2.8 Chemical Tests

REVISION NOTES

IGCSE Edexcel Chemistry

2.8 Chemical Tests

2.8.1 Describe tests for these gases:

Hydrogen
Oxygen
Carbon dioxide
Ammonia

Test of Gases

2.8.2 Describe how to carry out a flame test

FLAME TEST

When metal ions are heated in a flam, they produce a distinctive colour
The flame test is thus used to identify metal ions by observing the colour of the flame produced

Methods

Dip a metal wire loop made from unreactive metal (nichome or platinum) in concentrated acid
Then hold it in the blue flame of a Bunsen burner until there is no colour change
- This cleans the wire loop to avoid contamination
Dip the metal wire loop into the solid sample
It is then placed in the edge of the blue Bunsen burner flame
- It should not be heated for too long as it would glow red and affect the observation of flame colour

2.8.3 Know the colours formed in flame tests for these cations:

Li⁺ is red
Na⁺ is yellow
K⁺ is lilac
Ca²⁺ is orange-red
Cu²⁺ is blue-green.

2.8.4 Describe tests for these cations:

NH₄⁺ using sodium hydroxide solution and identifying the gas evolved
Cu²⁺, Fe²⁺ and Fe³⁺ using sodium hydroxide solution.

Test for Cations

2.8.5 Describe tests for these anions:

Cl^–, Br^– and I^– using acidified silver nitrate solution
SO₄^2–using acidified barium chloride solution
CO₃^2– using hydrochloric acid and identifying the gas evolved.

Test for Anions

Nitric acid is added first to react with and remove any other ions (e.g. carbonate ions) that may be present that would give a confusing precipitate with silver nitrate. (E.g. If carbonate ions are present, a white precipitate of silver carbonate would be produced which gives a false result of chloride ions)