Combustion is a chemical process in which a substance reacts with oxygen, releasing energy in the form of heat and light. This process is essential in everyday life, as it powers vehicles, generates electricity, and provides heat for cooking.
Combustion can occur in various forms, such as complete combustion, where the fuel burns completely with oxygen, and incomplete combustion, where the fuel burns partially, producing harmful substances like carbon monoxide. In this chapter, students will learn about the types of combustion, the factors affecting it, and the importance of fuels in daily life. Understanding combustion is crucial for managing energy use and reducing pollution.
Class 8 Science Ch 5 Combustion Oxford Textbook Answers
Objective Type Questions
A. Fill in the blanks with the correct words.
| Question | Answer |
|---|---|
| 1. …………… (Inflammable/Non-flammable) substances have low ignition temperature. | Inflammable substances have low ignition temperature. |
| 2. Non-luminous flame is of ……….. (blue/yellow) colour. | Non-luminous flame is of blue colour. |
| 3. …….. (Outer/Inner) zone is the coolest part of the flame. | Inner zone is the coolest part of the flame. |
| 4. ……. (Exhaustible/Inexhaustible) natural resources have limited availability in nature. | Exhaustible natural resources have limited availability in nature. |
| 5. …….. (Coal/Natural gas) was formed from the remains of plants. | Coal was formed from the remains of plants. |
| 6. ……….. (Extraction/Refining) of petroleum is done to separate petroleum into useful products. | Refining of petroleum is done to separate petroleum into useful products. |
B. Choose the correct option.
| Question | Answer |
|---|---|
| 1. The colour of the flame depends upon: a. Temperature b. Amount of air available c. Nature of the substance burning d. All of these | ✅ d. All of these |
| 2. What burns in a candle flame? a. Solid wax b. Liquid wax c. Wax vapours d. Oxygen | ✅ c. Wax vapours |
| 3. Which of the following is produced in incomplete combustion of hydrocarbons? a. Carbon dioxide b. Carbon monoxide c. Oxygen d. Methane | ✅ b. Carbon monoxide |
| 4. Which of the following is a characteristic of an ideal fuel? a. High calorific value b. Low ignition temperature c. High rate of combustion d. Should cause pollution | ✅ a. High calorific value |
| 5. Which of the following is the main constituent of natural gas? a. Oxygen b. Petrol c. Methane d. Carbon dioxide | ✅ c. Methane |
| 6. Which of the following is an air pollutant? a. Carbon monoxide b. Oxides of nitrogen and sulphur c. Soot particles d. All of these | ✅ d. All of these (Carbon monoxide, oxides of nitrogen and sulphur, soot particles) |
| 7. In which of the following cases, water should not be used for extinguishing fire? a. Burning diesel b. Burning petrol c. Electrical fire d. All of these | ✅ d. All of these (Water should not be used on oil fires or electrical fires.) |
| 8. Which of the following fuels is considered an ideal fuel for domestic use? a. Hydrogen b. Petrol c. Cow dung cakes d. Methane | ✅ d. Methane (Main component of LPG and natural gas) |
| 9. Which of the following types of coal is used for manufacturing coke? a. Lignite b. Peat c. Bituminous coal d. Anthracite | ✅ c. Bituminous coal |
| 10. Which of the following products of petroleum is used in candles, ointments and Vaseline? a. Fuel oil b. Paraffin wax c. Lubricating oil d. Asphalt | ✅ b. Paraffin wax |
| Column A | Column B |
|---|---|
| 1. Oxides of sulphur and nitrogen | d. Acid rain |
| 2. Domestic fuel | a. Methane |
| 3. Diesel | e. Heavy motor vehicles |
| 4. Spontaneous combustion | b. White phosphorous |
| 5. Fire extinguisher | c. Carbon dioxide |
Ans:
| Column A | Column B | Answer |
|---|---|---|
| 1. Oxides of sulphur and nitrogen | d. Acid rain | 1 → d |
| 2. Domestic fuel | a. Methane | 2 → a |
| 3. Diesel | e. Heavy motor vehicles | 3 → e |
| 4. Spontaneous combustion | b. White phosphorous | 4 → b |
| 5. Fire extinguisher | c. Carbon dioxide | 5 → c |
II. Very short answer type questions
A. Give one word for the following.
| Question | Answer |
|---|---|
| 1. Burning of a substance in the presence of air or oxygen with the liberation of heat and light | Combustion |
| 2. The combustion which takes place in adequate amount of air or oxygen | Complete combustion |
| 3. A region where combustion of fuel takes place | Combustion chamber |
| 4. Flame that emits a lot of light | Luminous flame |
| 5. The amount of heat produced in kilojoules when one gram of fuel is completely burned | Calorific value |
| 6. The resources which have limitless supply | Inexhaustible resources |
| Question | Answer |
|---|---|
| 1. Why is water not always used for extinguishing fire? |
Water is not always used for extinguishing fire because: • Electrical fires: Water is a good conductor of electricity and can cause electrocution if used on electrical fires. • Flammable liquids: Water can spread the fire if it is used on burning substances like oil or petrol, as these liquids are less dense than water and can float, spreading the fire further. |
| 2. State any two characteristics of an ideal fuel. |
• High calorific value: An ideal fuel should produce a large amount of heat when burned. • Low ignition temperature: It should catch fire easily at a lower temperature, making it convenient to use. |
| 3. Why are methane and LPG considered ideal for use as domestic fuels? |
Methane and LPG (Liquefied Petroleum Gas) are considered ideal for domestic use because: • High calorific value: They release a large amount of heat, making them efficient fuels. • Clean burning: They burn completely with a blue, non-luminous flame, producing little to no smoke or soot, which is beneficial for indoor use. • Easy to transport and store: They are available in cylinders, making them easy to store and transport. |
| 4. List the different types of coal with their carbon content and one use. |
• Peat: Carbon content: 60–70% Use: Primarily used as fuel for power plants in some regions, though it’s not very efficient. • Lignite (Brown Coal): Carbon content: 60–70% Use: Used in power generation, but it has a high moisture content and low energy output. • Bituminous Coal: Carbon content: 70–80% Use: Used extensively in power plants, steel manufacturing, and as domestic fuel. • Anthracite (Hard Coal): Carbon content: 80–95% Use: Used in home heating, as it burns very cleanly and efficiently. |
| 5. What are the various products obtained from refining of petroleum? Write one use for each. |
• Gasoline (Petrol): Used as fuel for cars, motorcycles, and other vehicles. • Kerosene: Used as a fuel for jet engines (aviation kerosene) and for heating and lighting in some areas. • Diesel: Fuel for trucks, buses, and other heavy vehicles. • Lubricating oil: Used to lubricate engines and machinery to reduce wear and tear. • Paraffin wax: Used in candles, ointments, and for coating materials. • Bitumen: Used for road construction (asphalt) and waterproofing. • Liquefied Petroleum Gas (LPG): Used as a domestic fuel for cooking and heating. |
| Question | Answer |
|---|---|
| 1. Write any one difference between the following: |
a. Combustible and non-combustible substances: Combustible substances can catch fire and burn when exposed to heat, while non-combustible substances do not burn under normal conditions. b. Rapid and spontaneous combustion: Rapid combustion occurs quickly with a visible flame when a substance burns in the presence of oxygen, while spontaneous combustion happens without an external flame or spark, often due to heat buildup inside a substance. c. Complete combustion and incomplete combustion: Complete combustion occurs when a substance burns with sufficient oxygen, producing carbon dioxide and water, while incomplete combustion occurs with insufficient oxygen, producing carbon monoxide, carbon (soot), and water. d. Blue and yellow flame: A blue flame indicates complete combustion, is hotter, and produces less soot, while a yellow flame indicates incomplete combustion, is cooler, and produces more soot. e. Exhaustible and inexhaustible natural resources: Exhaustible resources are limited in supply and can be depleted (e.g., coal, petroleum), while inexhaustible resources are available in unlimited quantities and cannot be depleted (e.g., solar energy, wind). |
| 2. Explain the different zones of a candle flame. |
A candle flame has three main zones: (i) Outer zone: Blue, hottest, and non-luminous part where complete combustion occurs. (ii) Middle zone: Yellow and luminous, partial combustion occurs here. (iii) Inner zone: Dark and least hot, consisting of unburnt wax vapors.  Diagram: Zones of a Candle Flame |
| 3. What are fossil fuels? How were coal and petroleum formed? |
Fossil fuels are energy sources formed from the remains of dead plants and animals buried under the earth millions of years ago. Formation of coal: Dead plants buried under soil got converted into peat and then coal due to high pressure and temperature over millions of years. Formation of petroleum: Marine plants and animals buried under sea sediments decomposed under heat and pressure to form petroleum and natural gas.  Diagram: Formation of Coal and Petroleum |
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