Polar solutes can form hydrogen bonds with the polar water molecules thereby dissolving in water. Non-polar solutes, on the other hand, do not make favorable interactions with the water molecules and clump together. Water can easily dissolve the charged solutes by hydrating and stabilizing the solute ions. Sodium chloride, for example, readily dissolves in water; the Na⁺ and Cl^- ions aquire greater freedom after leaving the salt crystal lattice. This increase in entropy makes the overall free energy change negative thereby favoring dissolution. The compounds that readily dissolve in water are termed hydrophilic while those that do not dissolve in water are termed hydrophobic. Certain molecules have spatially separated hydrophilic and hydrophobic regions; such molecules are termed as amphipathic compounds.

Chemical properties of water

Pure water undergoes weak ionization to yield H⁺ and OH^- ions.

(2.1)

Free H⁺ ions do not exist in water; they exist as hydrated ions called hydronium ion (H₃O⁺). Equation 2.1 can therefore be written as:

(2.2)

The dissociation of water molecule can be expressed in terms of the equilibrium constant, K_eq.

(2.3)

Dissociation of water is so weak that the concentration of undissociated water can be assumed to be constant i.e. 55.5 M. Equation 2.3 can therefore be written as follows:

(2.4)

where, K_w is called the ion product of water. The concentrations of H⁺ and OH^– at 25 °C for pure water are 1 × 10^-7 M each. Therefore, the ion product of water is 1 × 10^-14 M². When the concentration of H⁺ and OH^- are same, the water or any aqueous solution is said to be at neutral pH.