Explain the difference between wetting agents and water-proof agents with respect to the phenomenon of surface tension.

 

A substance that reduces the surface tension of a liquid, causing the liquid to spread across surface of a solid.

 

If such substances are added to the liquid, it lowers the surface tension and liquid can easily spread on the surface.

 

Contact angle is the angle a drop water makes with solid surface.

 

If contact angle is low liquid drop can easilly spread. Ex: water can easily wet the surface of a cottan cloth. Detergent also one example of wetting agent.

 

Water proof agents increases the surface tention hence the contact angle. Thus liquid can not spread on the surface. Ex: water beads on a wax or oily surface. water can not wet the oily surface because of high contact angle.