CHAPTER 18. REACTIONS IN AQUEOUS SOLUTION 18.4
Tests for anions ESAFS
We often want to know which ions are present in solution. If we know which salts precip-
itate, we can derive tests to identify ions in solution. Given below are a few such tests.
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Test for a chloride
Prepare a solution of the unknown salt using distilled water and add a small amount ofsil-
ver nitratesolution. If a white precipitate forms, the salt is either a chloride or a carbonate.
Cl−(aq)+Ag+(aq)+NO− 3 (aq)→AgCl(s)+NO− 3 (aq)(AgCl is white precipitate)
CO^23 −(aq)+ 2Ag+(aq)+ 2NO− 3 (aq)→Ag 2 CO 3 (s)+ 2NO− 3 (aq)(Ag 2 CO 3 is white precipitate)
The next step is to treat the precipitate with a small amount ofconcentrated nitric acid. If
the precipitate remains unchanged, then the salt is a chloride. If carbon dioxide is formed
and the precipitate disappears, the salt is a carbonate.
AgCl(s)+HNO 3 (ℓ)→(no reaction; precipitate is unchanged)
Ag 2 CO 3 (s)+2HNO 3 (ℓ)→ 2 Ag+(aq)+2NO− 3 (aq)+H 2 O(ℓ)+CO 2 (g) (precipitate disappears)
Test for bromides and iodides
As was the case with the chlorides, the bromides and iodides also form precipitates when
they are reacted with silver nitrate. Silver chloride is a white precipitate, but the silver
bromide and silver iodide precipitates are both pale yellow. To determine whether the
precipitate is a bromide or an iodide, we use chlorine water and carbon tetrachloride
(CCl 4 ).
Chlorine water frees bromine gas from the bromide and colours the carbon tetrachloride a
reddish brown.
2 Br−(aq)+Cl 2 (aq)→ 2 Cl−(aq)+Br 2 (g)
Chlorine water frees iodine gas from an iodide and colours the carbon tetrachloride purple.
2 I−(aq)+Cl 2 (aq)→ 2 Cl−(aq)+I 2 (g)
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