Hydrogen bonding is the strongest of the forces between molecules that have no formal charge. It is a force between a partially positive hydrogen atom, and a partially negative atom (either nitrogen or oxygen). Most often, hydrogen bonding occurs on molecules containing amino groups (NH2) and hydroxy groups (OH). Since the hydrogens are connected to highly electronegative atoms, their electron density is pulled away from them. This leaves them with a partially positive charge. The electronegative atom, having gained electron density, is partially negative. This allows for strong electrostatic interactions between the negative atom and a hydrogen on another molecule.

*Note* This is a vastly oversimplied model (water is far more disordered in reality) to demonstrate the affinity of partially negative oxygen atoms with the partially positive hydrogens.

Hydrogen bonds are essentially just the mother of all dipole interactions: in other words, the dipole interaction between a partially positive hydrogen attached to an oxygen atom with that of another oxygen atom is large enough to warrant its own special classification, as a hydrogen bond.

As described in the next section of the tutorial on solubility, the kinds and strength of the attraction between molecules plays an important role on many of their unique physical properties. Melting points, boiling points, and solubilities are very much defined by the attractions between molecules.

Since water is highly stabilized by the hydrogen bonds between different water molecules, it takes a very large amount of energy to disrupt these bonds and cause boiling. In fact, methane, which is similar in weight to water but has no hydrogen bonding capabilities, boils at -162 degrees celcius, wheras water, of course, boils at 100 degrees.