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Explain each of the following using the behaviour of molecules where possible:
(a) A steel needle placed on the surface of water does not sink
(b) When a small drop of detergent is placed on water, the needle moves rapidly away from it and sinks when more detergent is added. (Assume that the detergent does not affect the density of water)
(c) A match-stick rubbed at one end with soap starts moving immediately in one direction when placed on the surface of water
(d) When it is raining, is advisable not to touch a canvas tent from inside.
(e) Bristles of a paint brush spread when the brush is in water and cling together when it is taken out of the water.
(f) Water wets clean surfaces of glass but not waxed ones.
(g) Water rises up in narrow tubes but mercury, which is also a liquid, falls in narrow tubes to a level below the outside surface.

Answer Text:
(a)The surface of water acts like a fully stretched thin elastic skin hence when then the needle is placed on the surface it will not sink. This force that makes it to behave this way is due to force of attraction between the water molecules.
(b) When a small drop of detergent is placed, it lowers the surface tension at that particular point. The force holding the water molecules together weakens at this point hence the needle moves rapidly away from it. When more detergent is added, it breaks the surface tension and hence the needle sinks.
(c) The soap at the end of the stick lowers the surface tension at the end of the stick by weakening the force of attraction between the water molecules on that side. The tension on the other side is now greater because the surface tension has not been lowered yet hence it pulls the stick towards that direction. The movement gradually weakens and the whole surface covered with soap.
(d) Surface tension is drawn to the finger creating a winking point hence leakage.
(e) When the brush is taken out of the water, surface tension between water molecules tend to pull the bristles. When the brush is in water, the force between water molecules evens out hence it spreads.
(f) Water wets the glass surface because the adhesive forces between the water molecules and the glass molecules are greater than the cohesive forces between water molecules while in waxed surfaces, cohesive force is greater than the adhesive.
(g) Adhesive forces between the water and glass is greater than cohesive forces between the water molecules, the water rises up the tube so that more water molecules can be in contact with the glass.
On the other hand, the force of cohesion with the mercury is greater than the force of adhesion between glass and mercury. The mercury sinks to enable mercury molecules to keep together.