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    Mechanical Properties of Fluids
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    Large force is required to draw apart two glass plates enclosing a thin layer of water film. Explain why.

    The thin film formed between the glass plates has concave surface all around as shown

    The thin film formed between the glass plates has concave surface all

    in figure. Due to surface tension the pressure on convex side is less than that on concave side i.e. pi< p0. Therefore there is net inward force on the plates due to difference of pressure and require some force to separate them.
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    Why do the new earthen pots keep the water cooler than the old one?
    There are fine pores on the surface of new earthen pots. The water rises in these pores due to capillary action and spreads over the surface. The water from the surface of pot, when evaporates, cools down the pot and keeps the water cold. With time, the dust particles  fill the capillaries of pot and hence the above phenomenon do not take place in old pots. 
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    A capillary tube is dipped in water. The water rises to certain height. What will happen if we add some common salt in water?
    On adding highly insoluble impurities in water, the surface tension decreases. And, also the height to which  the liquid rises into the capillary tube is directly proportional to the surface tension of the liquid. As salt is added in water, surface tension of water increases. Therefore, the height to which the water rises in the capillary tube increases on adding the salt. 
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    Why does coating of glycerine over the glass window prevent raindrops from sticking to it?
    The angle of contact of water with clean glass is less than 90° i.e., acute. Therefore, the water sticks with glass window. By coating the layer of glycerine over glass window, the angle of contact becomes greater than 90° and makes the glass window non-sticky. 
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    Water rises to height 1.2 cm in capillary tube on the surface of earth. How the height of water column will be affected when the apparatus is taken to the moon?
    The height to which the liquid rises into the capillary tube is inversely proportional to the acceleration due to gravity. On the surface of the moon, acceleration due to gravity is 1/6th as that on the surface of the earth. Therefore, water will rise to a height 6 times than on the surface of the earth. 

    So, height of water on the surface of the moon = 7.2 cm
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