Table 2.4 reveals an important property of water. Its boiling point for its molecular weight (18) is high compared to other molecules. The reason is the strong tendency of water to form hydrogen bonds.

The electron arrangement of a single water molecule is shown in Figure 2.9a. Two of the outer six electrons of the oxygen atom are involved in covalent bonds to the hydrogens. The other four electrons exist in nonbonded pairs, which are excellent hydrogen bond acceptors. The OH groups in water are strong hydrogen bond donors. Each water molecule is simultaneously a hydrogen bond donor and a hydrogen bond acceptor, and a sample of water is a dynamic network of H-bonded molecules (Figure 2.9b).

The consequences of the extensive hydrogen-bonded network of water include:

1. High boiling point

2. High heat of vaporization (Table 2.4)

3. High viscosity (Table 2.5)

4. High surface tension

5. High dielectric constant

When water molecules freeze, the hydrogen bonding becomes most regular and clearly defined, creating a rigid tetrahedral molecular lattice in which each molecule is H-bonded to four others (Figure 2.10a). The lattice structure is only partially dismantled when ice melts, and some long-range order persists even at higher temperatures. The rather open structure of the ice lattice accounts for another of water's unusual properties - liquid water is denser than its solid form, because when the lattice breaks down, molecules can move closer together.

The processes of life require a wide variety of ions and molecules to move about in proximity, that is, to be soluble in a common medium. Water serves as the universal intracellular and extracellular medium, thanks to its remarkable solvent ability. This ability arises primarily from two properties of water: its tendency to form hydrogen bonds and its dipolar character. Substances that can take advantage of these properties so as to readily dissolve in water are called hydrophilic, or "water loving."

Hydrophobic molecules do not dissolve readily, however, in water, and form a clathrate, cage-like structure (Figure 2.13) when mixed with water.