Explain why the angle of contact of mercury with glass is obtuse, while that of water with glass is acute.
When a liquid comes in contact with solid, three surfaces are formed namely liquid-air, solid-air and solid-liquid. The surface tensions corresponding to these interfaces are TLA,TSA and TSL respectively Now angle of contact of solid with liquid is θ=TLATSA−TSL In case of mercury-glass TSA<TSL ,making cosθ negative and the angle of contact obtuse. 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. SSL,SLA,SSA 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, The angle of contact θ , is obtuse if SSA<SLA (as in the case of mercury on glass). This angle is acute if SSL<SLA (as in the case of water on glass).
