NCERT Solutions for Class 8th Science
Chapter 6. Combustion And Flame
Text Book Exercise
Text Book Exercise:
Q6. Make a labelled diagram of a candle flame.
Answer : Fig. 6.1 A candle flame
Q7. Name the unit in which the calorific value of a fuel is expressed.
Answer : The calorific value of a fuel is expressed in kilojoules per kg (kJ/kg).
Q8. Explain how CO2 is able to control .
Answer : 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.
Q9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer : Green leaves contain lot of water. So, when we try to burn green leaves, water contained in the leaves cools the combustible materials (leavss), so thaqt its temperature is brought below its ignition temperture. This prevents the burning of green leaves. In case of dry leaves, they do not contain any water. So when buring process starts, its temperature is raised drastically above its ignition temperature and the leaves catch fire easily.
Q10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Answer : The goldsmith uses the outermost zone of a flame with a metallic blow-pipe for melting gold and silver. The flame in the outermost zone has the highest temperature sufficient to melt the gold and silver.
Q11. 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.
Answer : Calorific value of fuels
= kilojoules/kg = (Total heat produced/Total mass burent)
Here, the mass of fuel =4.5 kg
The heat produced =180,000 kJ.
Calorific value of fuel =180,000 kJ/ 4.5 kg
= 40,000 kJ per kg.
Q12. Can the process of rusting be called combustion? Discuss.
Answer : Yes, process of rusting can be called combution, in fact, slow combution, because rusting also takes place in the air (O2) in presence of humidity in the atmosphere.
Q13. 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 : The water of Ramesh,s beaker will get heated with a shorter time because the outermost part of the flame the hottest.