Combustion and Flame Class 8 important questions with answers PDF download
Very Short Answer Questions: (1 Mark)
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.
5 Important Topics of Class 8 Science Chapter 4 You Shouldn’t Miss!
Benefits of Class 8 Chapter 4 Science Important Questions
Clear Understanding of Concepts: Important questions help students understand key ideas about combustion and its types.
Enhanced Critical Thinking: These questions encourage students to think about how combustion works in everyday life.
Improved Retention of Information: Answering questions helps students remember important facts about combustion and flames.
Better Exam Preparation: Practising these questions prepares students for exams by showing them what to focus on.
Real-World Application: The questions connect what students learn to real-life situations, showing the importance of combustion.
Development of Safety Awareness: Important questions cover fire safety, helping students understand how to stay safe around flames.
Increased Confidence: Regular practice helps students feel more confident in their knowledge for tests and discussions.
Conclusion
The important questions for Class 8 Science Chapter 4, "Combustion and Flame," are essential for understanding key concepts about combustion and flames. They help students learn about different types of combustion and the importance of fire safety. Practising these questions improves exam readiness and connects what they learn to real-life situations. Overall, these questions enhance the learning experience and strengthen knowledge about combustion.
Important Study Materials for Class 8 Science Chapter 4
CBSE Class 8 Science Important Questions for All Chapters
Additional Study Materials for Class 8 Science
FAQs on CBSE Important Questions for Class 8 Science Combustion and Flame - 2025-26
1. What types of important questions should I focus on from Chapter 4, Combustion and Flame, for the CBSE Class 8 Science exam 2025-26?
For the 2025-26 exams, you should prepare for a variety of question formats from this chapter. Expect questions such as:
Very Short Answer (VSA): Based on definitions like ignition temperature or calorific value.
Short Answer (SA): Explaining concepts like the conditions necessary for combustion.
Long Answer (LA): Requiring detailed explanations with diagrams, such as the structure of a candle flame.
HOTS (Higher Order Thinking Skills): Application-based questions that test your conceptual understanding.
Case-Study Questions: Scenarios related to fire safety or environmental pollution from fuels.
2. What are some important 3-mark questions related to the 'conditions necessary for combustion'?
Important 3-mark questions often test your understanding of the fire triangle. Be prepared to answer questions like:
Explain why a blanket wrapped around a person whose clothes have caught fire helps in extinguishing it. (Tests the 'supply of oxygen' concept).
Describe an experiment to prove that a combustible substance, oxygen, and ignition temperature are essential for burning.
Give reasons why a matchstick does not catch fire on its own at room temperature but does so when struck against a rough surface.
3. How can I score full marks on a question about the structure of a candle flame?
To score full marks, a well-labelled diagram is crucial. Your answer must clearly describe the three main zones:
Outermost Zone: Blue in colour, it is the hottest part due to complete combustion. Mention that goldsmiths use this zone.
Middle Zone: Yellow and luminous, this is where partial combustion occurs. It is moderately hot.
Innermost Zone: Black in colour, it contains unburnt wax vapours and is the least hot part of the flame.
4. Why is water not a suitable extinguisher for all types of fires? This is an important application-based concept.
This is a critical concept for exam questions. Water is not always suitable for two main reasons:
Electrical Fires: Since tap water is a good conductor of electricity, using it on an electrical fire can cause electrocution and spread the fire.
Oil Fires: Water is denser than oil. When poured on an oil fire, the water sinks below the oil, and the oil continues to burn on top, potentially spreading the fire as the water flows.
For such fires, a carbon dioxide or foam extinguisher is a more effective and safe choice.
5. What is the fundamental difference between 'rapid combustion' and 'spontaneous combustion', and why is it an important distinction for exams?
The fundamental difference lies in the trigger. Rapid combustion requires an external agent to initiate it, like lighting an LPG stove with a spark. In contrast, spontaneous combustion occurs without any apparent external cause, such as the self-ignition of white phosphorus at room temperature. This distinction is important because exam questions may ask you to classify examples or explain why forest fires can sometimes start on their own due to the heat of the sun (an example of spontaneous combustion).
6. What kind of Higher Order Thinking Skills (HOTS) questions are important from the 'Combustion and Flame' chapter?
HOTS questions from this chapter move beyond simple definitions. Expect questions that require you to apply concepts. For example:
A paper cup filled with water does not burn when heated. Why?
Can the process of rusting be called slow combustion? Justify your answer.
Why is it advised to not sleep in a closed room with a burning coal fire?
These questions test your deeper understanding of ignition temperature, the definition of combustion, and the products of incomplete combustion.
7. A goldsmith uses the outermost zone of a flame to melt gold. What is the scientific reason, and how could this be asked as a 5-mark question?
The scientific reason is that the outermost zone of a flame undergoes complete combustion, making it the hottest part of the flame. This high temperature is sufficient to melt gold and silver. A 5-mark question could be structured as:
(a) Why does a goldsmith use the outermost zone of a flame? (2 marks)
(b) Draw a neat, labelled diagram of a candle flame showing all its zones. (3 marks)
8. What are some important definitions and units I must know from this chapter for 1-mark questions?
For short, 1-mark questions, you must be very clear with the following definitions and their units:
Combustible Substances: Substances that undergo combustion.
Ignition Temperature: The lowest temperature at which a substance catches fire.
Calorific Value: The amount of heat energy produced on complete combustion of 1 kg of a fuel. Its unit is kilojoules per kilogram (kJ/kg). This unit is very important to mention.
Inflammable Substances: Substances with a very low ignition temperature that easily catch fire.
9. How might a case study on the 'harmful effects of fuels' be presented in the exam?
A case study could present a short paragraph about rising air pollution in a city. Based on this, you may be asked important questions like:
Which pollutants from burning fuels like coal and diesel lead to acid rain?
Explain how the incomplete combustion of fuel leads to the formation of a poisonous gas. Name the gas (Carbon Monoxide).
What is global warming, and how is it linked to the increased consumption of fuels?
Suggest a cleaner fuel alternative like CNG and justify your choice.
