Explain the rise of liquid in the capillary on the basis of pressure difference.

Explanation

When a capillary tube is partially immersed in a wetting liquid the liquid meniscus inside the tube is concave.
Consider four points A, B, C, D of which point A is just above the concave meniscus inside the capillary and point B is just below it. Points C and D are just above and below the free liquid surface outside.
Let `P_A, P_B, P_C and P_D` be the pressures at points A, B, C and D. respectively.
Here, `P_A = P_A`= atmospheric pressure
The pressure is the same on both sides of the free surface of a liquid, so that `P_C = P_D`
`therefore P_A = P_D`
The pressure on the concave side of a meniscus is always greater than that on the convex side, so that
` P_A gt P_B`
therefore `P_D gt P_B" "(therefore P_(A)=P_(D))`
The excess pressure outside pushes the liquid up the capillary tube until the pressures at B and D (at the same horizontal level) equalize, i.e.. `P_B` becomes equal to `p_D` Thus, there is a capillary rise.