Question

Explain why the angle of contact is acute for kerosene-glass pair and is obtuse for mercury-glass pair.

Answer 1

For a liquid which partially wets the solid e.g., kerosene in contact with glass. The adhesive force is very large compare to the cohesive force $(PC<\sqrt{2}PA)$. Therefore their resultant $\left(\overrightarrow{PR}\right)$ is directed into the solid, The free surface of a liquid at rest is always perpendicular to the resultant force on it.
Therefore, the liquid molecules like $P$ move upward till the liquid surface in contact becomes perpendicular to $\overrightarrow{PR}$. The liquid surface thus acquires a concave shape and the tangent to the makes an acute angle of contact, with the surface of solid, see fig.
For a non-wetting liquid (e.g. mercury in contact with glass) the cohesive force is very large compared to the adhesive force $(PC>\sqrt{2PA})$. Therefore, their resultant $\left(\overrightarrow{PR}\right)$ points are into the liquid. Hence the liquid surface curves down acquiring a convex shape until it becomes perpendicular to $\overrightarrow{PR}$. The tangent to it makes an obtuse angle of contact with the surface of the solid. See fig.