NCERT Solution For Class 8 Science, Chapter 6 – Combustion and Flame

NCERT Solutions is known as an extremely helpful resource for preparing for the exam. Takshila Learning provides its learners with access to a wealth of NCERT problems and their solutions. CBSE Class 8 Science NCERT Solutions are built by subject matter experts, so be sure to train learners for a good grade. The questions set out in the NCERT Books are prepared in compliance with the requirements of the CBSE.

NCERT Solution For Class 8 Science, Chapter 6 – Combustion and Flame provides us with all-inclusive information on all concepts. As students would have to learn the basics about the subject in class 8, this curriculum for class 8 is comprehensive study material, which explains the concepts in a great way.

1. List conditions under which combustion can take place.

Sol:

The burning of a substance in the presence of oxygen is defined as combustion.

The conditions under which combustion can take place are

• The presence of fuel plays a significant role.
• The presence of air or oxygen.
• Ignition temperature is maintained (It is defined as the substance that catches fire at its lowest 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___________ .

Sol:

(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.

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

Sol:

CNG played an important role in reducing pollution among automobiles for the following reasons

• The CNG can be an alternative for diesel, petrol and propane/LPG.
• It usually contains few undesirable gases than the other fuels mentioned above.
• The combustion of fuels like petroleum causes many un-burnt carbon particles along with carbon monoxide which leads to respiratory diseases.
• CNG is comparatively a cleaner fuel.

4. Compare LPG and wood as fuels

Sol:

Wood

• It is considered as a traditional fuel used for both domestic and industrial purposes.
• Wood produces a lot of smoke which pollutes the atmosphere and cause respiratory diseases.
• The usage of wood to a large extent causes deforestation.
• The calorific value of wood ranges between 17000 to 22000 kJ/kg
• However, wood may be used as a furnace, stove or fireplace in indoors while it is used for a campfire, furnace at outdoors.

      LPG

• The usage LPG (Liquefied petroleum gas) has replaced wood.
• It doesn’t release smoke and other pollutants
• It is a cleaner fuel
• The fuel efficiency of LPG is more than that of wood.
• The calorific value of LPG is 55000 kJ/kg
• Hence LPG is mostly preferred choice

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.

Sol:

a)

• Water is a good conductor of electricity.
• If added to an electrical fire, the water would just spread the electricity further.
• The person dousing the fire might get an electric shock

b)

• LPG is a cleaner fuel than wood smoke and other pollutants.• Wood, on the other hand, leaves a lot of smoke and the atmosphere is polluted by smoke, causing pollution and respiratory diseases.• Therefore LPG is a better domestic fuel than wood.

c)

• Paper in itself catches fire easily due to low ignition temperature• A piece of paper wrapped around an aluminum pipe does not catch fire because aluminum is a good conductor of electricity.• While the paper wrapped around the aluminum pipe increases the ignition temperature. So there is a transfer of heat from the paper to the aluminum pipe. Therefore it does not catch fire.

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

Sol:

Calorific value is defined as the energy contained in the fuel. It is expressed in the form kJ/kg

kJ-kilo joules

kg- kilogram

8. Explain how CO2 is able to control fires.

Sol:

CO2 is a non-combustible gas and extinguishes fire in two ways:

(i) It is heavier than oxygen and it covers the fire like a blanket and cuts off the contact between oxygen and fuel.(ii) CO2 is kept in liquid form in cylinders. When released, it expands too much. This brings down the temperature of the fuel, which helps control the fire.

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

Sol:

There is a lot of moisture in the pile of green leaves, so its ignition temperature is high. Therefore it does not catch fire easily.But there is no moisture in dry leaves, so its ignition temperature is low. Hence it easily catches fire.

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

Sol:

The goldsmith mainly uses a non-luminous flame called the outermost part of the flame. This part of the flame is used because the external flame undergoes complete combustion and is considered the hottest part of the flame.

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

Sol:

No, because corrosion is an external process because heat is released during corrosion. On the other hand combustion is a chemical process in which a substance reacts with oxygen to release energy in heat or light.

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?

Sol:

The water placed at the outermost part of the flame will heat up in a short time as it is a non-luminous flame and is considered the hottest part of the flame. So Ramesh’s beaker will be heated first. However, Abida puts the beaker in a glowing flame (yellow flame), comparatively less warm.

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