Lakhmir Singh & Manjit Kaur: Combustion and Flame- 3 | Lakhmir Singh & Manjit Kaur Solutions: Class 8 Science PDF Download

Q.56. Which of the following substance has the lowest ignition temperature?
(a) Kerosene
(b) Spirit
(c) Diesel
(d) Mustard oil

Correct Answer is Option (b)
Spirit/petrol has the lowest ignition temperature and can catch fire easily.

Q.57. One of the following is not a combustible substance. This one is:
(a) Alcohol
(b) Hydrogen
(c) Asbestos
(d) Chaff

Correct Answer is Option (c)
Asbestos is a non combustible substance and cannot catch fire on its own.

Q.58. Which of the following is not used in making matchsticks these days?
(a) Potassium chlorate
(b) White phosphorus
(c) Antimony trisulphide
(d) Red phosphorus

Correct Answer is Option (b)
White phosphorus undergoes spontaneous combustion. Hence it is not used in making matchsticks now days.

Q.59. Which of the following undergoes spontaneous combustion?
(a) Yellow sulphur
(b) Red phosphorus
(c) White phosphorus
(d) Brown sulphur

Correct Answer is Option (c)
White phosphorus undergoes spontaneous combustion. It catches fire on its own at room temperature.

Q.60. Which of the following statement is not correct about carbon dioxide acting as a fire extinguisher for electrical fires?
(a) It is heavier than air 
(b) It is lighter than air 
(c) It is not combustion 
(d) It does not support combustion

Correct Answer is Option (b)
Carbon dioxide is lighter than air and forms a layer around the burning substance. It covers the fire like a blanket due to which fresh air cannot reach the burning substance.

Q.61. Fires in underground coalmines usually occur due to the:
(a) Explosive combustion
(b) Deliberate combustion
(c) Spontaneous combustion
(d) Rapid combustion

Correct Answer is Option (c)
Spontaneous combustion fires in underground coalmines occur due to spontaneous combustion of coaldust.

Q.62. The calorific value of a fuel is 40000KJ/kg. This fuel is most likely to be:
(a) Biogas
(b) Methane
(c) Hydrogen gas
(d) Liquefied petroleum gas

Correct Answer is Option (a)
Biogas has calorific value of 40000Kj/Kg.

Q.63. Which of the following fuels has the highest calorific value?
(a) Natural gas
(b) Liquefied petroleum gas
(c) Coal gas
(d) Hydrogen gas

Correct Answer is Option (d)
Hydrogen gas has the highest calorific value.

Q.64. On a cold winter night, the person sleeping in a room with a closed door and windows with a coal fire burning inside may die due to the excessive accumulation of:
(a) Nitrogen monoxide
(b) Nitrogen dioxide
(c) Carbon dioxide
(d) Carbon monoxide

Correct Answer is Option (d)
Due to the burning of coal, the available oxygen gets depleted and it leads to incomplete burning of coal. Incomplete combustion of coal gives carbon monoxide gas. It is a very poisonous gas. It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill persons sleeping in that closed room.

Q.65. Which of the following burns without producing a flame?
(a) Camphor
(b) Coke
(c) Cooking gas
(d) Kerosene

Correct Answer is Option (b)
Coke does not vapourise during burning and hence do not produce flame.

Q.66. On a cold winter night, the persons sleeping in a room with a closed door and windows with a coal fire burning inside may die to the excessive accumulation of:
(a) Kerosene
(b) CNG
(c) Biogas
(d) LPG

Correct Answer is Option (b)

Q.67. Which of the following is the main cause of global warming?
(a) Nitrogen dioxide
(b) Sulphur dioxide
(c) Carbon dioxide
(d) Carbon monoxide

Correct Answer is Option (c)
Burning of fuels releases carbon dioxide in air in the environment. Increased percentage of carbon dioxide in air is causing global warming. Carbon dioxide gas in the air trap sun’s heat rays by producing green house effect.

Q.68. Which of the following gas does not contribute to the formation of an acid rain
(a) Nitrogen monoxide
(b) Carbon monoxide
(c) Sulphur dioxide
(d) Nitrogen dioxide

Correct Answer is Option (b)
Carbon monoxide does not contribute to the formation of an acid rain.

Q.69. Which of the following is the most environmentally friendly fuel to be used in automobiles?
(a) Petrol
(b) Diesel
(c) Natural gas
(d) Petroleum gas

Correct Answer is Option (c)
The use of petrol and diesel as fuels in automobiles is being replaced by CNG because it produces very small amount of harmful gases.

Q.70. Which of the following does not involve a combustion reaction?
(a) Production of heat and light from kerosene in a rocket
(b) Production of heat and light from hydrogen in a rocket.
(c) Production of heat and light from hydrogen in the sun
(d) Production of heat and light from wood in a bonfire.

Correct Answer is Option (c)
Production of heat and light from hydrogen in the sun is a nuclear reaction.

Q.71. A heap of green leaves is lying in one corner of a park. The green leaves in the heap burn with difficulty because:
(a) They contain a tough material called cellulose.
(b) They contain a lot of water.
(c) They contain a green pigment chlorophyll.
(d) They do not get sufficient oxygen for burning

Correct Answer is Option (b)
The green leaves contain a lot of water. This water does not allow the green leaves to get heated to their ignition temperature and makes burning of green leaves difficult.

Q.72. If the clothes of a person working in the kitchen catch fire, then to extinguish the fire:
(a) Sand should be thrown over the burning clothes
(b) Water should be thrown over the burning clothes.
(c) Polyester blanket should be used to cover the burning clothes
(d) Woolen blanket should be used to cover the burning clothes

Correct Answer is Option (d)
If the clothes of a person working in the kitchen catch fire, the person is immediately covered with a blanket. When the burning clothes of a person are covered with a blanket, the supply of air to the burning clothes is cut off and hence the burning stops.

Q.73. The outermost zone of a candle flame is the:
(a) Least hot part
(b) Coldest part
(c) Hottest part
(d) Moderately hot part

Correct Answer is Option (c)
In the outer zone complete combustion of the fuel takes place and the colour of the flame is blue and is the hottest part of the flame.

Q.74. The flame of a kerosene oil lamp (or lantern) has:
(a) Single zone
(b) Two zones
(c) Three zones
(d) Four zones

Correct Answer is Option (c)

Q.75. A lot of dry powder of one of the following chemicals can be released over a fire to extinguish it. This chemical is:
(a) Plaster of Paris
(b) Baking soda
(c) Washing soda
(d) Bitumen

Correct Answer is Option (b)
To obtain carbon dioxide for extinguishing a fire is to release a lot of dry powder of chemicals like sodium bicarbonate(baking soda) or potassium bicarbonate over the fire.

Q.76. An electric spark is struck between two electrodes placed near each other in a closed tank full of petrol. Will the petrol catch fire. Explain your answer. 

As it is a closed tank full of petrol, there will be no supply of air. Hence the petrol will not catch fire as air is necessary for the combustion of petrol.

Q.77. Give a reason for the following:
Paper by itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not. 

A piece of paper wrapped around aluminium pipe does not catch fire easily. This is because aluminium, being a metal, is a good conductor of heat and hence heat is conducted away. Also, the ignition temperature of paper is lower compared to that of aluminium. When paper is wrapped around an aluminium pipe; the ignition temperature increases. That is why paper itself catches fire easily whereas a piece of paper wrapped around an aluminium pipe does not.

Q.78. Abida and Ramesh want to heat water taken in separate beakers. 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? Why?

The yellow part of the flame is the middlemost part where Abida has kept the beaker. The fuel vapours burn partially in the middle zone because there is not enough air of burning in this zone. The partial burning of fuel in the middle zone produces carbon particles. It has moderate temperature. Hence, Abida’s beaker will take time to heat up.
Ramesh’s beaker is kept in the outermost part of the flame. In this zone, complete combustion of the fuel takes place because there is plenty of air around it. The outermost zone has the highest temperature in the flame. It is the hottest part of the flame. Hence Ramesh’s beaker gets heated in a shorter time.

Q.79. When a lot of dry powder of a substance X is released over a fire, the fire gets extinguished.
(a) Name the substance X.
(b) How does this substance extinguish the fire?
(c) Name another substance that behaves like X.

(a) Sodium bicarbonate (sodium hydrogen carbonate)
(b) The heat of fire decomposes sodium bicarbonate to produce carbon dioxide gas. This carbon dioxide covers the fire like a blanket and cuts off the supply of fresh air to the burning substance. Due to this, the fire gets extinguished
(c) Potassium bicarbonate(or Potassium hydrogen carbonate)

Q.80. What type of combustion is represented by:
(a) burning of white phosphorus in the air at room temperature?
(b) burning of LPG in a gas stove?
(c) ignition of a cracker?
(d) burning of coal dust in a coal mine?

(a) Burning of white phosphorus in the air at room temperature is spontaneous combustion. The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.
(b) Burning of LPG is combustion in which a substance burns rapidly and produces heat and light with the help of external heat.
(c) Ignition of a cracker is combustion in which a substance burns suddenly and produces heat, light and sound with the help of heat or pressure.
(d) Burning of coal dust in a coal mine is spontaneous combustion. The type of combustion in which a material suddenly bursts into flames, without the application of any apparent cause is called spontaneous combustion.