Third part:

61. Question: To prevent multiplication of mosquitoes, it is recommended to sprinkle oil in the ponds with stagnant water. Why?

Answer: Mosquitoes breed in stagnant water. The larvae of mosquitoes keep floating on the surface of water due to surface tension. However, when oil is sprinkled, the surface tension is lowered resulting in drowning and death of the larvae.


62. Question: Why does oil rise on a cloth tape of an oil lamp?

Answer: The pores in the cloth tape suck oil due to the capillary action of oil.


63. Question: Why are ventilators in a room always made near the roof?

Answer: The hot air being lighter in weight tends to rise above and escape from the ventilators at the top. This allows the cool air to come in the room to take its place.


64. Question: How does ink get filled in a fountain pen?

Answer: When the rubber tube of a fountain pen immersed in ink is pressed, the air inside the tube comes out and when the pressure is released the ink rushes in to fill the air space in the tube.


65. Question: Why are air coolers less effective during the rainy season?

Answer: During the rainy reason, the atmosphere air is saturated with moisture. Therefore, the process of evaporation of water from the moist pads of the cooler slows down thereby not cooling the air blown out from the cooler.


66. Question: Why does grass gather more dew in nights than metallic objects such as stones?

Answer: Grass being a good radiator enables water vapour in the air to condense on it. Moreover, grass gives out water constantly (transpiration) which appears in the form of dew because the air near grass is saturated with water vapour and slows evaporation. Dew is formed on objects which are good radiations and bad conductors.


67. Question: If a lighted paper is introduced in a jar of carbon dioxide, its flame extinguishes. Why?

Answer: Because carbon dioxide does not help in burning. For burning, oxygen is required.


68. Question: Why does the mass of an iron increase on rusting?

Answer: Because rust is hydrated ferric oxide which adds to the mass of the iron rod. The process of rusting involves addition of hydrogen and oxygen elements to iron.


69. Question: Why does milk curdle?

Answer: Lactose (milk sugar) content of milk undergoes fermentation and changes into lactic acid which on reacting with milk protein (casein) form curd.


70. Question: Why does hard water not lather soap profusely?

Answer: Hard water contains sulphates and chlorides of magnesium and calcium which forms an insoluble compound with soap. Therefore, soap does not lather with hard water.


71. Question: Why is it dangerous to have charcoal fire burning in a closed room?

Answer: When charcoal burns it produces carbon monoxide which is suffocating and can cause death.


72. Question: Why is it dangerous to sleep under trees at night?

Answer: Plants respire at night and give out carbon dioxide which reduces the oxygen content of air required for breathing.


73. Question: Why is a new quilt warmer than an old one?

Answer: In a new quilt the cotton is not compressed and as such it encloses more air which is bad conductor of heat. Therefore, it does not allow heat to pass.


74. Question: Curved rail tracks or curved roads are banked or raised on one side. Why?

Answer: Because a fast moving train or vehicle leans inwards while taking turn and the banked or raised track provides required centripetal force to enable it to move round the curve.


75. Question: How do bats fly in dark?

Answer: When bats fly they produce ultrasonic sound waves which are reflected back to them from the obstacles in their way and hence they can fly without difficulty.


76. Question: Water pipes often burst at hill stations on cold frosty nights. Why?

Answer: The temperature may fall below 00C during cold frosty nights which converts the water inside the pipes into ice, resulting in an increase in volume. This exerts great force on the pipes and as a result, they burst.


77. Question: Why are white clothes more comfortable in summer than dark or black ones?

Answer: White clothes are good reflectors and bad absorbers of heat, whereas dark or black clothes are good absorbers of heat. Therefore, white clothes are more comfortable because they do not absorb heat from the sun rays.


78. Question: Why does a rose appear red grass green in daylight?

Answer: Rose absorbs all the constituent colors of white light except red which is reflected to us. Similarly, grass absorbs all colors except green which is reflected t us.


79. Question: Why does a ship rise as it enters the sea from a river?

Answer: The density of sea water is high due to impurities and salts compared to river water as a result; the upthurst produced by the sea water on the ship is more than that of river water.


80. Question: Why are fuse provided in electric installations?

Answer: A safety fuse is made of a wire of metal having a very low melting point. When excess current flows in, the wire gets heated, melts and breaks the circuit. By breaking the circuit it saves electric equipment or installations from damage by excessive flow of current.


81. Question: Why is it easier to lift a heavy object under water than in air?

Answer: Because when a body is immersed in water, it experiences an upward thrust (Archimedes? Principle) and loses weight equal to the weight of the water displaced by its immersed potion, and hence, is easier to lift objects.


82. Question: If a highly pumped up bicycle tyre is left in the hot sunlight, it bursts. Why?

Answer: The air inside the tube increases in volume when heated up. As sufficient space for the expansion of the air is not available because the tube is already highly pumped, it may result in bursting of the tyre.


83. Question: What will be the color of green in blue light?

Answer: Grass will appear dark in color because it absorbs all other colors of the light except its own green color. The blue light falling on grass will be absorbed by it, and hence, it will appear dark in color.


84. Question: Why do two eyes give better vision than one?

Answer: Because two eyes do not form exactly similar images and he fusion of these two dissimilar images in the brain gives three dimensions of the stereoscopic vision.

Regards

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I am he, whom I Love. And he whom I Love is I.