Chemistry Reference
In-Depth Information
●
Notes on qualitative
analysis
The branch of chemistry that deals with the
identification of elements or grouping of elements
present in a sample is called
qualitative chemical
analysis
, or
qualitative analysis
for short. It does
not deal with anything to do with quantities.
The techniques employed in qualitative analysis
vary in their complexity, depending on the nature
of the sample under investigation. In some cases it
is only necessary to confirm the presence of certain
elements or groups for which specific chemical tests,
or 'spot' tests, applicable directly to the sample, may
be available. More often, the sample is a complex
mixture, and a systematic analysis must be made in
order that all the component parts may be identified.
Often, the first simple stages of qualitative analysis
require no apparatus at all. Things like colour
and smell can be observed without any need for
apparatus.
The following summary collects together
information from throughout the topic which would
allow you to carry out qualitative analysis.
Flame colours
If a clean nichrome wire is dipped into a metal
compound and then held in the hot part of a
Bunsen flame, the flame can become coloured
(Figure 16.3). Certain metal ions may be detected
in their compounds by observing their flame colours
(Table 16.2).
Appearance or smell
A preliminary examination of the substance will give
you a start. The appearance or smell of a substance
can often indicate what it might contain (see
Table 16.1).
Table 16.1
Deductions that can be made from a substance's appearance
or smell.
Observation on substance
Indication
Figure 16.3
The green colour is characteristic of copper.
Black powder
Carbon, or contains O
2−
ions (as in
CuO), or S
2−
ions (as in CuS)
Table 16.2
Characteristic flame colours of some metal ions.
Pale green crystals
Contains Fe
2
+
ions (as in iron(
ii
) salts)
Dark green crystals
Contains Ni
2
+
ions (as in nickel(ii)
ii
)
salts)
Metal
Flame colour
Group I
Lithium
Crimson
Blue or blue-green crystals
Contains Cu
2
+
ions (as in copper(ii)
ii
)
salts)
Sodium
Golden yellow
Potassium
Lilac
Yellow-brown crystals
Contains Fe
3
+
ions (as in iron(
iii
) salts)
Rubidium
Red
Smell of ammonia
Contains NH
4
+
ions (as in ammonium
salts)
Caesium
Blue
Group II
Calcium
Brick red
Strontium
Crimson
Barium
Apple green
Others
Lead
Blue-white
Copper (as Cu(
ii
))
Green
