NCERT Class 8 Science Chapter 6 - Combustion and Flame

Chapter 6 - Combustion and Flame

Question 1: List conditions under which combustion can take place.
Answer: Conditions under which combustion can take place are
• Presence of air (mainly oxygen)
• Presence of fuel
• Ignition temperature

Question 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 ____________________.
Answer:
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.

Question 3: Explain how the use of CNG in automobiles has reduced pollution in our cities.
Answer: The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas), because CNG produces the harmful products in very small amounts. CNG is a cleaner fuel.

Question 4: Compare LPG and wood as fuels.
Answer: For centuries, wood was used as domestic and industrial fuel. But now it has been replaced by coal and other fuels like LPG. In many rural parts of India, people still use wood as a fuel because of its easy availability and low cost. However, burning of wood gives a lot of smoke which is very harmful for human beings. It causes respiratory problem. Also, trees provide us with useful substances which are lost when wood is used as fuel. Moreover cutting of trees leads to deforestation which is quite harmful to the environment. The calorific value for wood is between 17000-22000 kJ/kg whereas the calorific value for LPG is 55000 kJ/kg. Therefore, LPG is preferred choice.

Question 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.
Answer:
a) Water is a good conductor of electricity. So if water is added to an electrical fire, the water would just spread the electricity further. The person dousing the fire might get an electric shock. Therefore, water is not used to control fires involving electrical equipment.

b) Burning of wood gives a lot of smoke which is very harmful for human beings. It causes respiratory problem. Also, trees provide us with useful substances which are lost when wood is used as fuel. Moreover cutting of trees leads to deforestation which is quite harmful to the environment. The calorific value for wood is between 17000-22000 kJ/kg whereas the calorific value for LPG is 55000 kJ/kg. Therefore, LPG is a better domestic fuel than wood.

c) The paper by itself catches fire easily because of its low ignition temperature. The piece of paper wrapped around an aluminium pipe doesn’t catch fire since aluminium is a good conductor of electricity. While the paper wrapped around an aluminium pipe results in an increase in ignition temperature. So, there is a transfer of heat from paper to the aluminium pipe. Therefore, it doesn’t catch fire.

Question 6: Make a labelled diagram of a candle flame.
Answer:

Question 7: Name the unit in which the calorific value of a fuel is expressed.
Answer: The amount of heat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value of a fuel is expressed in a unit called kilojoule per kg (kJ/kg).

Question 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. Carbon dioxide in most cases does not harm the electrical equipment.

Question 9: It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Answer: A heap of green leaves has a lot of moisture in it, so its ignition temperature is high. Therefore, it does not catch fire easily. Whereas dry leaves have no moisture in it, so its ignition temperature is low. Therefore, it catches fire easily.

Question 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. The outermost zone of a flame goes under complete combustion and is therefore considered hottest part of a flame.

Question 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.
Answer:
Weight of fuel = 4.5 kg
Heat produced by 4.5 kg fuel = 180000 kJ
Calorific value = kJ/kg
= 180000/4.5
= 40000 kJ/kg
Therefore, the calorific value of the fuel is 40000 kJ/kg.

Question 12: Can the process of rusting be called combustion? Discuss.
Answer: Combustion is a chemical process in which a substance reacts with oxygen and gives energy during the process in the form of light or heat or sometimes both. Rusting of iron is an exothermic process because heat is released during rusting. So, it is a kind of slow combustion.

Question 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: Water in Ramesh’s beaker will get heated in a short time because the outermost zone of a flame goes under complete combustion and is therefore considered hottest part of a flame. Water in Abida’s beaker will get heated in more time because it has been kept in innermost flame which is not much hotter.