NCERT solution class 8 science chapter 6 involves different questions related to combustion and flame. Hence it is important to go through the descriptive answers in simple language to develop conceptual understanding. The exercise-wide and chapter-wise availability of the solutions ensure that students can prepare well for the science class 8 chapter 6. Combining these solutions with details on combustion and flame in NCERT books makes students ready with the necessary material for preparing science subjects.
Different types of combustion and flame problems:
These include fill-in blanks, comparison, short and long answer questions. Let us start with these different types of NCERT Solutions for Class 8 Science Ch 6:
1, List the conditions under which combustion takes place.
Answer: Combustion is the process of burning a substance in the presence of oxygen. The different condition under which combustion takes place includes:
- The presence of fuel plays an important role in combustion.
- It is important to maintain the ignition temperature, which is the lowest temperature at which a substance catches fire.
- The presence of oxygen or air is a must for combustion to take place.
2. Fill in the blanks:
- The Burning of wood and coal causes __ of air.
- A liquid fuel used in homes is __.
- Fuel must be heated to its__ before it starts burning.
- The fire produced by oil cannot be controlled by __.
Answer:
- The Burning of wood and coal causes air pollution.
- A liquid fuel used in homes is Kerosene.
- Fuel must be heated to its Ignition Temperature before it starts burning.
- The 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: There are multiple ways in which CNG plays a crucial role in reducing automobile pollution in multiple cities. These include:
- It is a perfect alternative for LPG/propane, diesel, and petrol.
- Combustion of petroleum-based fuels causes the release of unburnt particles in the atmosphere, including carbon monoxide, that causes respiratory diseases.
- CNG contains fewer undesirable gases compared to petrol, diesel, propane, and LPG.
- It is a cleaner fuel compared to LPG/propane, diesel, and petrol.
4. Compare LPG and wood as fuels.
Answer: LPG:
- It does not release pollutants or smoke.
- LPG’s fuel efficiency is more than that of wood.
- LPG’s calorific value is 55,000 kJ/kg.
- It is a cleaner fuel.
Wood:
- It produces large amounts of smoke that can cause multiple respiratory diseases and pollute the atmosphere.
- Wood’s calorific value ranges between 17,000 to 22,000 kJ/kg.
- It is a traditional fuel for industrial and domestic purposes, including outdoor furnaces, campfires, stoves, indoor fireplaces, etc.
- The increased use of wood as fuel causes deforestation.
5. Give reasons.
- Water is not used to control fires involving electrical equipment.
- LPG is a better domestic fuel than wood.
- Paper catches fire easily, whereas a piece of paper wrapped around an aluminium pipe does not.
Answer:
- This is because water is a good conductor of electricity. Hence, if it comes in contact with a fire involving electrical equipment, it would spread the fire more. Further, the person using water to anguish the electrical equipment may get electrocuted.
- This is because it never pollutes or releases smoke like wood. Further, there is no threat of pollution and respiratory diseases compared to the use of wood.
- Paper has low ignition temperature and hence catches fire easily. However, aluminium is a good conductor of electricity. A piece of paper wrapped around an aluminium pipe will never catch fire due to the rise in ignition temperature. Aluminium is a good conductor of electricity. Hence, it causes heat transfer from paper to the aluminium pipe.
6. Make a labelled diagram of a candle flame.
Answer:
7. Name the unit in which the calorific value of a fuel is expressed.
Answer: The calorific value of a fuel is expressed in the form of kJ/kg, where kJ is kilojoules and kg is the kilogram. It stands for the energy contained in the fuel.
8. Explain how CO2 can control fires.
Answer: CO2 can control fire as it is a non-combustible gas. Some of the ways include:
- CO2 is stored in liquid form in cylinders, and when released, it expands enormously. It further brings the fuel temperature down and helps control fire.
- It covers fire like a blanket as it is heavier than oxygen. Hence, CO2 cuts off the contact between the fuel and oxygen.
9. It is difficult to burn a heap of green leaves, but dry leaves catch fire easily. Explain.
Answer: The ignition temperature of the heap of green leaves is high due to the presence of large quantities of moisture. The ignition temperature of the dry leaves is low due to small quantities of moisture. Hence, burning a heap of green leaves is difficult, but dry leaves catch fire easily.
10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer: A goldsmith uses the non-luminous flame part that is one of the hottest parts of the flame. This is because the outermost part of the flame undergoes complete combustion and hence is the hottest for melting gold and silver by a goldsmith.
11. In an experiment, 4.5 kg of fuel was completely burnt. The heat produced was measured to be 180,000 kJ. Calculate the calorific value of the fuel.
Answer: Heat produced by 4.5 kg of the fuel = 180000 kJ
Hence heat produced by 1 kg of the fuel = 180000/4.5*1 kJ/kg
= 40,000 kJ/kg
Thus, the calorific value of the fuel is 40,000 kJ/kg.
12. Can the process of rusting be called combustion? Discuss.
Answer: For any process to be called combustion, the substance must react to oxygen and release energy in the form of heat and light. The process of rusting is not combustion as only heat is released during rusting.
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 will be heated first as the outermost part of the flame is the non-luminous and hottest part. Abida’s beaker is in the luminous flame or yellow flame, which is the least hot part of the flame.
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