Q. Why are air coolers less effective during the rainy season?
A. During the rainy reason the atmospheric 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.
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Q. Why does grass gather more dew in nights than metallic objects such as stones?
A. Grass being a good radiator enables water vapour in the air to condense on it. Moreover, grass givesout 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 radiators and bad conductors.
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Q. If a lighted paper is introduced in a jar of carbon dioxide, its flame extinguishes. Why?
A. Because carbon dioxide does not help in burning. For burning, oxygen is required.
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Q. Why does the mass of an iron rod increase on rusting?
A. 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.
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Q. Why does milk curdle?
A. Lactose (milk sugar) content of milk undergoes fermentation and changes into lactic acid which on reacting forms curd.
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Q. Why does not hard water lather soap profusely?
A. Hard water contains sulphates andchlorides of magnesium and calcium which forms an insoluble compound with soap. Therefore, soap does not lather with hard water.
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Q. Why is it dangerous to have charcoal fire burning in a closed room?
A. When charcoal bums it produces carbon monoxide which is suffocating and can cause death.
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Q. Why is it dangerous to sleep undertrees at night?
A. Plants respire at night and give out carbon dioxide which reduces the oxygen content of air required for breathing.
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Q. Why does ENO's salt effervesce on addition of water?
A. It contains tartaric acid and sodium bicarbonate. On adding water,carbon dioxide is produced which when released into water causes effervescence.
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Q. Why does milk turn sour?
A. The microbes react with milk and grow. They turn lactose into lactic acid which is sour in taste.
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Q. Why is a new quilt warmer than an old one?
A. 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.
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Q. Eskimos live in double-walled ice houses. Why?
A. Because the air in between two icewalls does not allow heat to pass.
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Q. Curved rail tracks or curved roads are banked or raised on one side. Why?
A. 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.
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Q. How do bats fly in dark?
A. 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.
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Q. Water pipes often burst at hill stations on cold frosty nights. Why?
A. The temperature may fall below 0³C 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.
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Q. Why are 'white clothes more comfortable in summer than dark or black ones?
A. 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.
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Q. Why is rose red and grass green indaylight?
A. Rose absorbs all the constituent colours of white light except red which is reflected to us. Similarly, grass absorbs all colours except greenwhich is reflected to us.
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Q. Why does a ship rise as it enters the sea from a river?
A. The density of sea water is high due to impurities and salts compared to river water. As a result, the upthrust produced by the sea water onthe ship is more than that of river water.
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Q. Why are fuses provided in electric installations?
A. A safety fuse is made of a wire ofmetal 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.
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Q. Why is it easier to lift a heavy object under water than in air?
A. 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 portion, and hence, is easier to lift objects.
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Q. If a highly pumped up bicycle tyre is left in the hot sunlight, it bursts. Why?
A. The air inside the tube increases involume when heated up. As sufficientspace for the expansion of the air is not available because the tube is already highly pumped, it may result in bursting of the tyre.
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Q. What will be the colour of green grass in blue light?
A. Grass will appear dark in colour because it absorbs all other colours ofthe light except its own green colour. The blue light falling on grass will be absorbed by it, and hence, it will appear dark in colour.
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Q. Why do two eyes give better vision than one?
A. Because two eyes do not form exactly similar images and the fusionof these two dissimilar images in thebrain gives three dimensions of the stereoscopic vision.
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