Question

Explain why the angle of contact of water with glass is acute while that mercury with glass is obtuse?

Answer 1

S_SL + S_LA cosθ = S_SA

or cosθ = 

In case of mercury – glass, S_SA < S_SL

∴ cosθ is negative or θ > 90° i.e., obtuse.

In case of water – glass, S_SA > S_SL

cosθ is positive or θ < 90° i.e., acute.

Note:

S_SL:

Surface tension corresponding to solid-liquid interface.

S_LA:

Surface tension corresponding to liquid – air interface.

S_SA:

Surface tension corresponding to solid-air interface.