Science class 8 – Chapter 6- Combustion and Flame – Ncert Exercise Solution (Question-Answer)

Q1.  List conditions under which combustion can take place.

Answer- There are some conditions under which combustion takes place-

• Combustion takes place in the presence of a combustible substance.
• Presence of oxygen (oxygen is a supporter of combustion).
• Ignition temperature of the substance should be attained.

Q2.  Fill in the blanks.

(a) Burning of wood and coal causes  ________________ of air.

Answer– pollution

(b) A liquid fuel, used in homes is __________________.

Answer- LPG

(c) Fuel must be heated to its ___________  __________ before it starts burning.

Answer– ignition temperature

(d) Fire produce by oil cannot be controlled by ___________.

Answer- water

Q3. Explain how the use of CNG in automobiles has reduced pollution in our cities.

Answer- The use of CNG in automobiles has reduced pollution in our cities as given below-

• It produces less carbon dioxide, sulphur dioxide, and nitrogen dioxide. Due to this, CNG is very less responsible for acid rains, global warming, etc.
• After combustion, it doesn’t produce residue.

Q4. Compare LPG and wood as fuels.

Answer-

Q5. Give reasons.

(a) Water is not used to control fires involving electrical equipment.

Answer- Because water (not in pure form) is a good conductor of electricity. It allows to pass an electric current through it and may give electric shocks to the person who is trying to extinguish the fire.

(b) LPG is a better domestic fuel than wood.

Answer– Because LGP doesn’t produce gas and doesn’t leave any residue behind it. It has more calorific value than wood. It can be transported easily.

(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Answer- Because the ignition temperature of paper is low. So, it catches fire easily. But the paper wrapped around an aluminium pipe doesn’t catch fire because aluminium pipe absorbs heat. By this, the paper doesn’t able to get its ignition temperature.

Q6. Make a labelled diagram of a candle flame.

Answer-

Q7. Name the unit in which the calorific value of a fuel is expressed.

Answer- Kilo Joule per Kilogram ( KJ/ Kg).

Q8. Explain how CO₂ is able to control fires.

Answer- We already know that oxygen around us is a supporter of fire. But, Carbon dioxide (CO₂) is quite opposite to oxygen because it is heavier than oxygen. Therefore it forms a blanket of around fire. So, the supply of oxygen to fire is stopped. And hence, the temperature of the burning substance is getting down and in this way, CO₂ is able to control fires.

Q9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.

Answer- Water is present in green leaves in some amount. Therefore,  its ignition temperature is high and it is difficult to burn it.

But, in dry leaves, there is no preserve of water. Therefore it’s ignition temperature is low and thus can catch fire easily.

Q10. Which zone of a flame does a goldsmith use for melting gold and silver and why?

Answer- Gold and silver are used to melt often at high temperatures. Therefore, Goldsmith uses the outermost zone of a flame for melting gold and silver because it is the hottest zone of the flame and having more temperature.

Q11. 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.

Answer-  Calorific Value = Heat produced/ Amount of fuel

=  180,000 Kj / 4.5 Kg

= 40,000 KJ/ Kg

Q12. Can the process of rusting be called combustion? Discuss.

Answer- Yes, Because the rust is formed when iron slowly combines with the oxygen present in air ( in the presence of moisture ) to form iron oxide. The process of rusting of iron is an example of slow combustion.The rusting liberates very little heat but no light. ✔

Q13. 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?

Answer- The water gets heated in a shorter time which was put by Ramesh because he put the beaker nearer to the hottest zone of the flame.