WHY WATER WETS AND MERCURY DOES NOT?

Any substance has some physical/ chemical properties which include having positive/ negative electronic balance. If there are two substances next to each other, depending on the positive/negative nature, the molecules may attract or repel. Water in a glass – water molecules near glass get attracted hence it rises above the water level at the water edge. This property is called adhesion. So water forms a concave surface in a glass tube. An extreme of this nature is when a very narrow glass tube is inserted in a glass of water, water level in the narrow tube rises above the glass water level. This is known as capillary action.

The attraction between molecules of the same substance is called cohesion. This gives rise to surface tension. If water/ mercury is in air, the cohesive forces are more than adhesive forces with air and so water or mercury takes the shape of a spherical droplet on its own, sphere being the least energy consuming shape. But in contact with glass, water spreads. Mercury has about seven times surface tension than that of water. Mercury does not spread and remains spherical. While water is spread over a cloth/ paper/ sponge, it wets the item due to adhesive forces between them. If water is heated or mixed with disinfectant, its surface tension reduces further and so it gets more into the intermolecular space of the item. Mercury, having high surface tension, has low adhesive force and so it does not wet.

 

AUTHOR – Professor S. C. Misra


 


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