Why are some molecules polar while others are nonpolar? ?

Question

Also using the concept of molecular polarity why does oil and water cant mix?

Answer 1

it is related to the dipol moment.   if the forces that applied on the central atom are 0(vectorally) it is a non-polar molecule if not it is polar.

There is a rule that familiar molecules dissolves in each other. Water is a polar moecule but oil is not so they are not familiar. That's why oil doesn't dissolve in water.