# NCERT Solutions Science for Class 8 Chapter 6 COMBUSTION AND FLAME | Class8 Science Chapter 6

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NCERT Solutions Science for Class 8 Chapter 6 COMBUSTION AND FLAME। Below these solutions consist of answers to all the important questions in NCERT book class 8th science Chapter 6. Here we solve Ncert class 8 science Chapter 6 question answer concepts all questions with easy method with expert solutions. It help students in their study, home work and preparing for exam. Soon we provide

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# NCERT Solutions Science for Class 8 Chapter 6 COMBUSTION AND FLAME

## NCERT Solutions Science for Class 8 Chapter 6 COMBUSTION AND FLAME

#### science for class 8 Chapter 6 question answer

1. List conditions under which combustion can take place.
Answer : Required conditions for combustion
fuel,
heat (to raise the temperature of a fuel above its ignition temperature) and
presence of oxygen.

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 of air pollution.
(b) A liquid fuel, used in homes is LPG.
(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.
Answer : Diesel and petrol are now being replaced by CNG (compressed natural gas) as fuel in motor vehicles because CNG produces small amounts of oxides of sulfur and nitrogen. CNG is a cleaner fuel. This greatly reduces pollution.

4. Compare LPG and wood as fuels.
Answer : comparative study between LPG and wood as fuels

 LPG as Fuels Wood as Fuels It has a low ignition point. It has a high ignition point. It burns without smoke. However, burning of wood gives a lot of smoke. LPG not gives poisonous gas. Incomplete combustion of wood gives poisonous carbon monoxide gas. Its calorific value is high. Its calorific value is low. There is no residue left after burning. After burning, ash remains as a residue.

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.

(a) Water is not used to control fires involving electrical equipment.
The most common fire extinguisher is water. But water works only when things like wood and paper are on fire. If electrical equipment is on fire, water may conduct electricity and harm those trying to douse the fire. They are prone to electric shock. Therefore, water is not used to control fire associated with electrical equipment.

(b) LPG is a better domestic fuel than wood.
The ignition point of LPG is lower than that of wood. Due to LPG burns quickly. When wood is burnt, it generates more smoke and CO and CO2 gas is released on burning. Whereas LPG burns with no smoke and no poisonous gas is released. The wood turns into ash as a residue after burning. Whereas in LPG there is no residue left after burning. Hence LPG is a better domestic fuel than wood. So, 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.
Aluminum is a metal. Metal is a good conductor of heat. When the paper is burnt, the temperature of the paper starts increasing, at that time aluminum absorbs the heat of the paper and does not allow it to reach its ignition temperature. This is the reason why paper wrapped around aluminum does not catch fire when it is burnt.

6. Make a labelled diagram of a candle flame.
Answer : Different zones of candle फ्लेम

7. Name the unit in which the calorific value of a fuel is expressed.
Answer : The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

8. Explain how CO2 is able to control fires.
Answer : For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (CO2) is the best extinguisher. CO2, being heavier than oxygen, covers the fire like a blanket. Since the contact between the fuel and oxygen is cut off, the fire is controlled. The added advantage of CO2 is that in most cases it does not harm the electrical equipment.

9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer : Green leaves have more water content. Due to which the heat given to burn the leaves gets absorbed by the water and turns into vapour. Due to the absorption of heat, the leaves do not reach their burning temperature. So it is difficult to burn them.
In contrast, the amount of water in dry leaves is negligible. So that it immediately reaches the ignition temperature and burns.

10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer : Goldsmiths blow the outermost zone of a flame with a metallic blow-pipe for melting gold and silver. they use the outermost zone of the flame. This is because this region is the hottest region. Metals like gold and silver have high melting points. Due to which more heat is required to melt them. Therefore, to melt gold and silver, goldsmiths use the blue-flamed dark region of the flame.

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.
Value of heat per kg (calorific value) = $$\displaystyle \frac{{180000}}{{4.5}}$$