NCERT Solutions for Class 8 Science Chapter 6 Combustion and Flame
Question 1. List conditions under which combustion can take place.
Answer:- The condition under which combustion can take place are :-
- Presence of Oxygen.
- Presence of combustible substance.
- Ignition temperature of the substance
Question 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______________ .
(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.
Question 3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer:- Because of the following reasons the use of CNG in automobiles has reduced pollution in our cities:-
- CNG is a cleaner fuel.
- CNG can be used as a substitute for diesel, gasoline, and propane/LPG.
- CNG emits fewer global warming and acid rain causing gases like carbon dioxide, carbon monoxide, sulphur dioxide, and nitrogen dioxide.
- CNG does not leave any residue after combustion.
Question 4. Compare LPG and wood as fuels.
It does not cause pollution on burning.
It cause pollution on burning.
It doesn’t produced smoke.
It produced smoke.
It is found in Liquid form.
It is found in solid form.
It contains more calories (55000 kJ/kg).
It contains less calories (55000 kJ/kg).
Because it is stored in cylinders, it is easily transportable.
It is not as easily transportable as LPG fuels.
Question 5. Give reasons:
(a) Water is not used to control fires involving electrical equipment.
Answer:- (a) Because water is a good conductor of electricity, it may cause electric shocks to those trying to extinguish the fire.
(b) LPG is a better domestic fuel than wood.
Answer:- (b) LPG is a better home fuel than wood because it does not emit harmful gases and leaves no residue. It also has a higher calorific value than wood.
(c) Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.
Answer:- (c) Due to its low ignition temperature, the paper easily catches fire on its own. A piece of paper wrapped around an aluminium pipe, on the other hand, does not easily catch fire because the heat is absorbed by the pipe and the piece of paper does not reach its ignition temperature.
Question 6. Make a labelled diagram of a candle flame.
Question 7. Name the unit in which the calorific value of a fuel is expressed.
Answer:- The unit in which the calorific value of a fuel is expressed is kilojoules per kilogram (kJ/kg).
Question 8. Explain how CO2 is able to control fires.
Answer:- As Carbon dioxide is heavier than oxygen, it covers the flames like a blanket and prevents oxygen from reaching the fuel. In this way, CO2 plays an important role in controlling fires.
Question 9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer:- A heap of green leaves has a lot of moisture in it, therefore its ignition temperature is high. Hence, it does not catch fire easily.
However, dry leaves have no moisture in it, hence their ignition temperature is low. Hence, it catches fire easily.
Question 10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer:- The non-luminous outermost zone of a flame is used by a jeweller to melt gold and silver since it is the hottest zone of the flame and has the highest temperature.
Question 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.
Question 12. Can the process of rusting be called combustion? Discuss.
Answer:- The process of rusting generates heat during the formation of its oxide. So we can say the process of rusting as slow combustion.
Question 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?
Answer:– Ramesh’s beaker water placed in the outermost part of the flame will be heated in a short time because it is a non-luminous flame and the hottest part of the flame.