The capillary depression in mercury is on account of
Right Answer is:
Cohesion being greater than the adhesion
Capillary is a phenomenon by which a liquid (depending upon its; specific gravity) rises into a thin glass tube above or below its general level.”The rise of the liquid surface is known as capillary rise and the fall of the liquid surface is called capillary depression.
Cohesion refers to the attraction between particles of the same substance. That is, it is the intermolecular force between molecules of the liquid. It is maximum in solids, lesser in liquids, and least in gases.
Adhesion is the attraction between particles of different substances. Adhesion is seen in the ability of water to rise in thin tubes by capillary action. The Force of adhesion is different for different substances. For example, gum has greater adhesive force for a solid surface than water. When we pour water on a glass plate, the plate becomes wet because of the adhesive force between the molecules of glass and water.
When water spreads out and wets a glass surface the force of adhesion is greater than the force of cohesion. If we put mercury on a glass surface it does not behave in the same way as water. Mercury forms small droplets when it is poured onto the glass. The mercury molecules are attracted more strongly to other mercury molecules than they are to the glass. The force of cohesion is greater than the force of adhesion and we say that mercury is hydrophobic (water-hating).
Practical Examples of Cohesion e and Adhesive Forces.
(a) The ink sticks on paper. It is because the adhesive force between ink and paper is greater than the cohesive force of ink molecules.
(b) Water wets the glass. It is because the adhesive force between water molecules and glass molecules is greater than the force of cohesion between water molecules.
(c) Mercury does not wet the glass because the adhesive force between mercury molecules and glass molecules is less than the cohesive force between mercury molecules.
(d) We are able to write on the blackboard with a piece of chalk because the adhesive force between chalk molecules and wood molecules is much greater than the cohesive force between chalk molecules.