What chemical compounds might be present in drinking water?
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# Glossary

Amphoteric
  An amphoteric compound dissolves in and reacts with both acidic and alkaline solutions.

Bronsted-Løwry
  The Bronsted-Løwry theory of acids and bases states that an acid is a substance from which a proton can be removed and a base is a substance that can accept a proton from an acid.

Anions
  A negatively charged ionic species.

Catena-
  A prefix distinguishing a linear condensed anion.

Cations
  A positively charged ionic species.

Chelating
  A chemical species (a ligand) that contains two or more atoms with lone pairs of electrons that may bind to a metal ion. Their complexes show enhanced stability compared to ones where each ligand only binds at one position to the metal ion.

Coordination number
  The number of roughly equidistant nearest-neighbour atoms that surround another atom.

Cyclo-
  A prefix to denote a cyclic condensed anion.

Eutrophication
  The term given to the death of life forms in bodies of water, such as lakes due to excess phosphate and nitrate fertilisers in the water.

General formula of oxoacids
  In an oxoacid there may be one or more terminal oxygen atoms, so the general formula of oxoacids can be applied, A(O) t (OH) n , where t can equal 0. For example, sulfuric acid, H2SO4, is written as S(O)2(OH)2 (where t = 2, n = 2).

Hard and soft acids and bases
  Hard acids and hard bases have low polarisability (they are small compact atoms or ions), whereas soft acids and soft bases are more polarisable (they are larger atoms and ions). Hard acids include the cations of the alkali metals, the alkaline earths and the first-row transition metals in high oxidisation states, and hard bases include ammonia and the fluoride ion. Soft acids include the copper(I), silver(I) and mercury(II) ions, and the larger halogens. Soft bases include the cyanide ion, phosphines and the iodide ion. Hard acids tend to bond most effectively to hard bases, and soft acids to soft bases.

Hydrophilic
  Literally, 'water-loving'. A hydrophilic compound, such as an ionic species, will dissolve in water.

Hydrophobic
  Literally, 'water-hating'. A hydrophobic compound, such as a non-polar organic compound, will be totally immiscible with water.

Inert pair effect
  The tendency of the typical elements at the bottom of Groups 13-16 to adopt an oxidation number two less than the Group number.

Negatively charged ions
  A negatively charged ionic species, also referred to as anions.

Nitrogen fixation
  A collective term for all the natural processes that convert atmospheric N2 into useful compounds such as ammonia and nitrates.

Oxidisation number
  To assign the oxidisation numbers, or oxidisation states, within a compound we imagine that the shared electron pairs in covalent bonds between different elements are completely transferred to the more electronegative of the two elements. For example, in sodium chloride, sodium has an oxidisation state of +1 and chlorine has has an oxidisation state of -1. In covalent compounds, the sign reflects the relative electronegativity of the different elements, for example in water, H2O, the H is +1 and the O is -2.

Polybasic
  A polybasic acid contains more than one ionisable proton, H+ ion.

Polyprotic
  A compound that contains several ionisable protons, H+. For example sulphuric acid is a polyprotic acid.

Positively charged ions
  A positively charged ionic species, often referred to as a cation.

Redox
  A reaction in which oxidation and reduction occur.

Stoichiometry
  This can have either of two meanings, depending on the context in which the term is used. In inorganic chemistry, with NaCl, for instance, if there are n atoms in the crystal lattice, then there are n octahedral holes, and so the Na : Cl ratio will be 1 : 1. The stoichiometry in this case is 1 : 1. In the context of chemical reactions, the stoichiometry represents the relative amounts of reactants and products taking part, as expressed by the balanced chemical equation.

VSEPR theory (valence-shell electron-pair repulsion theory)
  A theory for predicting molecular shapes by postulating that electron pairs in molecules will be kept as far away from each other as possible.

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