2.4 Water

Water is a covalent but polar molecule. Oxygen attracts the shared electrons more strongly than hydrogen, so the oxygen end is slightly negative (δ−) and the hydrogen ends are slightly positive (δ+), while the whole molecule remains electrically neutral.

• Within one molecule: Covalent bonds share electrons between oxygen and hydrogen.
• Between molecules: The δ+ hydrogen of one water molecule is attracted to the δ− oxygen of a neighbouring molecule; this intermolecular attraction is a hydrogen bond.
• Network effect: Hydrogen bonds continually break and reform, but many together give water a cohesive network.
• Property link: This network helps explain water’s solvent action, cohesion and surface tension, relatively high specific heat capacity, relatively high latent heat of vaporisation, and why solid water is less dense than liquid water.

Unequal electron sharing creates polarity; polarity creates attractions between neighbouring molecules; the resulting hydrogen-bond network gives water properties that are important in living systems. The property is therefore a consequence of intermolecular attraction, not of replacing the covalent bonds inside each molecule.

Do not draw full ionic charges on water, call the molecule an ion, or place a hydrogen bond inside one molecule. A hydrogen bond is the attraction between the δ+ hydrogen of one molecule and the δ− oxygen of another.

Water is useful in living organisms because its polarity and hydrogen-bond network create distinct properties. Each property should be linked to the biological job it makes possible, rather than memorised as an isolated list.

• Solvent → reactions and transport: Water’s polarity allows many ionic and polar substances, such as salts and glucose, to dissolve. Dissolved particles can move freely enough for metabolic reactions and transport; non-polar substances do not dissolve readily.
• High specific heat capacity → temperature buffering: Many hydrogen bonds absorb energy before water’s temperature rises substantially, helping cells and bodies resist rapid temperature change and keep enzyme conditions more stable.
• High latent heat of vaporisation → cooling: A large energy input is needed for water molecules to escape during evaporation. Water leaving sweat or a transpiring leaf therefore removes thermal energy and produces cooling.
• Cohesion/surface tension → a connected water surface: Hydrogen bonds attract neighbouring water molecules, giving cohesion and surface tension. This helps water remain together rather than separating at a surface.

The common cause is intermolecular hydrogen bonding, but the applications differ: polarity supports dissolution and movement of solutes; many hydrogen bonds buffer temperature; and breaking hydrogen-bond attractions during evaporation removes heat. Keep these mechanisms separate so “specific heat capacity” is not confused with “latent heat of vaporisation.”

Water is not a solvent for every substance, and “high specific heat capacity” describes resistance to temperature change, not the energy required for a phase change. “High latent heat of vaporisation” describes evaporation and cooling. These properties explain biological usefulness without adding unsupported ecological or physiological claims.