9.2 , surface tension — Physics STD 11 Science — Question

b
Now lets see, any surface is said to be water proof, if it does not allow water to pass through it. So when we look at the raincoat, it practically does not allow water to pass through it. In other words we can also say that it does not show the property of getting wet.

Now that does not mean that water may not be collected on the outer surface of the raincoat.

Now if we look at the property of water, we know it well that it has the property of wetting the surface it comes in contact with. This is one of the reasons why water wets the surface of the glass.

There is a simple scientific explanation to this...

When speaking of water molecules, they have strong attraction force between its own molecules.

When water molecules come in contact with other surface, the molecules are attracted to the molecules of that surface.

So this is best explained as cohesive force and adhesive force.

Cohesive force is the type of force that is responsible for water molecules to stick together, while the adhesive force is the type of force that forces water molecules to stick to the surface of other material molecules. In case of rain coat, it is obvious that cohesive force in water molecules is much stronger as compared to adhesive force, so water does not stick to the raincoat surface.