Important Questions Class 8 Science Chapter 6

Important Questions Class 8 Science Chapter 6 – Combustion and Flames

Fire is an essential element used in daily activities like domestic, industrial, and commercial purposes. Class 8 Science Chapter 6 talks about ‘Combustion and Flames’. This chapter discusses topics around combustion, spontaneous combustion, flammable substances, ignition temperature, and essential requirements of fire and its control. It also describes good fuel and its characteristics, the harmful effects of burning fuel in  the environment and its ill effects. 

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Our question bank Important Questions Class 8 Science Chapter 6 provides students with solutions to essential questions that cover most of the topics discussed in the chapter. The students get a vivid description of what the chapter is all  about, and it helps them learn more about fire, combustion, and various other terminology and practical uses. The stepwise solutions help students to prepare for their upcoming examinations, broadly covering  the key aspects of the chapter.

Class 8 Science Chapter 6 Important Questions boost students confidence level in achieving a high grade in the exam.. The Science faculty at Extramarks have compiled a set of Important Questions Class 8 Science Chapter 6, which helps students revise and clear doubts regarding the chapter which come useful when they are faced with tricky questions which most students miss out due to careless slips. 

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Important Questions Class 8 Science Chapter 6 – With Solutions

A complete list of Important Questions Class 8 Science Chapter 6 has been curated for all the students by the Extramarks team. These questions have been compiled by following NCERT books while adhering to r the CBSE syllabus and guidelines… These questions also encourage and create  an understanding among the students about their exam question pattern and prepare accordingly. Science Class 8 Chapter 6 important questions are solutions to various questions, including MCQs, short answers, long answers, etc. By registering on the Extramarks website, students are granted access to all these explicit materials.

Extramarks believes in incorporating joyful learning experiences through its own repository.We understand the importance of solving important questions and we take our role seriously to provide the best resource to the students and help them excel in life. Here are some of the questions and their solutions from our question bank Important Questions Class 8 Science Chapter 6.

Question 1. Which of the following are products of combustion?

1. Carbon dioxide
2. Nitrogen
3. Water
4. Energy

 Answer1. The correct option is A Carbon dioxide

Combustion, also a term for burning, is a high-temperature exothermic chemical reaction between a fuel (carbon) and an oxidant, usually, atmospheric oxygen, that produces carbon dioxide and other compounds, often a gaseous mixture termed smoke. Water is produced along with smoke and other chemicals. Combustion also gives out heat energy.

Nitrogen does not participate in combustion.

An example of combustion is given below.

CH4(Methane)+2O2⟶CO2+2H2O+Heat

Question 2. State the conditions under which combustion occurs. Why is it more difficult to burn some combustible substances than others?

Answer 2: Combustion of fuel requires:

• The presence of any flammable substance, such as coal, natural gas, or wood.
• The presence of air provides oxygen.
• Heat is available for raising the fuel temperature above its particular ignition point.

Because these different substances catch fire at different temperatures, certain flammable substances are much more difficult to burn than others. Because some of these compounds have a lower ignition temperature, they easily catch fire. The temperature at which a substance ignites is critical in assessing its combustibility.

Question 3. Explain how CO2 can control fires.

Answer 3:

• Oxygen is a supporter of combustion.
• Carbon dioxide is heavier than oxygen and forms a blanket around the fire.
• Thus, cutting off the supply of oxygen to the fire.
• It also reduces the temperature of the fuel.

CO2 is a non-combustible gas. It is also a non-supporter of combustion. CO2 extinguishes fire in two ways: Since it is heavier than oxygen, it forms and covers the fire like a blanket form and cuts off the contact between oxygen and fuel. In cylinders, CO2 is mostly kept in liquid form. When released, it expands enormously and cools down. This brings down the fuel’s temperature, which helps control the fire.

Question 4. Which flame zone does a goldsmith use to melt gold and silver, and why?

Answer 4: The goldsmith uses the outermost flame zone for melting gold and silver because it is considered the hottest portion of the flame and hence undergoes complete uniform combustion.

• The outermost zone of a flame is blue: It is a non-luminous zone.
• In this zone, the fuel’s complete combustion occurs because there is plenty of air surrounding it.
• The outermost part has the highest temperature in the flame.
• It is s the hottest part of the flame at that time.
• It is quite thin and sleek as compared to the middle zone.

Question 5. Give Reason: Paper by itself catches fire easily, whereas a piece of paper wrapped around an aluminium pipe does not.

Answer 5: 

• The lowest temperature when the substance catches fire is called ignition temperature.
• A substance cannot catch fire unless it reaches its initial temperature.
• The Ignition temperature of paper is very low, so it catches fire easily.
• The ignition temperature increases when a paper is wrapped around an aluminium pipe so it does not catch fire easily.
• This happens because aluminium is a metal. As a result, it is a very good conductor of heat, and when it tries to burn, it absorbs all the heat energy.
• Therefore, aluminium is a good conductor of heat; when the paper is wrapped around, it does not catch fire.

Question 6. What are the main three essential requirements to produce fire? How a fire extinguisher is useful for controlling fire.

Answer 6:  Three essential requirements to produce gas are:

1. Oxygen.
2. Combustible substance or fuel.
3. Heat to acquire ignition temperature.

Fire extinguisher:

• A fire extinguisher is used to control small fires.
• A fire extinguisher contains chemicals such as Sulfuric acid (H2SO4) and Sodium bicarbonate (NaHCO3)

Working of Fire extinguisher:

• When a fire extinguisher is operated by pressing the knob, the sulphuric acid reacts with sodium bicarbonate solution, producing carbon dioxide gas (CO2).
• The carbon dioxide is non-combustible. The fire is controlled when the contact between the fuel and oxygen is cut off.

The fire extinguishers are very useful for controlling the fire in the following ways:

1. Fire extinguishers extinguish the fire by turning  down the supply of air, bringing down or lowering the temperature of the fuel, or both.
2. Water is the most commonly used fire extinguisher. But water works only when things like paper and wood are on fire. If an electrical equipment is on fire, water can conduct electricity and harm those things trying to douse the fire. Water is also not very suitable for fires involving oil and petrol. Water is heavier than oil. Hence, it sinks below the oil, and the oil keeps burning on top.
3. For fires that are involved in electrical equipment and combustible materials like petrol or diesel, carbon dioxide (CO2) is the best extinguisher. CO2, being more heavier than oxygen, covers the fire like that of a blanket. Since the contact established between the fuel and oxygen is snapped  fire can be   controlled. The additional advantage of CO2 is that, in most of the cases, it does not harm electrical equipments.

Question 7. Choose the right statement about inflammable substances from the following options given below. They have

1. low ignition temperature and cannot catch fire easily
2. high ignition temperature and can catch fire easily
3. low ignition temperature and can catch fire easily.
4. high ignition temperature and cannot catch fire easily.

Answer 7: The correct option is C. low ignition temperature and can catch fire easily.

The explanation for the correct option:

Option (C): Low ignition temperature and can catch fire easily

1. Combustible substances burn in the presence of air to produce heat and light. Example: petrol, coal
2. Inflammable substances are highly combustible, i.e. they catch fire easily as they have low ignition temperatures.
3. The lowest temperature when a substance will catch fire and undergoes combustion, releasing heat and light, is called the ignition temperature.
4. Therefore, both conditions should be fulfilled for a substance to be inflammable.
5. Hence, it is the correct option.

Question 8. People usually keep Angethi or burning coal in their closed rooms during winter. Why is it advised to keep the door open?

Answer 8:

• When Angethi/ Coal is burned, various hazardous chemicals, including carbon dioxide and carbon monoxide, are released.
• This process tends to reduce oxygen flow in the atmosphere, which mainly causes severe respiratory problems inside the locked room.
• This explains why it is advised to keep the door open while burning a call inside the house, allowing the proper flow of air and oxygen in the atmosphere.
• Angethi produces heat while burning coal and has glowing coal or charcoal bits but forms little or no flames when used.
• Also, the Coal-fired power stations contribute severely to air pollution because they emit sulphur dioxide, which is known as a hazardous chemical connected to acid rain, in addition to the emission of toxic carbon monoxide gas and  hazardous metals like mercury.

Hence, the reasons mentioned above explain why keeping the door open while burning coal is advisable.

Question 9. Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

Answer 9:  The following reasons can be considered to discourage the use of wood as a fuel-

• It has a high ignition temperature, so the combustion process of wood would be a bit tough.
• The ignition temperature of a substance is said to be the minimum temperature at which the substance catches fire for spontaneous combustion.
• Burning wood produces many harmful gases such as Carbon monoxide (CO) and carbon dioxide (CO2). Excess of both can kill a person, and it also causes  global warming.
• To use wood as fuel, we would have to cut trees.
• Trees are essential for any living being.

Question 10. The cracker on the ignition produces sound. Why?

Answer 10: Upon ignition, the cracker releases gas, which is heated by the amount of heat generated by the cracker. This led to an explosion resulting in the crackling  sound.

Question 11. Forest fire produces a lot of air pollution. Write a brief about the reasons for forest fires.

Answer 11: Forest is a large geographical area dominated by trees, animals of various species, aquatic biomes and microorganisms. Forest fire is synonymous with disaster. Even before human involvement, natural, low-intensity wildfires occurred every few years to burn fuel, plant debris, and dead trees. The reasons for forest fires are listed below.

1. Dry grass in the forest, when it gets very dry, high temperature helps it to burn, which causes a forest fire.
2. Lightning from the sky could  also be a reason.
3. Sometimes when villagers use fire to ward off animals, it burns the dry grass and results in forest fires.
4. Friction between bamboo produces heat and burns  bamboo, resulting in  forest fire.

Question 12. What do you understand by fuel efficiency?

Answer 12: Fuel efficiency:

• Fuel efficiency is usually expressed in terms of the calorific value, which is expressed in units of kilojoule per kg.
• The amount of heat that is produced on the complete combustion for 1 kg of a fuel is called its calorific value.
• The calorific value of fuel is kilojoule per kg (kJ) that is expressed in a unit. 

Example:

• The calorific value of coal is 25000-33000 kJ/kg

Question 13. You are provided with three watch glasses containing each milk, petrol and mustard oil. Suppose you bring burning candles near these materials; which material will instantly catch fire and why?

Answer 13: Petrol will catch fire instantly because it is highly flammable. Mustard oil and milk have very high ignition temperatures. So they will not catch fire instantly.

• Petrol is highly volatile, i.e. it forms vapour at low temperatures. Hence, it is more flammable.
• The higher the chaining in chemical composition, the lower the volatility, hence the flammability.
• Kerosene is a petroleum product, so kerosene  ignition temperature is low compared to other given substances.
• The ignition temperature is also known as the ignition point. The ignition temperature of fuel can be termed as the minimum temperature to which fuel should be heated so that it can catch fire and start burning.

Question 14. If a person’s clothes catch fire, the best way to extinguish the fire is to: 

• throw water on the clothes. 

• use the fire extinguisher. 

• cover the person with a woollen blanket. 
•  cover the person with a polythene sheet.

Answer 14.

1. cover the person with a woollen blanket.

If a person’s clothes are catches  fire, the best and most used way to extinguish the fire is the method to cover the person with a woollen blanket. By covering it with a blanket, we can  prevent  oxygen supply, which supports combustion. Throwing water on him  is not desirable  if the cause of fire was electricity.  

1. Educate everyone on risks of fire and safety
2. Keep the lawn clippings and other  flammable Items in safe places.
3. Check electrical wiring frequently.

Question 15. Give two examples each for solid, liquid and gaseous fuel, along with some important uses.

 Answer 15. Any substance that, upon combustion, produces a usable amount of energy is known as fuel.

For Solid

Fuels that are found in a solid state at room temperature are generally referred to as Solid Fuels. For fuels, we can take coal and wood.

• Coal is frequently used in factories and industries, while wood is mostly used in homes.
• They are used for cooking purposes.

For Gases

Besides being manufactured from solid and liquid fuels, gaseous fuels are available in nature. CNG and LPG are considered gaseous fuels.

• CNG is also used to drive vehicles. LPG is used in vehicles and homes.
• LPG is used like a fuel for industries.

For Liquid

Most liquid fuels are obtained from the fossilised remains of dead animals and plants by exposure to pressure and heat present in the Earth’s crust. Petrol is an example of liquid fuel.

• Kerosene is mostly used in stoves for cooking food, while petrol is used in vehicles.
• Kerosene is also used in lamps.

Question 16. Manu was heating oil to fry some potato chips. The cooking oil suddenly caught fire; he poured water to extinguish the fire. Do you think this particular action was suitable? If yes, why? If not, why not? In such a condition, what should have Manu done?

Answer 16: No. Because water is not really suitable for fires involving oil.

Oil is lighter than water. When one pours water on burning oil, it’ll settle at the bottom of the vessel. But, pouring soil or sand may help extinguish the fire by reducing the airflow. The particles will not allow the air to pass through the oil, and the fire will stop. This happens due to the flow of oxygen reduces . And without the presence of oxygen, burning of fire is not possible. Manu should have turned  off the burner’s flame and put a lid on the frying pan. Doing this, the contact established between fuel and oxygen is cut off, and the flame extinguishes

Question 17. Why is the use of petrol and diesel for fuels in automobiles being replaced by (CNG) Compressed Natural Gas in big cities?

Answer 17: CNG (Compressed Natural Gas) is a much better fuel than any other fuel present right now. It has no smoke and doesn’t produce any harmful gas that harms society and the environment. So CNG is replacing various other fuel options such as petrol and diesel.

The properties of CNG make it a safer fuel. It is stored and preserved in high gauge seamless cylinders, which are certified, so there is negligible chance of its leakage. It is lighter than the air, so in case of a leak, it just rises and disperses into the atmosphere and mixes with the air easily and evenly.

• It is relatively more abundant than the rest of the fossil fuels, i.e. coal and petroleum.
• It is also a safer fuel, lighter than air, and dissipates rather than explodes.
• It provides instant energy, which is why it is used in the oven cooking, as it does not require preheating.

Question 18. Which among the following is considered the cleanest fuel 

• Petrol

• Kerosene

•  Hydrogen Gas
•  Cow Dung Cake

Answer 18. The correct answer is (c) Hydrogen Gas

Hydrogen gas is considered as the cleanest fuel because it produces water vapour when it burns in the air.

Benefits of Solving Important Questions Class 8 Science Chapter 6

Important Questions Class 8 Science Chapter 6 provides simplified and accurate  answers to each question. All the questions mentioned are more likely to be asked in exams, so students are advised to revise and go through them thoroughly. Step-by-step solutions and MCQ or fill-in-the-blank explanations help students understand the concepts discussed in each chapter clearly and clarify doubts about the particular concept. After revising MCQs, short answers, medium answers, and long answer questions provided with simplified solutions, students will get well versed with their question pattern and it encourages the students to master the topic and increase their confidence in achieving a higher grade.  . 

Furthermore, the several benefits of referring to Important Questions Class 8 Science Chapter 6 are:

• The stepwise solutions to every question curated by the Extramarks team help students to revise and understand all the important theoretical concepts discussed in the chapter and score good marks in their examinations.
• Students can entirely rely on the  solutions as they have been compiled by following NCERT books while adhering to  the latest CBSE syllabus and guidelines…
• The questions given are expected  in the examinations. This will help students get a basic idea of the questions that are repeatedly asked  in the exams.Subject experts have created several pointwise notes that help students conveniently recall the crucial points in the exam and use these solutions for last-minute revisions. 
• The Science faculty at extramarks have years of  teaching experience who diligently follow the latest CBSE guidelines have prepared these study materials. Needless to say, they completely understand what is legitimate as per the board’s standards

•  The experts  understand the key topics which students might find challenging while studying, therefore the study materials are prepared in such a way that they can get to the point answers without wasting much time on a single subject.
• The questions mentioned in Important Questions Class 8 Science Chapter 6 are collected from various sources to ensure that they are fully authentic in content and free from omissions, errors, ambiguities, etc.

To browse through and practice the Important Questions Class 8 Science Chapter 6, students are advised to register on the Extramarks website. 

Furthermore, students can also get access to several other study materials by clicking on the links given  below:

• NCERT books
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• cbse past  years’ question papers
• Important formulas 
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