Combustion

Chapter 7: Science - Class 8

Topic: Combustion

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Fill in Blanks Match Following Tick Correct Brief Answers Detailed Answers

A. Fill in the Blanks

1. A good fuel should have its ignition temperature above the room temperature.

2. Burning of cowdung cakes is an example of slow combustion.

3. The innermost zone of a candle flame is the coldest.

4. The non-luminous zone of a candle flame is the hottest part of the flame.

5. During incomplete combustion of a fuel, carbon monoxide gas is formed.

B. State True or False for the following statements

1. Coal is an inflammable fuel. – False

2. The outermost part of a candle flame contains unburnt carbon particles. – False

3. The heat produced, by burning completely one kilogram of a given fuel, is known as the efficiency of that fuel. – True

4. All solid fuels have a higher calorific value than liquid and gaseous fuels. – False

5. Sulphur dioxide gas, produced during combustion of fossil fuels, causes acid rain. – True

C. Tick the correct option.

1. An example of a non-combustible substance is –

i. wood

ii. paper

iii. petrol

iv. iron ✅

2. A fuel starts burning only when it is heated to –

i. its boiling point

ii. 100°C

iii. its ignition temperature ✅

iv. its melting point

3. The highest temperature zone of a candle flame is its –

i. yellow zone

ii. dark zone

iii. blue zone ✅

iv. inner zone

4. The amount of heat produced on complete burning of 10 kg of a given fuel in pure oxygen equals H kilojoule. The calorific value of that fuel would equal –

i. H kJ/kg

ii. 10H kJ/kg

iii. 0.1H kJ/kg ✅

iv. H/10 kg

5. The gas produced due to incomplete combustion of carbon fuels is –

i. oxygen

ii. carbon monoxide ✅

iii. carbon dioxide

iv. nitrogen

D. Answer the following questions in brief.

1. What is meant by a combustible material? Give two examples.

Answer: A combustible material is a substance that catches fire easily when heated in the presence of air or oxygen. It produces heat and light during combustion.

Examples of combustible materials include coal and wood.

2. What are the three conditions necessary for combustion to occur?

Answer: The three conditions required for combustion are:

a) A combustible material is present.

b) There is a continuous supply of air.

c) The temperature of the combustible material is higher than its ignition temperature.

3. Under what condition does slow combustion take place?

Answer: Slow combustion occurs when there is an insufficient supply of air.

4. What is the calorific value of a fuel?

Answer: The calorific value of a fuel is the amount of heat energy produced when one kilogram of the fuel is completely burned in pure oxygen.

5. What are two characteristics of an ideal fuel?

Answer: Two characteristics of an ideal fuel are:

a) It should be cheap, readily available, and easy to transport.

b) It should have a high calorific value.

6. Why is water not used to control electrical fires?

Answer: Water should not be used to extinguish electrical fires because the electric current can flow through the water, potentially causing a severe electric shock that could be fatal.

E. Answer the following questions.

1. Draw a diagram showing the different zones of candle flame. Label the zone which contains:

Answer:

(a) unburnt vapours of wax.

Answer: The innermost zone of the candle flame contains unburnt vapours of wax. It is also known as the black zone and is the least hot part.

(b) unburnt carbon particles.
Answer: The middle zone of the candle flame contains unburnt carbon particles. This zone is yellow and luminous and is moderately hot.

👉 Suggested Diagram:
Draw image given in the book

Innermost zone: Unburnt wax vapours (Black)

Middle zone: Luminous zone with unburnt carbon particles (Yellow)

Outer zone: Complete combustion, non-luminous (Blue)

2. Calorific value of wood is 18,000 kJ/kg. How much of wood is required to produce 360,000 kJ of heat energy?

Answer:

3. Why are gaseous fuels considered better than liquid or solid fuels.

Answer: Gaseous fuels are considered better than liquid or solid fuels because:

a) They are easy to transport using cylinders or pipelines.

b) They release a large amount of energy on combustion.

c) They do not leave any ash or solid residue after burning.

d) They produce the least amount of harmful gases, so they are more environment-friendly.

4. Why do we say that ‘burning of fuels’ can cause ‘health hazards’?

Answer: We say that ‘burning of fuels’ can cause health hazards because:

a) Incomplete combustion of carbon fuels produces carbon monoxide, a poisonous gas that can cause breathing problems and even death.

b) Burning of coal and diesel releases sulphur dioxide, which is a suffocating gas and causes acid rain. This can harm buildings, plants, and human health.

c) Burning of fuels releases carbon dioxide, a greenhouse gas that causes global warming. It melts polar glaciers and increases sea level, causing floods in coastal areas.