Q62. Place a piece of burning wood or charcoal on an iron plate
or Tawa. Cover it with a glass jar or a tumbler, or a transparent plastic jar. Observe
what happens. Does charcoal stop burning after sometime? Can you think of the reason
why it stops burning?
When a burning wood or charcoal is covered with a glass jar or a tumbler, it
stops burning after sometime, that is, the charcoal fire gets extinguished
after some time. This is so, because when we cover the burning wood or charcoal,
the supply of supporter of combustion (i.e. air) to the burning wood or
charcoal is cut off and hence the charcoal fire stops.
Q63. Explain how the use of CNG in automobiles has reduced
pollution in our cities.
Ans. Burning of fuel like petroleum releases unburnt
carbon particles and gases like carbon monoxide, carbon dioxide, sulphur
dioxide and nitrogen oxides into the air. These pollutants can cause many
respiratory diseases and environmental hazards. CNG produces the harmful
products in very small amounts. CNG is a cleaner fuel. Thus, the use of CNG in
automobiles has reduced pollution in our cities.
Q64. Arrange the following fuels in the increasing order of
their calorific value.
cow dung cake, petrol, wood, LPG, hydrogen
Fuel Calorific Value
Cow dung cake
Q65. Make a labelled diagram of a candle flame.
Ans. Different Zones of candle flame
Image from NCERT
Q66. What is matchstick made of?
How do matchsticks work?
Does a matchstick burn by itself? How does it burn?
Ans. These days the head of the safety match contains
only antimony trisulphide and potassium chlorate. The rubbing surface has
powdered glass and a little red phosphorus (which is much less dangerous). When
the match is struck against the rubbing surface, some red phosphorus gets
converted into white phosphorus. This immediately reacts with potassium
chlorate in the matchstick head to produce enough heat to ignite antimony
trisulphide and start the combustion.
Q67. Compare LPG and wood as fuels.
Why is LPG a better domestic fuel than wood?
Ans. LPG and wood as fuels
For centuries, wood was used as domestic
and industrial fuel. But now it has been replaced by coal and other fuels like
LPG has much higher calorific value than
wood, so it produces much more heat on burning than an equal mass of wood.
LPG burns without producing any smoke but
burning of wood produces a lot of smoke which is very harmful for human beings.
LPG burns completely without leaving behind
any solid residue but wood leaves behind a lot of ash on burning. Thus, LPG is
a cleaner fuel compared to wood.
LPG does not causes environmental problem
but cutting of trees for wood leads to deforestation which is quite harmful to
Q68. What are the harmful effects of burning fuels?
Ans. The increasing fuel consumption has harmful
effects on the environment.
Carbon fuels like wood, coal, petroleum
release unburnt carbon particles. These fine particles are dangerous pollutants
causing respiratory diseases, such as asthma.
Incomplete combustion of these fuels gives
carbon monoxide gas. It is a very poisonous gas.
Combustion of most fuels releases carbon
dioxide in the environment. Increased concentration of carbon dioxide in the
air is believed to cause global warming.