Box 41 Properties of water and hydrogen bonds

The water molecule H2O is triangular in shape, with each hydrogen (H) bonded to the oxygen (O) as shown in Fig. 1. The shape results from the geometry of electron orbits involved in the bonding. Oxygen has a much higher electronegativity (Box 4.2) than hydrogen and pulls the bonding electrons toward itself and away from the hydrogen atom. The oxygen thus carries a partial negative charge (usually expressed as S-), and each hydrogen a partial positive charge (S+), creating a dipole (i.e. electrical charges of equal magnitude and opposite sign a small distance apart). At any time a small proportion of water molecules dissociate completely to give H+ and OH- ions.

for which the equilibrium constant is:

aH2O

The activity of pure water is by convention unity (1), so equation 2 simplifies to:

The polar nature of the water molecule allows the ions of individual water molecules to interact with their neighbours. The small hydrogen atom can approach and interact with the oxygen of a neighbouring molecule particularly effectively. The interaction between the hydrogen atom, with its partial positive charge, and oxygen atoms of neighbouring water molecules with partial negative charges, is particularly strong — by the standards of intermolecular interactions - though weaker than within covalent bonding. This type of interaction is called hydrogen bonding.

The molecules in liquid water are less randomly arranged than in most liquids because of hydrogen bonds. The polarity of the bonds makes water an effective solvent for ions; the water molecules are attracted to the ion by electrostatic force to form a cluster around it. Moreover, ionic-bonded compounds, with charge separation between component ions, are easily decomposed by the force of attraction of the water dipole. Hydrogen bonding gives water a relatively high viscosity and heat capacity in comparison with other solvents. Hydrogen bonds also allow water to exist as a liquid over a large temperature range. Since most biological transport systems are liquid, this latter property is fundamental to supporting life.

vegetation influences the global cycling of atmospheric gases and hence climate. It is clear that humans have a duty to manage soils judiciously. If soils are lost through erosion, bad agricultural practice or contamination they no longer fulfil their fundamental functions. While soils can be destroyed over a period of years or decades, their regeneration may require hundreds or even thousands of years.

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