Topic outline

    • Combustion and Flame

      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?

      Ans. 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

                               i.        For centuries, wood was used as domestic and industrial fuel. But now it has been replaced by coal and other fuels like LPG.

                              ii.        LPG has much higher calorific value than wood, so it produces much more heat on burning than an equal mass of wood.

                             iii.        LPG burns without producing any smoke but burning of wood produces a lot of smoke which is very harmful for human beings.

                             iv.        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.

                          v.        LPG does not causes environmental problem but cutting of trees for wood leads to deforestation which is quite harmful to the environment.

      Q68. What are the harmful effects of burning fuels?

      Ans. The increasing fuel consumption has harmful effects on the environment.

                                i.        Carbon fuels like wood, coal, petroleum release unburnt carbon particles. These fine particles are dangerous pollutants causing respiratory diseases, such as asthma.

                               ii.        Incomplete combustion of these fuels gives carbon monoxide gas. It is a very poisonous gas.

                              iii.        Combustion of most fuels releases carbon dioxide in the environment. Increased concentration of carbon dioxide in the air is believed to cause global warming.


    • Download to practice offline.