Why mercury does not rise in capillary tubes?

Answer 1

A liquid has two properties (amongst many) called cohesion and adhesion. Cohesion is the force of attraction between molecules/atoms of the liquid itself. Adhesion is the force of attraction between the molecules/atoms of the liquid and other substances. Therefore, if the molecules' attraction for another substance is greater than the attraction between the molecules, the molecules will preferentially interact with the other substance. When you have a substance that is more attracted to the walls of a capillary tube than itself (i.e. adhesion > cohesion), the substance will exhibit capillary action and form a concave meniscus. Mercury, however, has a stronger cohesive force between its atoms than adhesive force to the walls of a capillary tube, and therefore will not preferentially interact with the tube, thus not demonstrating capillary action and forming a convex meniscus. Curved surfaces have a higher pressure (called LaPlace pressure) on the concave side of the curve than on the convex side. Because mercury has a convex meniscus it has a lower LaPlace pressure in the capillary than the surrrounding liquid. It will therefore show a capillary drop rather than the more common capillary rise seen with materials that have a concave meniscus like water.