NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame

NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame will help in building a great foundation of knowledge and make easy for the students to understand basics. Student can rely on these answers as they come from experts who have been teaching for quite some time and know what’s expected in the test papers.

Chapter 6 CBSE Class 8 Science NCERT Solutions will be useful in increasing concentration skill and score better marks in the exams. You can use these to score maximum marks in your exams.

Chapter 6 Combustion and Flame Class 8 Science NCERT Solutions

Exercises

1. List conditions under which combustion can take place.

Solution

Conditions under which combustion can take place are as follows:
(i) Fuel
(ii) Air
(iii) Heat to raise its temperature beyond its ignition temperature.

2. Fill in the blanks :
(a) Burning of wood and coal causes of air.
(b) A liquid fuel, used in homes is
(c) Fuel must be heated to its before it starts burning.
(d) Fire produced by oil cannot be controlled by

Solution

(a) pollution
(b) LPG
(c) ignition temperature
(d) water

3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

Solution

The combustion of fuel like petrol, diesel release many unburnt carbon particles along with poisonous gases like carbonmonoxide, sulphurdioxide, etc. which pollutes the air and cause many harmful disease. As a gaseous fuel, CNG (Compressed Natural Gas) is a clean and efficient fuel. It burns completely in atmospheric air without producing any harmful gases or residue. Therefore, its use in automobiles reduces city pollution.

4. Compare LPG and wood as fuels.

Solution

LPG	Wood
It is in the gaseous state.	It is in the solid state.
It burns easily with a blue flame.	It does not burn easily.
It has low ignition temperature.	It has high ignition temperature.
It does not produce smoke on burning.	It releases a lot of smoke on burning.
It has high calorific value.	It has low calorific value.

5. Give reasons:
(a) Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Solution

(a) Using water to control electrical fires is not recommended because water conducts electricity easily and can result in electric shock, which can harm those attempting to douse the fire.
(b) LPG is a better domestic fuel than wood because it is a cleaner fuel than wood. It neither produces smoke nor does it leave any residue after burning. It has high calorific value. While wood produces a lot of smoke on burning and leaves behind ashes on burning. Wood has low calorific value.
(c) If paper wrapped around an aluminium pipe is brought near a flame, it does not burn because the heat gets transferred to the aluminium pipe and the paper does not reach the ignition temperature.

6. Make a labelled diagram of a candle flame.

Solution

7. Name the unit in which the calorific value of a fuel is expressed.

Solution

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

8. Explain how CO₂ is able to control fires.

Solution

As carbon dioxide is heavier than oxygen, it covers combustible substances like a blanket. The carbon dioxide cuts off contact between combustible substances and air. This extinguishes the fire.

9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Solution

Due to the high moisture content of green leaves, they have the highest ignition temperature. In contrast, dry leaves have a low ignition temperature, so a heap of leaves does not easily catch fire.

10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Solution

The outermost layer, or non-luminous zone, of the flame is usually used by goldsmiths for melting gold and silver since it is the hottest.

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.

Solution

Calorific value = Total heat produced/Total mass burnt
Mass = 4.5 kg
Heat produced = 1,80,000 kJ Calorific value = 1,80,000/4.5
=40,000 kJ/kg

12. Can the process of rusting be called combustion? Discuss.

Solution

The process of rusting is referred to as combustion. Combustion occurs when a substance reacts with oxygen and produces heat and light. Iron rusting is also a chemical reaction in which iron reacts with oxygen slowly and emits heat, so it can be viewed as a slow combustion process.

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?

Solution

Ramesh’s beaker will get heated in a shorter time because the outermost part of the flame is the hottest part.