Water, H₂O, undergoes self ionization, to form the oxonium ion, H₃O⁺, and the hydroxide ion, OH^-.

The oxonium cation is the standard acid medium in aqueous solution, although other species, such as H₉O₄⁺ have been proposed.

Water is a strongly solvating solvent, due to the large dipole moment, and separation of charge along the H-O bond, H^δ+-O^δ-. This separation of charge leads to the formation of hydrogen bonds, when the proton is interposed between the oxygen and another electronegative element. This leads to there being high melting and boiling points in water and other H-bond containing compounds (the H-bonds contribute to increased attractive, coulombic, interactive forces, and this is reflected in the increased temperatures of melting and boiling - in these processes the attractive forces have to be overcome).

Hydrogen Bonding

This is defined as:

The interaction between H bound to an electronegative element and another electronegative element which has lone pairs.

It is strongest when both of the electronegative elements are first row elements.

When it occurs, the X-H-Y separation (where X and Y are the electronegative elements) is less than the sum of the van der Waals radii of species X and Y.

H-bonds are generally linear, or approximately so.

The H-Y stretching frequency is shifted to lower frequencies: this is because the bond strength is reduced when the electrons making up the bond are delocalized over three atoms (X, H and Y) rather than just two (H and Y), and hence the bonding interaction is reduced. The decreased bond strength means that it is more easily stretched, and the stretching frequency decreases.

There are two contributions to H-bonding: the first is electrostatic interaction between the positively charged proton, H^δ+, and the negatively charged proton acceptor. The second is the formation of a bond of covalent character due to the transfer of charge between the H-Y HOMO (highest occupied molecular orbital) and the H-X LUMO (lowest unoccupied molecular orbital).

The Hydrogen bond is generally weaker than a covalent bond, but stronger than van der Waals interactions, and has a strength of about 20 kJmol^-1. The species HF₂^- though possesses the strongest H-bond at 165 kJmol^-1, and contains a linear F-H-F arrangement, with the H positioned midway between the two F atoms, ie. the H-F covalent bond is equal to the H-F H-bond in HF₂^-.

Bond strengths of Hydrogen bonds (kJmol-1)
System	H-bond:	Covalent:
	7	363
	17	386
	22	464
	29	565
	55	428
	165	565

The directional nature of the H-bond has a large effect on the structure of water. In the condensed phases, each O atom is tetrahedrally coordinated by H atoms, and as the O-H-O system is linear, there is also a tetrahedral arrangement of O atoms around the O atoms. The requirement for the O-H-O system to be linear means that ice has a very open structure, and hence has a lower density than liquid water.

On melting, this open structure can collapse and the density falls. Water is one of the few substances whose density decreases with temperature. One form of ice, ice-I, has the O atoms arranged in the positions occupied by C in diamond, and the H atoms are located on the lines joining adjacent O atoms.