Q. Why is it the boiling point of sea water more than that of pure water?
A. Sea water contains salt, and other impurities which cause an elevation in its boiling point.
Q. Why Does ink leak out of partially filled pen when taken to a higher altitude?
A. As we go up, the pressure and density of air goes on decreasing. A Partially filled pen leaks when taken to a higher altitude because the pressure of air acting on the ink inside the tube of the pen is greater than the pressure of the air outside.
Q. Why does milk curdle?
A. Lactose (milk sugar) content of milk undergoes fermentation and changes into lactic acid which on reacting with milk protein (casein) form curd.
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. A piece of chalk when immersed in water emits bubbles. Why?
A. Chalk consists of pores forming capillaries. When it is immersed in water, the water begins to rise in the capillaries and air present there is expelled in the form of bubbles.
Q. Ice wrapped in a blanket or saw dust does not melt quickly. Why?
A. Both wood and wool are bad conductors of heat. They do not permit heat rays to reach the ice easily.
Q. Why are water or mercury droplets always round when dropped on a clean glass?
A. The surface of a liquid is the seat of a special force as a result of which molecules on the surface are bound together to form something like a stretched membrane. They tend to compress the molecules below to the smallest possible volume, which causes the drop to take a round shape as for a given mass he sphere has minimum volume.
Q. Why does an ordinary glass tumbler crack when very hot tea or milk is poured in it?
A. When a hot liquid is poured into a tumbler, the inner layer of the tumbler gets heated, it expands before the outer layer and an unequal expansion of both layers causes the tumbler to crack.
Q. Why is water from a hand pump warm in winter and cold in summer?
A. In winter, the outside temperature is lower than that of water flowing out of the pump, and therefore, the water is warm. Whereas in summer, the outside temperature is higher than the water of the pump, and therefore, it feels cold.
Q. Why does a swimming pool appear less deep than is actually is?
A. The rays of light coming from the bottom of the pool pass from a denser medium (water) to a rarer medium (air) and are refracted (bend away from the normal). When the rays return to the surface, they form an image of the bottom of the pool at a point, which is little above the real position.
Q. Why does blotting paper absorb ink?
A. Blotting paper has fine pores, which act like capillaries. When a portion of blotting paper is brought in contact with ink, ink enters the pores due to surface tension (capillary action f liquids) and is absorbed.
Q. Why does a small 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 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. When a needle is placed on a small piece of blotting paper which is place 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 cans 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.