Chapter 6: Science - Class 8
Topic: Sources of Energy
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A. Fill in the Blanks
1. Coal and petroleum are non-renewable sources of energy.
2. Wood is a major renewable, natural resource.
3. The process, in which coal is strongly heated in the absence of air, is known as destructive distillation.
4. In oil wells, natural gas is always found above the layer of petroleum.
5. Coal tar is a black viscous liquid, obtained by destructive distillation of coal.
B. State True or False for the following statements
1. Fossil fuels are renewable natural resources. – False
2. Anthracite has higher carbon content than lignite. – True
3. Coke can be used to make fuel gases. – True
4. Natural gas is found below the petroleum layers. – False
5. Burning of fossil fuels is a major source of pollution. – True
C. Tick the correct option.
1. The highest grade of coal is known as –
i. Peat
ii. Lignite
iii. Bituminous
iv. Anthracite ✅
2. Coke and coal tar are obtained from –
i. Petroleum
ii. Coal ✅
iii. Natural gas
iv. Crude oil
3. Different types of coal generally have the –
i. Same moisture content
ii. Equal heating value
iii. Different content of volatile material as well as different percentage of carbon ✅
iv. Same carbon content
4. A product, that is not obtained through the refining of petroleum, is –
i. Diesel
ii. Petrol
iii. Lubricating oil
iv. Coal gas ✅
5. Which of the following is a renewable source of energy?
i. Natural gas
ii. Tidal energy ✅
iii. Coal
iv. Petroleum
D. Answer the following questions in brief.
1. What is meant by the term 'fuel'?
Answer: A fuel is a substance that produces heat when it burns in air.
Answer: Two renewable natural sources of energy are wind energy and water energy.
Answer: Carbonisation is the process in which dead plant materials are converted into coal over millions of years.
4. What are the three main products obtained from the destructive distillation of coal?
Answer: The three main products obtained from the destructive distillation of coal are coke, coal tar, and coal gas.
5. What are two uses of asphalt?
Answer: Asphalt is used for making roads. It is also used to coat the underside of electric poles to protect them from rusting.
6. What are two uses of paraffin wax?
Answer: Paraffin wax is used as grease. It is also used to make vaseline, ointments, and candles.
7. What are the full forms of CNG, LPG, and PNG?
Answer: The full form of CNG is Compressed Natural Gas. The full form of LPG is Liquified Petroleum Gas. The full form of PNG is Piped Natural Gas.
E. Answer the following questions.
1. Describe, in brief, the process of ‘coal formation’.
Answer: Coal was formed from the remains of plants that got buried deep under the earth’s crust. Over millions of years, these plant remains went through physical and chemical changes due to heat and pressure inside the earth. This slow geological process transformed them into a solid carbon-rich material known as coal.
2. Differentiate between lignite and bituminous coal.
Answer:
Lignite- It has about 38 percent carbon content. It contains high moisture, produces less heat energy, and is mainly used for electric power generation. It is considered the lowest grade of coal.
Bituminous- It has a carbon content of about 65 percent. It has lower moisture, produces more heat energy, and is used both for power generation and for making coke. It is considered a middle-grade coal.
3. How do petroleum and natural gas occur in nature?
Answer: Petroleum is usually found at depths ranging from 500 meters to 2000 meters, trapped between two layers of impermeable rocks. It is often found mixed with salt water, and since petroleum is lighter than salt water, it floats above it. Natural gas is found above the petroleum layer and is trapped between the rock cap and the petroleum.
4. State the main advantages of using natural gas.
Answer: Natural gas has several advantages. It burns easily and has a high calorific value. It can be transported and stored conveniently. It undergoes complete combustion and is easy to control while burning. Being a cleaner fuel, it releases relatively low amounts of harmful gases and does not leave ash after burning.
5. Keeping in mind their availability in nature, explain the need of using coal and petroleum judiciously.
Answer: Coal and petroleum are fossil fuels that take millions of years to form, which means they are non-renewable resources. Their supply is limited, and if we continue using them carelessly, they might get exhausted. Therefore, it is important to use coal and petroleum wisely so that future generations can also benefit from these resources.