Chapter 4 Combustion And Flame Question Answers

Class 8 Ncert Question and Answers, NCERT Class 8 Science Question/Answers / By

Important Terms to Learn Before Starting Making Notes 🗒

Acid Rain (अम्लीय वर्षा): Rainfall made acidic by atmospheric pollution, causing environmental harm, particularly to forests and lakes.
Calorific Value (ऊष्मीय मान): The amount of energy produced by the complete combustion of a material or fuel.
Combustion (दहन): A chemical process in which a substance reacts with oxygen to give off heat.
Deforestation (वन-विनाश): The clearing or thinning of forests by humans.
Explosion (विस्फोट): A violent and destructive shattering or blowing apart of something, as is caused by a bomb.
Flame (ज्वाला): The visible, gaseous part of a fire.
Fire Extinguisher (अग्निशामक यंत्र): A portable device that discharges a jet of water, foam, gas, or other material to extinguish a fire.
Fuel (ईंधन): A material that is burned or altered to obtain energy.
Fuel Efficiency (ईंधन दक्षता): A measure of how effectively a fuel is converted into energy.
Global Warming (वैश्विक ऊष्मीकरण): The gradual increase in the overall temperature of the Earth’s atmosphere due to the greenhouse effect.
Ideal Fuel (आदर्श ईंधन): A fuel that has high efficiency, is cost-effective, and causes minimal environmental impact.
Ignition Temperature (ज्वलन तापमान): The minimum temperature at which a fuel must be heated for it to catch fire.
Inflammable Substances (ज्वलनशील पदार्थ): Materials that can easily catch fire and burn quickly.

Exercise Question Starts Here: Start Working 🖊️

1. List Conditions Under Which Combustion Can Take Place.

Combustion can take place under the following conditions:

  1. Presence of a Combustible Substance: There must be a substance that can burn.
  2. Presence of Oxygen: Oxygen (or an oxidizing agent) must be available to support the combustion process.
  3. Ignition Temperature: The combustible substance must be heated to its ignition temperature for combustion to start.

2. Fill in the Blanks

(a) Burning of wood and coal causes pollution of air. (b) A liquid fuel, used in homes is kerosene. (c) Fuel must be heated to its ignition temperature before it starts burning. (d) Fire produced by oil cannot be controlled by water.

3. Explain How the Use of CNG in Automobiles Has Reduced Pollution in Our Cities.

The use of CNG (Compressed Natural Gas) in automobiles has significantly reduced pollution in our cities because:

  1. Lower Emissions: CNG burns more cleanly than petrol or diesel, producing fewer pollutants like carbon monoxide, hydrocarbons, and nitrogen oxides.
  2. Reduced Greenhouse Gases: CNG combustion results in lower emissions of carbon dioxide, a major greenhouse gas.
  3. Less Particulate Matter: CNG engines emit negligible particulate matter compared to diesel engines, which helps improve air quality.

4. Compare LPG and Wood as Fuels.

LPG:

  1. Clean Burning: LPG burns cleanly with a blue flame, producing very little smoke or soot.
  2. High Calorific Value: LPG has a higher calorific value, meaning it produces more heat per unit mass.
  3. Convenience: LPG is easy to store and transport in cylinders and provides instant heat.
  4. Less Pollution: Burning LPG produces fewer pollutants compared to wood.

Wood:

  1. Smoky Combustion: Wood burns with a lot of smoke, producing soot and ash.
  2. Lower Calorific Value: Wood has a lower calorific value compared to LPG.
  3. Less Convenient: Wood is bulkier to store and requires more effort to handle and use.
  4. More Pollution: Burning wood releases more pollutants and particulate matter into the air.

5. Give Reasons.

(a) Water is not used to control fires involving electrical equipment: Water is a good conductor of electricity. Using water to extinguish an electrical fire can lead to electric shocks and further spread the fire.

(b) LPG is a better domestic fuel than wood: LPG burns more cleanly, producing less smoke and soot, has a higher calorific value, and is more convenient to store and use. It also contributes less to indoor air pollution, making it safer for health.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminum pipe does not: The aluminum pipe conducts heat away from the paper, preventing it from reaching its ignition temperature. Thus, the paper wrapped around the aluminum pipe does not catch fire easily.

6. Make a Labelled Diagram of a Candle Flame.

A candle flame can be divided into three zones:

  1. Outer Zone: Blue in color and the hottest part of the flame due to complete combustion.
  2. Middle Zone: Yellow and moderately hot, where partial combustion takes place.
  3. Inner Zone: Dark or black, consisting of unburnt wax vapors.

Here is a simple diagram:

7. Name the Unit in Which the Calorific Value of a Fuel is Expressed.

The calorific value of a fuel is expressed in kilojoules per kilogram (kJ/kg).

8. Explain How CO2 is Able to Control Fires.

CO2 controls fires by displacing oxygen around the flame. Since oxygen is essential for combustion, removing it suffocates the fire. Additionally, CO2 is non-combustible and cools the flame by absorbing heat, further helping to extinguish the fire.

9. It is Difficult to Burn a Heap of Green Leaves But Dry Leaves Catch Fire Easily. Explain.

Green leaves contain a high amount of moisture, which needs to evaporate before the leaves can reach their ignition temperature. This makes them difficult to burn. Dry leaves, on the other hand, have little to no moisture and can easily reach their ignition temperature, catching fire quickly.

10. Which Zone of a Flame Does a Goldsmith Use for Melting Gold and Silver and Why?

A goldsmith uses the outer zone of the flame for melting gold and silver because it is the hottest part of the flame. This zone provides sufficient heat for melting metals due to complete combustion.

11. In an Experiment, 4.5 kg of a Fuel Was Completely Burnt. The Heat Produced Was Measured to Be 180,000 kJ. Calculate the Calorific Value of the Fuel.

The calorific value of the fuel can be calculated using the formula:

Calorific Value=Total Heat Produced (kJ)/Mass of Fuel Burnt (kg)

Calorific Value=180,000 kJ/4.5 kg

Calorific Value=40,000 kJ/kg

12. Can the Process of Rusting Be Called Combustion? Discuss.

No, the process of rusting cannot be called combustion. Although both processes involve oxidation, combustion is a rapid process that produces heat and light, whereas rusting is a slow process where iron reacts with oxygen and moisture to form iron oxide without producing heat or light.

13. Abida and Ramesh Were Doing an Experiment in Which Water Was to Be Heated in a Beaker. Abida Kept the Beaker Near the Wick in the Yellow Part of the Candle Flame. Ramesh Kept the Beaker in the Outermost Part of the Flame. Whose Water Will Get Heated in a Shorter Time?

Ramesh’s water will get heated in a shorter time because he kept the beaker in the outermost part of the flame, which is the hottest part due to complete combustion. Abida kept the beaker in the yellow part of the flame, which is less hot due to incomplete combustion.

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