NCERT Class VIII Science Solutions: Chapter 6-Combustion and Flame Part 6

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LONG ANSWER QUESTIONS

Question 31:

You are provided with three watch glasses containing milk, petrol and mustard oil, respectively. Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why?

Answer:

The watch glass containing petrol will catch fire instantly because its ignition temperature is very low. Also, petrol is an inflammable substance, it can easily catch fire with a flame.

Question 32:

Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable? If yes, why? If not, why not? In such a condition what should Manu have done?

Answer:

  • Water is not suitable for fires involving oil.

  • Manu should have switched off the flame of the burner and put a lid on the frying pan. By doing this the contact between fuel and oxygen is cut off and the flame will go off.

Question 33:

What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire.

Answer:

Three essential requirements.

  1. Fuel

  2. Air

  3. Heat to acquire the ignition temperature

The job of fire extinguisher is to cut off the supply of air or to bring down the temperature of fuel or both.

The most common fire extinguisher is water. But water works only when things like wood and paper are on fire. Water extinguisher fire by cooling the burning substance.

Question 34:

Give two examples each for a solid, liquid and gaseous fuel along with some important uses.

Answer:

  1. Types of fuels

    Solid fuel - Coal, wood, etc.

    Liquid fuel - Kerosene oil, petrol etc.

    Gaseous fuel - CNG, LPG etc.

  2. Uses

    Coal – cooking etc.

    Kerosene oil - Fuel for stoves, lamps etc.

    LPG – Fuel for industry etc.

Add more uses.

Image of importance of fuel cell

Image of Importance of Fuel Cell

Image of importance of fuel cell

Question 35:

The calorific values of petrol and CNG are and , respectively. If you have vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?

Answer:

CNG, because the calorific value of CNG is higher than that of petrol. Therefore CNG will be more economical. At the same time it produces the least air pollutants.

Question 36:

Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.

Answer:

  1. Wood produces lot of air pollution.

  2. Use of wood as fuel encourages cutting of trees leading to deforestation.

  3. Trees provide us many useful substances. To obtain fuel wood, when trees are cut down, then all useful substances which can be obtained from trees are lost.