tion 2.7, NaCl is an electrically neutral compound, whilst on the right side sodium
and chloride each carry a single but opposite charge so that the charges cancel
(neutralize) each other.
In this book the symbol , placed underneath the reaction arrow (e.g. see
Box 4.16), is used to denote the presence of microorganisms involved in the reac-
tion. We use the symbol where the presence of microorganisms is proven,
although we should remember that in many other reactions microorganisms are
often suspected to play a part.2.5 Describing amounts of substances: the mole
Chemists have adopted a special unit of measurement called the mole (abbrevi-
ation mol) to describe the amount of substance. A mole is defined as 6.022 136 7
¥ 1023 molecules or atoms. This is chosen to be equivalent to its molecular weight
in grams. Thus 1 mol of sodium, which has an atomic weight of 23 (if one were
to be very accurate, it would be 22.9898), weighs 23 g and contains 6.022 136 7
¥ 1023 molecules. This special number of particles is called the Avogadro number,
in honour of the Italian physicist Amedeo Avogadro.
The mole is used because it always refers to the same amount of substance, in
terms of the number of molecules, regardless of mass of the atoms involved. Thus
a mole of a light element like sodium, and a mole of a heavy element like uranium
(U), contain the same numbers of atoms.
Units of concentration are, for this reason, also expressed in terms of the mole.
Thus a concentration is given as the amount of substance per unit volume as mol
dm-^3 (the unit of molarity) or per unit weight as mol kg-^1 (molality). The latter
is now used frequently in chemistry because it has a number of advantages (such
as it does not depend on temperature). However, at 25°C, in a dilute solution,
mol kg-^1 , mol dm-^3 and mol l-^1 are almost equivalent. Although mol l-^1 is not an
accepted SI unit it remains widely used in the environmental sciences, which is
the reason we have decided to use it throughout this book.2.6 Concentration and activity
When environmental chemists measure the amounts of chemical substances, for
example in a water sample, they are usually measuring the concentration of that
substance, for example the concentration of calcium (Ca) in the water. It is very
easy to assume that the analysis has measured all of the free Ca^2 +ions in the
sample, but in fact it will almost certainly have measured all of the calcium-
bearing dissolved species, called ion pairs, as well. Ions in solution are often suf-
ficiently close to one another for electrostatic interactions to occur between
oppositely charged species. These interactions reduce the availability of the free
ion to participate in reactions, thereby reducing the effective concentration of the
free ion. Collisions between oppositely charged ions also allow the transient for-
mation of ion pairs, for example:22 Chapter Two