Angle of contact between a liquid and a solid is defined as the angle enclosed between the tangents to the liquid surface and the solid surface inside the liquid, both the tangents being drawn at the point of contact of the liquid with the solid.

q < 90o concave meniscus. Liquid wets the solid surface

q = 90o plane meniscus. Liquid does not wet the solid surface.

q > 90o convex meniscus. Liquid does not wet the solid surface.

Important Points

 (i) Its value lies between 0^o and 180^o

(ii) It is particular for a given pair of liquid and solid. Thus the angle of contact changes with the pair of solid and liquid.

(iii) It does not depends upon the inclination of the solid in the liquid.

(iv) On increasing the temperature, angle of contact decreases.

(v) Soluble impurities increases the angle of contact.

(vi) Partially soluble impurities decreases the angle of contact.