classic example is a water molecule... a water molecule isn't actually neutrally charged about the molecule. to understand why you have to understand where the electrons are |dw:1444257220818:dw|
the center atom is oxygen, whereas the ones ont he side are hydrogen. Now, hydrogen is the smallest atom, and it's made up of 1 proton and 1 electron. But, if you strip the electron from the proton, it's just a proton. A proton is positively charged, so, the resultant ion will be positive. In the case of water, the electron is NOT stripped from the proton completely. However, because it is participating in a bonding with the oxygen, there is a side of the proton that is entirely exposed.
as you may imagine, this results in a positive charge. It's not completely equal to 1 since it's less than a proton, but it's partial - sometimes you'll see a \(+\delta\) represetnting the partial charge.
why does this matter? well, that partial positive charge can participate in bonds. if you think about water, the reason it has such a high boiling point for a molecule its size is because of its h-bonds. the central oxygen atom has 2 lone pairs at the top of the molecule, and these make it negatively charged. note that because of this the positive ends of the water molecule will want to stick to the negative center oxygen atom, and as such the molecules have good cohesion. because of this, it is hard to separate and boil them.
h bonds are not just limited to water, but water is the standard example because it is so ubiquitous.
h bonds are the strongest of INTERmolecular bonds, which are secondary weak bonds. That being said, while an h bond might be around an order of magnitude or two weaker than a primary bond in terms of separation energy, in comparison to other bonds such as fluctating dipole and permanent dipole, the H bond is much, much stronger.
water is so good at dissolving things because those partial charges can tear apart molecules with charge - in the case of table salt, the water sticks itself to the sodium and chlorine ions and helps dissociate them. yet, the reason why water cannot be miscible with many oils is that the oils are so neutral in charge that the water would rather stick to itself than to the oils. There are ways around this of course... you can create an emulsion of very fine oil droplets (i believe) inside the liquid water, or you might use a surfectant with a hydrophilic head (charged) and a hydrophobic tail (uncharged). This is how soaps help remove oils when used in conjunction with water.
First, you need to have a good understanding of what an electronegativity is. Second, you need to have a good understanding of what an electron cloud density is. Third, you need to have a good understanding of what a polar molecules are. Fourth, you need to understand the difference between covalent and ionic bonds. Finally, you need to understand that the presence of partially charged Hydrogen (usually as a result of being covalently bonded to more highly electronegative atom) in a polar molecule is electrostatically attracted to a highly electronegative atom. It is not a complete bond as in the case of covalent bonding, but rather an attraction between opposite charges much like how magnets work.