Water's Solvent Properties
Water is considered the universal solvent due to its ability to dissolve or dissociate most compounds. Since water is a polar molecule with slightly positive and slightly negative charges, ions and polar molecules can readily dissolve in it. Therefore, water is referred to as a solvent: a substance capable of dissolving other polar molecules and ionic compounds. The charges associated with these molecules form hydrogen bonds with water, surrounding the particle with water molecules. This is referred to as a sphere of hydration, or a hydration shell, and serves to keep the particles separated or dispersed in the water.
When ionic compounds are added to water, the individual ions react with the polar regions of the water molecules, and their ionic bonds are disrupted in the process of dissociation. Dissociation occurs when atoms or groups of atoms break off from molecules and form ions. Consider table salt (NaCl, or sodium chloride): when NaCl crystals are added to water, the molecules of NaCl dissociate into Na+ and Cl– ions, and spheres of hydration form around the ions . The positively-charged sodium ion is surrounded by the partially-negative charge of the water molecule's oxygen. The negatively-charged chloride ion is surrounded by the partially-positive charge of the hydrogen on the water molecule.
Since many biomolecules are either polar or charged, water readily dissolves these hydrophilic compounds. On the other hand, water is a poor solvent for hydrophobic molecules such as lipids. The nonpolar molecules in water undergo hydrophobic interactions: the water changes its hydrogen bonding patterns around the hydrophobic molecules.