Explain why
(a) The angle of contact of mercury with glass is obtuse , while of water with glass is acute .

Explanation

Let a drop of a liquid (L) is on a solid surface S placed in air A. The air is the medium around drop . The surface tensions are shown on drop .

`S_(SL)=` The surface tension corresponding to solid -liquid layer .
`S_(LA)` = The surface tension corresponding to liquid - air layer.
`S_(SA)` = The surface tension corresponding to solid - air layer.
`theta` = angle of contact
`S_(LA)costheta+S_(SA)=S_(SL)`
For the mercury glass interface , `S_(SA)ltS_(SL)` Therefore , `costheta` is negative . Thus `theta` is an acute angle.