can be obtained from trees are lost.
Page No 35:
Question 37:
Forest fire produces a lot of air pollution. Write in brief about the reasons of forest fires.
Answer:
Reasons of forest fires are as given below :
(i) At high temperature, sometimes dry grass catches fire which spreads throughout the forest.
(ii) Camp fires can also lead to forest fires.
(iii) A spark of lightning from the sky.
(iv) The use of fires by villagers to ward off wild animals.
(v) Fire lit intentionally by people living around forests for recreation.
Page No 35:
Question 38:
Complete the crossword in the given figure with the help of the clues :
Across Down
1. Non-metal which catches 2. A chemical process in which
fire if exposed to air (10) a substance reacts with oxygen to give off heat. (10)
3. The lowest temperature at which 4. Petrol is used as a _________in automobiles. (4)
fire is produced from a substance is called its 6. It is as hard as stone and black in colour. (4)
__________temperature. (8)
5. The most common fire extinguisher. (5)
Answer:
Across
1. Phosphorus
3. Ignition
5. Water
Down
2. Combustion
4. Fuel
6. Coal
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