NCERT Class 8 Science Chapter 6 Solution
Brief Information: This post contains the solution for Chapter 6 Combustion and Flame of Class 8 of the CBSE and also Gujarat Board (GSEB) books by GCERT (based on new NCERT).
Q1. List conditions under which combustion can take place.
Ans. A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The Combustion requires following conditions:
1.) Presence of combustible substance also known as fuel.
2.) Stagnant supply of Oxygen in air.
3.) Certain level of temperature called Ignition temperature. Because fuel cannot combust fire below ignition temperature.
Q.2 Fill in the blanks.
(a) Burning of wood and coal causes pollution of air.
(b) A liquid fuel, used in homes is LPG (Liquefied Petroleum Gas).
(c) Fuel must be heated to its ignition temperature before it starts burning.
(d) Fire produced by oil cannot be controlled by water.
Q.3. Explain how the use of CNG in automobiles has reduced pollution in our cities.
Ans. The use of petrol and diesel leads to release of harmful gases like carbon dioxide and oxides of nitrogen and sulphur. They pollute air and are responsible for the problems like causing respiratory diseases ( such as asthma), acid rain and global warming. CNG (Compressed Natural Gas) is cleaner fuel as it release very small amount of these harmful gases. Therefore, use of CNG in automobiles has reduced pollution in our cities.
Q.4. Compare LPG and wood as fuels.
|(a) It does not cause pollution on combustion.||(a) It pollutes air on its combustion.|
|(b) No smoke is produced.||(b) It produces smoke.|
|(c) It is a liquid fuel.||(c) It is a solid fuel.|
|(d) It has more calorific value (55000 kJ/kg).||(d) It has less calorific value (17000 kJ/kg).|
|(e) It can be easily transported, as it is stored in cylinders.||(e) It can’t be transported easily like LPG fuels.|
Q.5. Give reasons.
(a) Water is not used to control fires involving electrical equipment.
Ans. Water is good conductor of electricity. If water is used to extinguish fire involving electrical equipment, the person throwing water can get electric shock. The person may either get injured or die. Thus, Water is not used to control fires involving electrical equipment.
(b) LPG is a better domestic fuel than wood.
Ans. Liquefied Petroleum Gas (LPG) has higher calorific value (55000 kJ/kg) than wood (17000-22000 kJ/kg). Therefore, LPG produces more heat energy than the wood. Moreover, LPG is cleaner form of fuel as it release were less amount of smoke and harmful gases on combustion. That is the reason why 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.
Ans. The Ignition temperature of paper is low, therefore it catches fire easily. On the other hand aluminium is good conductor of heat. It absorbs the heat provided to piece of paper wrapped by it and does not allow to reach the ignition temperature. Thus, a piece of paper wrapped around an aluminium pipe does not catch fire.
Q.6. Make a labelled diagram of a candle flame.
Q.7. Name the unit in which the calorific value of a fuel is expressed.
Ans. The calorific value of a fuel is expressed in unit – kilojoules per kilogram (kJ/kg).
Q.8. Explain how CO2 is able to control fires.
Ans. For fire, continuous supply of oxygen is one the requirement. Carbon dioxide is heavier than the oxygen. When spread over fire, it replaces the surrounding air and forms blanket around the fire. The disconnection of supply of oxygen extinguishes of fire.
Q.9. It is difficult to burn a heap of green leaves but dry leaves catch fire easily. Explain.
Ans. The green leaves contain water. It increase the ignition temperature of the leaves. On the other hand, dry leaves do not contain moisture so their ignition temperature is low. Therefore, It is difficult to burn a heap of green leaves but dry leaves catch fire easily.
Q.10. Which zone of a flame does a goldsmith use for melting gold and silver and why?
Ans. Goldsmith uses non-luminous outer zone for melting gold and silver. Outer zone of flame is hottest therefore it is used for melting gold and silver.
Q.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.
Q.12. Can the process of rusting be called combustion? Discuss.
Ans. The process of rusting emits heat during the formation of its oxide. So we can call the process of rusting as slow combustion.