Chemistry Reference
In-Depth Information
Testing for different salts
Sometimes we want to analyse a salt and find out
what is in it. There are simple chemical tests which
allow us to identify the anion part of the salt. These
are often called
spot tests
.
More about salts
You have already seen on p. 122 in Table 8.2 that
salts are useful substances. Some of the salts shown in
that table occur naturally and are mined, for example
calcium sulfate (gypsum) and calcium carbonate
(limestone). Many of the others must be made by the
chemical industry, for example ammonium nitrate,
iron(ii)
ii
) sulfate and silver bromide.
With acids such as sulfuric acid, which has two
replaceable hydrogen ions per molecule, it is
possible to replace only
one
of these with a metal
ion. The salt produced is called an
acid salt
. An
acid salt is one in which not all of the replaceable
hydrogen ions of the acid have been replaced by
metal ions or the ammonium ion. Some examples of
acid salts are shown in Table 8.5.
Testing for a sulfate (SO
4
2−
)
You have seen that barium sulfate is an insoluble
salt (p. 125). Therefore, if you take a solution of a
suspected sulfate and add it to a solution of a soluble
barium salt (such as barium chloride) then a white
precipitate of barium sulfate will be produced.
barium ion + sulfate ion → barium sulfate
Ba
2+
(
aq
) + SO
4
2−
(
aq
) → BaSO
4
(
s
)
A few drops of dilute hydrochloric acid are also added to
this mixture. If the precipitate does not dissolve, then it is
barium sulfate and the unknown salt was in fact a sulfate.
If the precipitate does dissolve, then the unknown salt
may have been a sulfite (containing the SO
3
2−
ion).
Testing for a chloride (Cl
−
), a bromide (Br
−
)
or an iodide (I
−
)
Earlier in this chapter you saw that silver chloride is
an insoluble salt (p. 125). Therefore, if you take a
solution of a suspected chloride and add to it a small
volume of dilute nitric acid, to make an
aqueous
acidic solution
, followed by a small amount of a
solution of a soluble silver salt (such as silver nitrate),
a white precipitate of silver chloride will be produced.
chloride ion + silver ion → silver chloride
Cl
−
(
aq
) + Ag
+
(
aq
) → AgCl(
s
)
If left to stand, the precipitate goes grey (Figure 8.18).
Table 8.5
Examples of acid salts.
Acid
Type of acid salt
Example
Carbonic acid
(H
2
CO
3
)
Hydrogencarbonate
Sodium hydrogencarbonate
(NaHCO
3
)
Sulfuric acid
(H
2
SO
4
)
Hydrogensulfate
Potassium hydrogensulfate
(KHSO
4
)
Sodium hydrogencarbonate is the acid salt used as
the raising agent in the baking of cakes and some
bread, and is often called baking soda (Figure 8.17).
Figure 8.18
If left to stand the white precipitate of silver chloride goes
grey. This photochemical change plays an essential part in black and
white photography.
Figure 8.17
Sodium hydrogencarbonate is used as a raising agent in
bread and muffins.
