Because of the polar nature of their bonds, water molecules can interact with ions, thereby making them soluble.

Take common table salt, NaCl, which normally exists as a familiar crystalline solid. The bonds between the Na and Cl atoms are totally ionic.

To remove a Na or Cl ion from a salt crystal involves separating positive from negative charges.

In water the ion's charge is partially neutralized by interactions with the partially charged groups of the water molecules.

As salt dissolves, the Na⁺ and Cl^- ions become surrounded by a shell of oriented water molecules.

Water does the same thing to itself. It stabilized its dissociation into OH^- (hydroxyl) and H⁺ ions.

The H⁺ ion, a naked proton, does not exist alone, but is associated with water molecules, forming H₃O⁺, H₅O₂⁺ and larger ions.

In pure water, the concentrations of OH- and H+ are equal to one another and are 10-7Molar.

You should know the meaning of a Molar solution, the definition of pH, and what it means to be an acid or a base.

There is a good
pH tutorial here!

Why does pH matter, anyway?

Biological molecules typically contain weak, rather than strong acidic and basic groups.

The most prominent examples are the amino acids, the basic subunits that combine to form polypeptides and proteins.

Each amino acid contains a carboxylic acid (-COOH) and basic amino (-NH₂) group.

Different amino acids have different side chains, referred to generically as "R-groups".

Some of these R-groups contain carboxylic acid or amino groups, and so act as weak acids and bases.

Depending on the pH of the solution they are in, these groups may be protonated or unprotonated.

Whether a group is charged or uncharged can have dramatic effects on the structure, and therefore the activity, of a protein.

By regulating pH, an organism can modulate protein activity.

There are, in fact, compartments within eukaryotic cells that are maintained at low pH in part to regulate protein activity.

Water as a reactant

Water takes part in a wide range of reactions within the cell. Water molecules are an integral part of all proteins and can play a direct role in their catalytic activities.

Water is directly involve in two major classes of chemical reactions, known as condensation and hydrolysis reactions.

In a condensation reaction, such as occurs during the assembly of amino acids into a polypeptide, two molecules are joined together and a molecule of water is released.

In a hydrolysis reaction, the reverse occurs. The addition of a water molecule is associated with splitting the original molecule into two.