Combustion and Flame - Exemplar Solutions 4

CBSE Class 8 Science
NCERT Exemplar Solutions
CHAPTER 6
Combustion and Flame
LONG ANSWER QUESTIONS
31. You are provided with three watch glasses containing milk, petrol and mustard oil, respectively. Suppose you bring a burning candle near these materials one by one, which material(s) will catch fire instantly and why?
Ans. Petrol will catch fire instantly because it is highly inflammable and its ignition temperature is very less as compared to other substance. Ignition temperature is the lowest temperature at which the substanve catches fire.
32. Manu was heating oil to fry potato chips. The cooking oil all of a sudden caught fire; he poured water to extinguish the fire. Do you think this action was suitable? If yes, why? If not, why not? In such a condition, what should Manu have done?
Ans. 
  • Water is not suitable for fires involving oil. Water is heavier than oil , so when water is poured over oil; oil comes on top and keeps on burning.
  • Manu should have switched off the flame of the burner and put a lid on the frying pan. By doing this the contact between fuel and oxygen is cut off and the flame will go off. 
33. What are the three essential requirements to produce fire? How fire extinguisher is useful for controlling the fire.
Ans. Three essential requirements.
a. Fuel
b. Air
c. Heat to acquire the ignition temperature
Flames need fuel, oxygen and heat in order to burn. Fire extinguishers are designed to remove one of these elements by applying an agent that either cools the burning fuel, or removes or displaces the surrounding oxygen.
34. Give two examples each for a solid, liquid and gaseous fuel along with some important uses.
Ans. Types of fuels
Solid fuel – Coal, wood, etc.
Liquid fuel – Kerosene oil, petrol etc.
Gaseous fuel – CNG, LPG etc.
Uses
Coal – coal has been used as an energy resource, primarily burned for the production of electricity and heat, and is also used for industrial purposes, such as refining metals.
Wood - Wood fuel can be used for cooking and heating, and occasionally for fueling steam engines and steam turbines that generate electricity. Wood may be used indoors in a furnace, stove, or fireplace, or outdoors in a furnace, campfire, or bonfire.
Kerosene oil – Fuel for stoves, lamps etc.
Petrol - For running vehicles.
LPG – Fuel for industry etc.
35. The calorific values of petrol and CNG are 45000 kJ/kg and 50,000 kJ/kg, respectively. If you have vehicle which can run on petrol as well as CNG, which fuel will you prefer and why?
Ans.  CNG, because the calorific value of CNG is higher than that of petrol. Therefore, CNG will be more economical. At the same time, it produces the least air pollutants.
36. Although wood has a very high calorific value, we still discourage its use as a fuel. Explain.
Ans. (i) Burning of wood produces lot of air pollution.
(ii) Use of wood as fuel encourages cutting of trees leading to deforestation.
37. Forest fire produces a lot of air pollution. Write in brief about the reasons of forest fires.
Ans. Forest fire always start because of two reasons:
  1. Natural cause :
  • At high temperature, sometimes dry grass catches fire which spreads throughout the forest.
  • Lightning.
  1. Human cause
  • Smoking
  • Camp fire
  • Recreation
38. Complete the crossword Fig. 6.1 with the help of the clues:

Across
Down
1. Non-metal which catches fire if exposed to air (10)
2. Chemical process in which a substance reacts with oxygen to give off heat. (10)
3. The Lowest temperature at which a substance catches fire is called its ____ temperature. (8)
4. Petrol is used as a ___ in automobiles. (4)
5. The most common fire extinguisher. (5)
6. It is as hard as stone and black in colour. (4)
Ans.