Question

Define angle of contact.

Answer 1

When a liquid is in contact with solid, angle between solid surface and the tangent to the free surface of liquid at that point of contact, measured from inside the liquid is known as angle of contact.

see figure, here $\theta$ is angle of contact.

when the liquid surface is curved as concave upwards, the angle of contact is acute and when the liquid surface is curved as convex upwards , the angle of contact is obtuse.

Answer 2

The angle which the tangent make to the free surface of the liquid at the point of contact with the walls of the vessel inside the liquid is called the angle of contact.