Question

Explain why The angle of contact of mercury with glass is obtuse, while that of water with glass is acute

Answer 1

The angle between the tangent to the liquid surface at the point of contact and the surface inside the liquid is called the angle of contact (θ), as shown in the given figure.

 

S_la, S_sa, and S_sl are the respective interfacial tensions between the liquid-air, solid-air, and solid-liquid interfaces. At the line of contact, the surface forces between the three media must be in equilibrium, i.e.,

`cos theta = (S_(sa) - S_(sl))/S_(la)`

The angle of contact θ , is obtuse if S_sa < S_la (as in the case of mercury on glass). This angle is acute if S_sl < S_la (as in the case of water on glass).

Answer 2

Let a drop of a liquid L be poured on a solid surface S placed in air A. If T_SL,and T_SA be the surface tensions corresponding to solid-liquid layer, liquid-air layer and solid-air layer respectively and θ be the angle of contact between the liquid and solid ,then

T_LA Cos θ + T_SL =T_SA

=>Cos θ=T_SA-T_SL/T_LA

For the mercury-glass interface, T_SA< T_SL.

Therefore, cos 0 is negative. Thus θ is an obtuse angle. For the water-glass interface, T_SA > T_SL. Therefore cos 0 is positive. Thus, θ is an acute angle.