Combustion and Flame is a chapter in science books for 8th class students. There are many books on science for class 8, for example, ‘NCERT Science’ and ‘Science Matters.’
For the preparation of this chapter, you need to have a complete understanding of the concepts given in the chapter, and together with that, you must have a collection of questions and answers that may be asked in the exams.
Here, you will get answers to the questions given at the end of the chapter Combustion and Flame from NCERT and the book ‘Science Matters’.
Combustion and Flame
NCERT Exercises (Combustion and Flame)
1. List conditions under which combustion can take place.
2. Fill in the blanks.
(a) Burning of wood and coal causes ___________ of air.
(b) A liquid fuel, used in homes, is ________.
(c) Fuel must be heated to its _______ _______ before it starts burning.
(d) Fire produced by oil cannot be controlled by __________ .
3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Let Us Exercise: Science Matters
1. Choose the correct option.
a. 6.12 kg of fuel was burnt to produce 4,49,820 kJ of energy. The calorific value of the fuel is:
(i) 7.35 × 103 kJ/kg
(ii) 7.35 × 102 kJ/kg
(iii) 7.35 × 104 kJ/kg
(iv) 7.35 × 101 kJ/kg
Answer
The calorific value of a fuel is the amount of heat liberated by the fuel per kg on its complete combustion.
Here, energy produced by burning 6.12 kg of fuel = 4,49,820 kJ
So, energy produced by burning 1 kg of fuel = kJ/kg
b. The gas formed during incomplete combustion is _________ .
(i) carbon dioxide
(ii) nitrogen oxide
(iii) carbon monoxide
(iv) sulphur dioxide
Answer
carbon monoxide
(i) liquified petroleum gas
(ii) petrol
(iii) camphor
(iv) charcoal
Answer
charcoal
(i) Rising sea levels
(ii) Fall in overall temperature of the Earth
(iii) Melting of polar ice caps
(iv) Flooding of coastal areas
Answer
Fall in overall temperature of the Earth
2. Assertion - Reasoning questions.
For questions (i) and (ii), two statements are given - one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below.
(a) Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of the Assertion (A).
(b) Both Assertion (A) and Reason (R) are true, and Reason (R) is not the correct explanation of the Assertion (A).
(c) Assertion (A) is true, but Reason (R) is false.
(d) Assertion (A) is false, but Reason (R) is true.
(i) Assertion (A): Food is fuel for the body.
Reason (R): A substance that undergoes combustion is called fuel.
Answer
(b) As both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
A true Reason for this Assertion would be that food gives energy to the body.
(i) Assertion (A): Water-based extinguishers cannot be used to extinguish oil fires.
Reason (R): Oil floats on water as it is less denser than water.
Answer
(a) As both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of the Assertion (A).
3. Answer the following questions in one sentence.
a. What is combustion?
Answer
Combustion is a chemical process in which a substance burns in the presence of oxygen to release energy in the form of heat and light.
b. What units are used to measure the heat liberated by fuels?
Answer
The heat generated by fuels on burning is measured in joules or calories.
c. Define the term 'ignition temperature'.
Answer
The lowest temperature at which a substance catches fire is called its ignition temperature.
d. What is global warming?
Answer
Global warming is a rise in the overall temperature of the Earth's atmosphere and surface. Click for more
4. Give reasons for the following statements.
a. Goldsmith blows air into the outer zone of the kerosene lamp flame with a blowpipe onto the gold or silver.
Answer
It is because the temperature of the outer zone is extremely high, and it can melt gold or silver.
b. LPG is a cleaner fuel.
Answer
Because LPG does not produce carbon dioxide and leave any residue on burning due to its high calorific value.
c. More care is needed to store kerosene than coal, though both are fuels.
Answer
Because kerosene has a lower ignition temperature than coal.
5. Answer the following questions in 3-4 sentences.
a. List any four characteristics of an ideal fuel.
Answer
The four characteristics of an ideal fuel are:
1. High calorific value
2. Moderate ignition temperature
3. Low moisture content
4. Does not produce any harmful gases or leave any residue on combustion.
b. Discuss the harmful effects of burning fuels on the environment.
Answer
Burning fossil fuels such as coal and petroleum leads to the release of harmful products into the environment. Some of the harmful effects of burning fossil fuels on the environment are:
Global warming: Burning of fossil fuels releases carbon dioxide into the atmosphere. Carbon dioxide, being a greenhouse gas, traps the Sun's heat on Earth's surface.
Air pollution: Burning of fossil fuels releases harmful pollutants like oxides of sulfur and nitrogen and carbon particles.
Acid rain: Oxides of sulfur, nitrogen, and carbon mix with rainwater to form sulphuric acid, nitric acid, and carbonic acid, respectively. These acids, as acid rain, damage soil, trees, buildings, and monuments.
c. Explain how a carbon dioxide extinguisher works.
Answer
Carbon dioxide extinguishers store carbon dioxide at high pressure in a liquid state. Carbon dioxide is heavier than oxygen. It covers the fire, like a blanket cutting off the oxygen supply and extinguishing it.
Extra Questions and Answers on Combustion and Flame
This set of questions has been created on the chapter Combustion and Flame for preparing the chapter comprehensively.
You can score 100% in the Combustion and Flame Chapter if you prepare all the given questions.
Multiple Choice Questions (Combustion and Flame)
1. Which of the following is not a combustible substance?
(i) Wood
(ii) Coal
(iii) LPG
(iv) Asbestos
Answer
(iv) Asbestos
2.Which of the following conditions is not necessary for combustion?
(i) Presence of a combustible substance
(ii) Presence of oxygen
(iii) Presence of Sunlight
(iv) Attainment of ignition temperature
Answer
(iii) Presence of Sunlight
3. No substance can burn below its ________ .
(i) ignition temperature
(ii) melting point
(iii) boiling point
(iv) None
Answer
(i) ignition temperature
4. What are the by-products of complete combustion?
(i) Carbon monoxide and water
(ii) Carbon dioxide and water
(iii) Sulfur dioxide
(iv) Nitrous oxide
Answer
(ii) carbon dioxide and water
5. What are the by-products of complete combustion?
(i) Carbon monoxide and water
(ii) Carbon dioxide and water
(iii) Sulfur dioxide
(iv) Nitrous oxide
Answer
(ii) carbon dioxide and water
6. Bursting of firecrackers is an example of ________ .
(i) Complete Combustion
(ii) Incomplete Combustion
(iii) Explosion
(iv) Spontaneous Combustion
Answer
(iii) Explosion
7. Which of the following is not an element of the Fire Triangle?
(i) Fuel
(ii) Heat
(iii) Oxygen
(iv) Nitrogen
Answer
(iv) Nitrogen
8. Water cannot be used to extinguish _______ .
(i) Wood fires
(ii) Paper fires
(iii) Cloth fires
(iv) Oil fires
Answer
(iv) Oil fires
9. Which colour of a flame indicates complete combustion?
(i) Blue
(ii) Red
(iii) Yellow
(iv) Pink
Answer
(i) Blue
10. Which zone of the flame is the coldest?
(i) Dark zone or the innermost zone
(ii) Luminous zone or the middle zone
(iii) Non-luminous zone or the outer zone
Answer
(i) Dark zone or the innermost zone
11. In which zone of the flame does complete combustion take place?
(i) Dark zone or the innermost zone
(ii) Luminous zone or the middle zone
(iii) Non-luminous zone or the outer zone
Answer
(iii) Non-luminous zone or the outer zone
12. In which zone of the flame does partial combustion take place?
(i) Dark zone or the innermost zone
(ii) Luminous zone or the middle zone
(iii) Non-luminous zone or the outer zone
Answer
(iii) Luminous zone or the middle zone
13. The hottest zone of the flame is ______.
(i) Dark zone or the innermost zone
(ii) Luminous zone or the middle zone
(iii) Non-luminous zone or the outer zone
Answer
(iii) Non-luminous zone or the outer zone
Very Short Answer Questions (Combustion and Flame)
1. What are combustible substances?
Answer
Substances that, on heating, catch fire easily, producing heat and light in the presence of air, are called combustible substances. For example, wood, paper, cow dung cakes, kerosene, cooking gas, etc.
2. What are non-combustible substances?
Answer
Substances that do not catch fire on heating in the presence of air are called non-combustible substances. For example, stone, clay, glass, cement, bricks, soil, sand, water, iron, copper, asbestos, etc.
3. Write the conditions necessary for combustion.
Answer
The conditions necessary for combustion are:
(i) Presence of a combustible substance
(ii) Presence of a supporter of combustion
(iii) Attainment of ignition temperature
4. Why does a matchstick catch fire immediately on rubbing it at the side of the matchbox?
Answer
A matchstick catches fire immediately on rubbing it at the side of the matchbox because the chemicals that make up the side of the matchbox have a low ignition temperature, and it attains it by the heat produced by friction.
5. Why can a coal fire not be started by using a lighted matchstick directly?
Answer
A coal fire cannot be started by using a lighted matchstick directly, as the small amount of heat produced by the burning matchstick is not sufficient to heat the coal to its ignition temperature.
6. Why does petrol catch fire faster than kerosene?
Answer
Petrol catches fire faster than kerosene because petrol has a lower ignition temperature than kerosene.
7. Why can petrol not be used in stoves but kerosene can be?
Answer
Due to its exceptionally low ignition temperature, petrol is not used in stoves used for domestic purposes, while kerosene, which has a comparatively higher ignition temperature, can be used.
8. What is complete combustion?
Answer
A combustion that occurs in the presence of enough oxygen is called complete combustion. No soot (carbon) is produced as a by-product.
Carbon dioxide and water are the by-products of complete combustion.
9. What is incomplete combustion?
Answer
A combustion that occurs in the presence of limited oxygen is called incomplete combustion.
Carbon dioxide and soot (carbon) are produced as the by-products of incomplete combustion.
10. What is slow combustion?
Answer
A combustion that takes place at a slow rate and takes longer to release energy is called slow combustion. For example, cellular respiration, digestion, rusting, etc.
11. What is rapid combustion?
Answer
A combustion that takes place rapidly and heat and light energy are produced is called rapid combustion. For example, burning of matchsticks, LPG, CNG, kerosene, etc.
12. What is spontaneous combustion?
Answer
A combustion that takes place on its own without the help of any external heat is known as spontaneous combustion. For example, burning of white phosphorus at room temperature, burning of coal dust in coal mines, and forest fires due to the heat of the sun or spark of lightning.
13. Name the by-products of incomplete combustion.
Answer
The by-products of incomplete combustion are carbon monoxide and soot (carbon).
14. What type of combustion is the burning of a firecracker?
Answer
The burning of a firecracker is an example of explosion.
15. Give an example of slow combustion
Answer
An example of slow combustion is digestion.
16. What type of combustion does not need any external heat?
Answer
Spontaneous combustion does not need any external heat.
17. Name the three essential ingredients for igniting a fire.
Answer
The three essential ingredients for igniting a fire are:
⚬ fuel
⚬ heat
⚬ oxygen
18. Mention the different methods to extinguish a fire.
Answer
The different methods that can be employed to extinguish fire are:
⚬ Cooling the burning material: Reducing the heat and bringing down the substance's temperature below its ignition point is one of the most effective ways to extinguish a fire. Water is used to cool a burning material.
⚬ Starving the fire: Fuel is the food of fire. If we stop the source of fuel, it will starve the fire. For example, removing any wood or dry grass around a bonfire will cause it to extinguish.
⚬ Smothering the fire: Fire grows by breathing oxygen. It cannot survive a minute without oxygen. If we cut off the oxygen at the burning place, the fire will be smothered and stop burning.
19. Why do some substances burn with a flame and others do not?
Answer
A flame is produced by those substances that vapourise on burning, while substances that do not vapourise on burning do not produce a flame. The cause of flame on burning is the vapour formation.
20. Name the solids that produce vapours on heating.
Answer
The solids that produce vapours on heating are camphor, wax, oily solids, etc.
21. What are fuels?
Answer
Fuels are substances that produce energy on burning.
22. Name the greenhouse gas that causes global warming.
Answer
Carbon dioxide is the greenhouse gas that causes global warming as it traps the Sun's heat on Earth's surface.
21. What should be considered while choosing a good fuel?
Answer
The following things should be considered while choosing a good fuel.
• high calorific value
• moderate ignition temperature
• low moisture content
• does not produce any harmful gases or leave any residue on combustion
• low cost and easy availability
• easy to transport
• controllable combustion, and
• low storage cost and convenient to store.