Force acting on a polar molecule What kind of force does act on a polar molecule? I mean, there must be a force that is acting on the molecule to keep the negative and positive centre away ( like in H2O). Why the positive and negative centre of charges do not attract and make it a polar molecule? Also energy is needed to keep the positive and negative centre away from each other. Is the energy supplied in the form of heat at the time of formation of the molecule or there is something else i am not understanding?
 A: The picture of a positive center and negative center is an effective description, and not a literal one. Because indeed as you thought, electric charges cannot be maintained at a stable stationary equilibrium (i.e. without external forces). This is called Earnshaw's theorem in electrostatics.
To understand precisely what's happening in molecules, you should use the laws of quantum 
mechanics to determine what probabilistic configuration minimizes energy. In the case of $H_2 O$ you will find a tetrahedral configuration for electronic orbitals, two of which form covalent bonds with hydrogen. As we know from geometry the angle between hydrogen atoms should then be $109.5$ (it is actually a bit less due to lone pairs...).
However consider this classically metastable example from electrodynamics. A positive sphere slowly orbiting a negative sphere of much larger magnitude of charge (this is metastable because of electromagnetic radiation, but for low velocities we can ignore this effect for an appropriately short time interval). If the orbit is circular you don't have any polarity. But if it is an ellipse for example you can easily see how the positive center of charge is at the center of the ellipse, whereas the negative is located at one of the foci, then the polarity will be along the major axis pointing from the winter towards the summer solstice. 
