Question

What is meant by an angle of contact? Explain why it is acute in case of water and glass pair and obtuse in case of mercury and glass pair

Answer 1

Angle of contact:

It is the angle between the tangent to the liquid surface at the point of contact and the surface inside the liquid  

When a little amount of a liquid is poured on a solid, three types of interfaces exists :

1.liquid-air,

2 air and

3 solid-liquid are formed.  

If θ is the angle of contact between solid and liquid then,

SSL+SLAcos θ=SSA

Cos θ = SSA - SSL / SLA------(i)

For mercury and glass SSA< SSL, therefore {from Eq. (i) cos θ  is negative and therefore θ > 90° i.e., obtuse.

For water and glass SSA, > SSL, therefore from Eq. (i) cos θ  is positive and therefore θ < 90° i.e., acute.

Answer 2

Answer:

The adhesive force between the molecules of Glass and water is more than the cohesive force between the molecules of water.this results in the formation of a concave meniscus for the surface of water.the angle made by the water from the Glass surface is always acute.