(a) Angle of contact between a liquid and solid is the angle enclosed between the tagents 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 solid.
In case of a mercury in glass capillary tube, the force of cohesion between mercury molecules is greater than the force of adhesion between mercury molecules and glass molecules. Due to it, the surface tension between solid (glass)-liquid (mercury) interface (i.e., `S_(sl)`) is greater than the surface tension between liquid (mercury)-air(i.e., `S_(la)`) interface. It means , `S_(sl) gt S_(la)`. As a result of it, the mercury dies not wet the glass tube. the shape of mercury meniscus in the capillary tube is convex upwards. fig. then the angle of contact between mercury and glass is more than `90^(@)`, i.e., obtuse angle.
In case of a water in glass capillary tube, the force of cohesion between wate molecules in less than the force of adhesion between water molecules and glass modecules. Due to it, the surface tension between solid (glass)-liquid (water) interface (i.e., `S_(sl)`) is less than the surface tension between liquid (water) - air interface (i.e., `S_(la)`) . it means `S_(sl) gt S_(la)`. ltbr. As a result of it, the water wets the glass tube. The shape of the meniscus in the capillary tube is concave upwards , fig. then the angle of contact between water and glass is less than `90^(@)`, ie., acute angle.
(b) For mercury - glass, angle of contact is obtsue. in order to achieve this obtuse value of angle of contact, the water tends to spread.
(c) Surface tension of liquid is the force acting per unit length on a line drawn tangentially to the liquid surface at rest. since this force is independent of the area of liquid surface, therefore, surface tension is also independent of the area of the liquid surface.
We know that the cloth has narrow spaces in the form of capillaries. the rise of liquid in a capillary tube is directly proportional to `cos theta`. if `theta` will be large. due to which, capillary rise will be more and so the detergent will penetrate more in cloth.
(e ) In the absence of external forces, teh surface of the liquid drop tends to acquire the minumum surface area due to surface tension. since for a given volume, the surface are of sphere is least, hence the liquid drop takes the spherical shape.
