Summary GCSE Chemistry - Test for Cations and Anions (Qualitative Analysis)

Qualitative Analysis



Qualitative Analysis
A compound is made up of a cation (positive ion) and an anion (negative ion). For example,
in the compound Sodium Chloride (NaCl), Sodium (Na+) is the cation, while the anion is the
Chloride (Cl-).

If a person is given a sample of a substance and is asked to identity it, said person can perform
a number of tests to identify the cation and the anion present.

The cation is the positive ion and is always in front when writing a chemical formula, so we
will test for it first.



Test for the Cation
When testing for the cation, one can perform three different tests:

 A flame test.
 Adding a soluble alkali to a solution of the unknown compound: first a little, then
excess.
 Adding a solid alkali to the solid unknown compound and heating.

It is important to take in consideration the states of the substances.

If the flame test is done and no observations are seen, one can continue by performing the
next test. If observations are seen and conclusions are made, one will move on to test for the
anion present.




1. Flame Test



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, Qualitative Analysis


The flame test is used to identify the metal cations by the colour of the flame they produce.
The flame test will help us identify the Lithium, Sodium, Potassium, Calcium, Barium, Lead
and Copper cations.

Method:

 Take a piece of unreactive wire such as nichrome wire/platinum wire.
 Moisten it with distilled water and dip it into the substance being tested.
 Insert the nichrome wire covered with the substance into the Bunsen flame.
 Note any observations.

Results:

If the flame turns:

 Deep red, Lithium (Li+) cations are present.
 Golden yellow, Sodium (Na+) cations are present.
 Lilac, Potassium (K+) cations are present.
 Brick red, Calcium (Ca+2) cations are present.
 Apple green, Barium (Ba+2) cations are present.
 Steely blue, Lead (Pb+2) cations are present.
 Blue-green, Copper (Cu2+) cations are present.




Flame test results
Sources of Error/Precautions:

 Impurities in metal ion solutions can alter results.
 If the wire is dipped into a solution, it must have a high concentration of the metal
ions.

If no colour is impaired to the flame, none of the above cations are present in the compound.
In that case, we will pass on to the next test.

2. Adding a soluble alkali to a solution of the unknown compound: first
a little, then excess.
The alkalis can be either NaOH(aq) (Sodium Hydroxide Solution) or NH4OH(aq) (Ammonium
Hydroxide solution). If the substance is not in solution (i.e. in solid form), add distilled water
and mix to dissolve it.


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