Q.   Why does a small ball of iron sink in water but a large ship float?
A.   The weight of water displaced by an iron ball is less than its own weight, whereas water displaced by the immersed portion of a ship is equal to its weight (Archimedes' Principle).
 

Q.   Why does ice float on water?
A.   The weight of the ice block is equal to the weight of the liquid displaced by the immersed portion of the ice.
 

Q.   Why does moisture gather outside a tumbler containing cold water?
A.   The water vapour in the air condenses on cooling and appears as droplets of water.


 

Q.   Why does kerosene float on water?
A.   Because the density of kerosene is less than that of water. For the same reason cream rises in milk and floats at the top.
 

Q.   Why is the water in an open pond cool even on a hot summer day?
A.   As the water evaporates from the open surface of a pond, heat is taken away in the process, leaving the surface cool.
 

Q.   Why is it less difficult to cook rice or potatoes at higher altitudes?
A.   Atmospheric pressure at higher altitudes is low and boils water below 100°C. The boiling point of water is directly proportional to the pressure on its surface.
 

Q.   Why is it difficult to breathe at higher altitudes?
A.   Because of low air pressure at higher altitudes the quantity of air is less, and so also that of oxygen.
 

Q.   Why are winter nights and summer nights warmer during cloudy weather than when the sky is clear?
A.   Clouds being bad conductors of heat do not permit radiation of heat from land to escape into the sky. As this heat remains in the atmosphere, the cloudy nights are warmer.
 

Q.   Why is a metal tyre heated before it is fixed on wooden wheels?
A.   On heating, the metal tyre expands by which its circumference also increases. This makes fixing the wheel easier and thereafter cooling down shrinks it; thus fixing the tyre tightly.
 

Q.   Why is it easier to swim in the sea than in a river?
A.   The density of sea water is higher; hence the upthrust is more than that of river water.
 

Q.   Who will possibly learn swimming faster–a fat person or a thin person?
A.   The fat person displaces more water which will help him float much more freely compared to a thin person.
 

Q.   Why is a flash of lightening seen before thunder?
A.   Because light travels faster than sound, it reaches the earth before the sound of thunder.
 

Q.   Why cannot a petrol fire be extinguished by water?
A.   Water, which is heavier than petrol, slips down permitting the petrol to rise to the surface and continue to burn. Besides, the existing temperature is so high that the water poured on the fire evaporates even before it can extinguish the fire. The latter is true if a small quantity of water is poured.
 

Q.   Why does water remain cold in an earthen pot?
A.   There are pores in an earthen pot which allow water to percolate to the outer surface. Here evaporation of water takes place thereby producing a cooling effect.
 

Q.   Why do we place a wet cloth on the forehead of a patient suffering from high temperature?
A.   Because of body's temperature, water evaporating from the wet cloth produces a cooling effect and brings the temperature down.
 

Q.   When a needle is placed on a small piece of blotting paper which is placed on the surface of clean water, the blotting paper sinks after a few minutes but the needle floats. However, in a soap solution the needle sinks. Why?
A.   The surface tension of clean water being higher than that of a soap solution, it can support the weight of a needle due to its surface tension. By addition of soap, the surface tension of water reduces, thereby resulting in the sinking of the needle.
 

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

Q.   Why does oil rise on a cloth tape of an oil lamp?
A.   The pores in the cloth tape suck oil due to the capillary action of oil.
 

Q.   Why are ventilators in a room always made near the roof?
A.   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.
 

Q.   How does ink get filled in a fountain pen?
A.   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.
 

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.
 

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 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 radiators and bad conductors.
 

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.
 

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.
 

Q.   Why does milk curdle?
A.   Lactose (milk sugar) content of milk undergoes fermentation and changes into lactic acid which on reacting forms curd.
 

Q.   Why does not hard water lather soap profusely?
A.   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.
 

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.
 

Q.   Why is it dangerous to sleep under trees at night?
A.   Plants respire at night and give out carbon dioxide which reduces the oxygen content of air required for breathing.
 

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.
 

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.
 

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.
 

Q.   Eskimos live in double-walled ice houses. Why?
A.   Because the air in between two ice walls does not allow heat to pass.
 

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.
 

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.
 

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.
 

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.
 

Q.   Why is rose red and grass green in daylight?
A.   Rose absorbs all the constituent colours of white light except red which is reflected to us. Similarly, grass absorbs all colours except green which is reflected to us.
 

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 on the ship is more than that of river water.
 

Q.   Why are fuses provided in electric installations?
A.   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.
 

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.
 

Q.   If a highly pumped up bicycle tyre is left in the hot sunlight, it bursts. Why?
A.   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.
 

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 of the 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.
 

Q.   Why do two eyes give better vision than one?
A.   Because two eyes do not form exactly similar images and the fusion of these two dissimilar images in the brain gives three dimensions of the stereoscopic vision.