Which produces the better cleaning action-clear water or soap solution?

Soap solution produces the better cleaning action than clear water. Surface tension of soap solution is less as compared to that of clean water.
Therefore, the molecules of soap solution can penetrate more into the fine pores of the cloth, making the cleansing action more effective. 

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Water spreads over a clean glass plate but mercury does not. Why?

Let us consider a liquid drop on the surface in equilibrium.



Let us consider a liquid drop on the surface in equilibrium.For water


For water the surface tension force between glass plate and air is greater than the surface tension between liquid and solid, therefore angle of contact is less than 90° and it will spread over the glass plate. For mercury the surface tension force between glass plate and mercury is less than the surface tension between liquid and solid, therefore angle of contact is greater than 90° and hence mercury will not spread over the glass plate.
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How the soldering paste makes the soldering process easy?

Adding of soldering paste to molten tin decreases the surface tension of the molten tin. Hence, it becomes easy to spread over the junction where soldering has to be done. 
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Oil is poured to calm the sea waves. Explain why.


Oil when poured over water spreads over the surface of water because of surface tension. When the wind blows, the surface film of oil is carried in the forward direction, leaving behind the water of higher surface tension. This left behind water exerts a backward pull thus, opposing the motion of the surface by the wind. 
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Why a glass rod coated with wax does not become wet when dipped in water?

Adhesive force  between water and wax is very less as compared to the cohesive forces between water molecules. Therefore, a glass rod coated with wax does not become wet when dipped in water.
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