Question

Why are water molecules polar?

Answer 1

For two reasons. To be polar, there must be a charge separation between the positive end and the negative end.
First, there is an electronegative difference between H and O; that is, H and O don't have the same attraction for the electrons that make up the bond between the H atoms and the O atom. (O has an EN of 3.5 and H is 2.1; thus, the O atom pulls the electrons closer than H atoms which makes the H atoms lightly positive and the O atom slightly negative.)
Second, the H-O-H angle is NOT 180 degrees; that is, it is not a linear molecule. If it were a straight line molecule the H-O dipole on the left side would exactly cancel the 0-H bond on the right side and the net dipole moment would be zero. When we measure the dipole moment of water, we find it has a permanent dipole moment; therefore, we know it isn't linear. It is a bent molecule in geometry; something like this.

O-H
|
H

I hope this helps.