Important Questions for CBSE Class 8 Science Chapter 4 - Combustion and Flame

Very Short Answer Questions: (1 Marks)

1. ________ do not produce flames but generate heat. 

A. Kerosene

B. Oil

C. Petrol

D. Coal

Ans: D. Coal

2.  Which of the following is the product of combustion? 

A. Carbon dioxide and water

B. Oxygen and Water 

C. Only Carbon Dioxide

D. Only Oxygen

Ans: A. Carbon Dioxide and Water

3. Which one of the following is a combustible substance? 

A. Wood

B. Nail

C. Steel spoon

D. All of These

Ans: A. Wood

4. __________ is produced when a charcoal is burnt in a closed room? 

A. carbon dioxide

B. Nitrogen Dioxide 

C. Carbon Monoxide

D. All of These

Ans: C. Carbon Monoxide

5. What is used as fuel to run automobiles? 

A. Petrol

B. Diesel

C. Kerosene

D. Both A and B

Ans: D. Both A and B

6. A good fuel is one which is ______________. 

A. Cheap

B. Readily Available 

C. Produces Large Amount of Heat

D. All of These

Ans: D. All of these

7. How many zones are there in the flame? 

A. One

B. Three

C. Two

D. Four

Ans: B. Three

8. What are the requirements essential for producing fire?

Ans: Requirements for producing fire:

• Fuel

• Air (Oxygen)

• Ignition Temperature

9. Charcoal burns in air to produce ______________, _______________ and ___________.

Ans: Carbon Dioxide, Heat and Light.

10. Name any one chemical substance which helps to put off the fire other than carbon dioxide.

Ans: Bicarbonates of sodium or potassium will help us to put off the fire.

11. In the Sun, heat and light are produced by ___________.

Ans: Nuclear Reaction.

12. Expand LPG.

Ans: Liquified Petroleum Gas

13. The lowest temperature at which a substance catches fire is called ___________.

Ans: Ignition Temperature

14. The unit of a calorific value of a fuel is _________________.

Ans: Kilojoules per Kilogram.

Short Answer Questions: (3 Marks)

15. Classify the following into combustible and non-combustible substances. 

iron rod, wood, steel spoon, stone, newspaper, petrol

Ans:

16. Kerosene oil while burying gives flames on the other hand charcoal does not produce flames? Give a reason.

Ans: Only substances which vaporise during burning can produce flames such as kerosene whereas charcoal doesn’t vaporise when starting burning. Thus, charcoal doesn’t produce any flame.

17. If a fire breaks out in the petrol station, how does a fire extinguisher help to put off the fire?

Ans: A combination of liquid and gaseous carbon is used in fire extinguishers. Because carbon dioxide is heavier than oxygen, it blankets the fire. Thus, the fire is controlled because there is no contact between the fuel and the oxygen.

18. Petrol can be considered as a good fuel. Why?

Ans: Reasons petrol is considered a good fuel:

a. It is affordable.

b. It is easily available.

c. It has a high calorific value.

d. It is easy to transport.

e. It burns easily and produces more heat.

19. What will happen when a paper cup with water is heated?

Ans: When we heat a paper cup containing water, the water will absorb the heat of the flame. Hence, the ignition temperature of paper will not reach and it will not start burning easily.

20. Give four examples of inflammable substances.

Ans: Four inflammable substances:

a. Kerosene

b. Petrol

c. LPG

d. Diesel

21. How does combustion start in a matchstick?

Ans: Antimony trisulphide and potassium chlorate are found in the matchstick's head. The rubbing surface has powdered glass & red phosphorus. Red phosphorus is transformed to white phosphorus when the match is hit against the rubbing surface. This combines with the potassium chlorate in the matchstick head to provide enough heat to ignite antimony trisulphide and start the combustion process.

22. Define the following. 

a. Combustion: Combustion is a chemical reaction in which a material combines with oxygen present in air to produce heat.

b. Fuel: Fuels are the substances which undergo combustion.

c. Calorific Value: The calorific value of a fuel is the quantity of heat energy produced when 1 kg of fuel is completely burned.

23. Why is it not advised to use water if an electrical equipment is on fire?

Ans: Water is a good conductor of electricity which increases the risk to the helpers. Therefore it is advised not to use water if there is any electrical equipment on fire.

24. Label the different zones of a candle flame.

Ans: 

A – outer zone of complete combustion (Blue coloured) 

B – Middle zone of partial combustion (yellow coloured) 

C – Innermost zone of unburnt wax vapours (black coloured)

25. In an experiment, 1 kg of fuel was completely burnt. The heat produced was measured to be 9,000 kJ. 

a. What will be the head produced for 7 kg of fuel?

Ans: If the heat produced for 1 kg of fuel is 9000 kJ. 

Then heat produced for 7 kg is = 7 X 9000 

         = 63,000 kJ

b. Calculate the calorific value of the fuel.

Ans: Calorific value of the fuel = Amount of heat energy produced / weight of fuel burnt 

        = 63000/8 kJ/kg 

        = 9,000 kJ/kg

Long Answer Questions: (5 Marks)

26. List the characteristics of an ideal fuel.

Ans: Characteristics of an ideal fuel:

a. It should be affordable.

b. It should be easily available.

c. It should have high calorific value.

d. It should be easy to transport.

e. It should burn easily and produce more heat.

f. It should not leave any unwanted residue behind on burning.

g. It should be more environmentally friendly.

27. With the help of an experiment show that air is necessary for combustion.

Ans: 

i. Take two lighted candles on a table. 

ii. Put a glass chimney over one candle s in such a way that air can enter the chimney. 

iii. On the other hand, put a chimney over another candle and place a glass plate over the chimney. 

iv. We observe that the first candle continues to burn whereas flame in the second candle extinguishes. 

This is because the air supply was cut off after placing the glass plate over the chimney and the candle was not able to burn in the absence of air. This shows that air (oxygen) is necessary for combustion.

28. Explain the different types of combustion with examples.

Ans: There are three different types of combustions:

a. Rapid Combustion: It's a form of combustion in which a substance burns quickly, releasing heat and light. Example: LPG.

b. Spontaneous Combustion: The sort of combustion in which a substance spontaneously erupts into flames without any apparent cause. 

Example: Phosphorus. It burns in air at ambient temperature without igniting.

c. Explosion: When a substance is ignited, it undergoes a rapid reaction that produces heat, light, and sound. The reaction produces a significant amount of gas and heat, which is then released. This type of combustion is called an explosion.

Example: Bursting of firecrackers.

29. Explain in detail the harmful effect caused by burning of fuel.

Ans: Harmful effects of burning of fuels:

a. The carbon based wood, coal etc leaves unburnt substances which causes air pollution. They can be harmful for humans too, causing respiratory diseases.

b. Most of the fuel releases carbon dioxide on burning. This process slowly increases carbon content in the atmosphere leading to bigger problems like global warming.

c. Incomplete combustion of fuel can produce carbon monoxide instead of dioxide which when inhaled can rapidly combine with our blood and work as poison.

d. In a few cases sulphur dioxide is produced on burning of fuel which is harmful to both humans and the environment.

Download Class 8 Science Chapter 4 Important Questions PDF

Important questions will help the students in ways that they should not struggle to prioritize the topics while their exam preparation. Class 8 Science Chapter 4 important questions are created following the recent CBSE guidelines.

Topics covered in the Class 8 Science Chapter 4 are materials combustion and flame, what is combustion? How do we control fire? Types of combustion, flame, the structure of a flame, What is fuel? Fuel efficiency, etc.

Chapter 4 - Combustion and Flames

• Combustion

It is a type of chemical process, in which a substance reacts with oxygen to produce heat and light is called combustion. One example of combustion is the burning of wood. There are two types of substances mainly combustible substances and non-combustible substances. Substances that easily catch fire are known as combustible substances, some of the examples of combustible substances are paper, coal, and wood. The substances which do not catch fire readily are known as incombustible substances, some of the examples are sand, water, and glass.

There are two types of combustion mainly rapid combustion and spontaneous combustion. When something burns rapidly and produces heat and light like LPG gas is known as rapid combustion.

And when something bursts into flames without application of any other thing is known as spontaneous combustion. The spontaneous reaction can be seen, when phosphorus is in direct contact with air at room temperature.

• Flame

It is the visible and gaseous part of the fire, the flame is the light energy released during the process of combustion. There are different zones of flames, the outer zone is of complete combustion, the middle zone is of partial combustion, the innermost zone is of uncombusted substance.

Smoke is an example of solid unburnt particles, which disappears in the gas. Smoke is of black colour due to the presence of unburnt carbon particles in the smoke. Fire can be controlled by removing all the factors of the combustion like fuel, oxygen, and by lowering the ignition temperature. A fire extinguisher is a device that is used to control the fire, by cutting off the supply of oxygen or by bringing down the temperature of the fuel.

Download CBSE Class 8 Science Chapter 4 - Combustion and Flame Important Questions PDF

Get the free PDF version of these important questions and answers. Solve these questions at your convenience at home and use them as a mock test tool. Focus on how the experts have solved these questions and learn to use the scientific concepts of this chapter well. Sharpen your answering skills by following the answers given in the solutions and stay ahead of the competition.

Conclusion 

The compilation of important questions for CBSE Class 8 Science Chapter 4, "Combustion and Flame," plays a pivotal role in the educational journey of students. These questions serve as invaluable aids for a comprehensive understanding of the subject. They offer a structured approach to learning, highlighting key concepts related to combustion, types of flames, and fire safety measures. As an integral part of exam preparation, these questions help students identify and focus on the most critical topics, ensuring readiness for assessments. By engaging with these questions, students not only consolidate their knowledge but also hone critical thinking skills. In essence, important questions facilitate a deeper exploration of combustion and flame, contributing to enhanced understanding and academic excellence.