Modern inorganic chemistry

GROUP VI 273

(b) sodium can be melted in dry chlorine without reaction; in the

presence of a trace of moisture, violent reaction occurs.

HATURAL WATER

Because of its excellent solvent properties naturally-occurring water

is never pure. During its passage through the air, rain water absorbs

carbon dioxide, small amounts of oxygen and nitrogen, and in

urban areas, small quantities of other gaseous oxides such as those

of sulphur. On reaching the ground it can absorb more carbon

dioxide from decaying animals and vegetable material and dissolve

any soluble salts. The dissolved carbon dioxide can attack limestone

or other rock containing the carbonates of calcium and magnesium:

CaCO 3 (s) + CO 2 (aq) + H 2 O -> Ca^2 + (aq) + 2HCO 3 ~ (aq)

Such water, and also that containing salts of multipositive metals,

(usually sulphates), is said to be hard since it does not readily

produce a lather with soap. Experiments with alkali metal salts

can be performed to verify that the hardness is due to the presence

of the multipositive metal ions and not to any of the anions present.

The hardness due to calcium and magnesium hydrogencarbonates

is said to be temporary since it can be removed by boiling:

Ca2+ + 2HCO- -^- CaCO 3 i + CO 2 T + H 2 O

whilst that due to other salts is called permanent hardness and
is unaffected by boiling. Soap, essentially sodium stearate
C 17 H 35 COO~Na*, gives stearate and sodium ions in solution. The
metal ions causing hardness form insoluble stearates which appear
as scum, using up soap needed to wsolubilise" the fats and oils mainly
responsible for 'dirt'. The metal stearate precipitates—scum—may
be slightly coloured, and water for washing and laundering must
be softened, or a detergent used as an alternative to soap.
Detergents are made by, for example, treating petroleum hydro-
carbons with sulphuric acid, yielding sulphonated products which
are water soluble. These can also "solubilise' fats and oils since, like
the stearate ion, they have an oil-mistible hydrocarbon chain and
a water-soluble ionic end. The calcium salts of these substances,
however, are soluble in water and, therefore, remove hardness
without scum formation.
However, the deposition of salts from temporarily hard water in
boilers, and so on (for example the 'fur' found in kettles) makes it
desirable to soften such water for domestic and industrial use. Very