Summary AQA A Level Chemistry - All Chemical Tests with Reagents and Observations

Testing for Cations
Testing for Group 2 Ions
Using Hydroxide Ions:

• Reagent: NaOH
• Observations:

Group 2 Ion Initial Solution 10 Drops of NaOH Excess NaOH
2+
Magnesium, Mg Colourless Slight White Precipitate White Precipitate
Calcium, Ca2+ Colourless Slight White Precipitate Slight White Precipitate
Strontium, Sr2+ Colourless Slight White Precipitate Slight White Precipitate
Barium, Ba2+ Colourless No Change No Change

• Group 2 hydroxides become more soluble down the group.
• Mg(OH)2 is insoluble in water, so forms a white precipitate.
• Ca(OH)2 is partially soluble in water, so forms a slight white precipitate.

Using Sulfate Ions:

• Reagent: H2SO4
• Observations:

Group 2 Ion Initial Solution 10 Drops of H2SO4 Excess H2SO4
Magnesium, Mg2+ Colourless Slight White Precipitate Colourless
Calcium, Ca2+ Colourless Slight White Precipitate Slight White Precipitate
Strontium, Sr2+ Colourless White Precipitate White Precipitate
Barium, Ba2+ Colourless White Precipitate White Precipitate

• Group 2 sulfates become less soluble down the group.
• BaSO4 and SrSO4 will form white precipitates due to their low solubilities..

Testing for Group 3 Ions
• Reagent: NaOH
• Observation: white precipitate of aluminium hydroxide:
o [Al(H2O)6]3+ (aq) + 3OH– (aq) ! Al(H2O)3(OH)3 (s) + 3H2O (l)

• Reagent: Excess NaOH
• Observation: The precipitate will dissolve in excess sodium hydroxide to form a colourless solution:
o Al(H2O)3(OH)3 (s) + OH– (aq) ! [Al(OH)4] – (aq) + 3H2O (l)

Test for Ammonium Ions
• Reagent: NaOH
o Adding hydroxide ions to a solution with ammonium ions will form ammonia gas:
• Observation: Ammonia gas is alkaline, so will turn damp red litmus paper blue.
o NH4+ + OH–! NH3 + H2O.