Qualitative analysis

Question 1

How do you test for carbonates?

Answer 1

Add hydrochloric acid. If carbonate ions are present there will be effervescence (bubbling) due to CO2 gas being present.
Test if the gas is carbon dioxide by bubbling it through limewater [Ca(OH)2]. If CO2 is present, limewater turns cloudy, indicating presence of carbonate ions.

Question 2

How do you test for sulphates?

Answer 2

First add hydrochloric acid to remove any carbonates.
Add a solution containing Ba2+ ions, such as barium chloride. Barium ions will react with sulphate ions if they are present. Barium sulphate will form, which is a white precipitate.
Ba2+(aq) + SO4^2-(aq) –> BaSO4 (s)

Question 3

How do you test for ammonium ions?

Answer 3

Add sodium hydroxide. If NH4^+ ions are present, ammonia gas (NH3) will form but will be dissolved in water.
Heat the test tube to release ammonia. You can identify due to a pungent distinctive smell. Also test using damp red litmus paper which will turn blue
NH4^+(aq) + OH- (aq) –> NH3 (g) +H20(l)

Question 4

How to test for halides?

Answer 4

First add nitric acid to remove any anions (negative ions) that aren’t halides. Then add silver nitrate (AgNO3). You will get a coloured precipitate (ppt).
Chloride: white ppt
Bromide: cream ppt
Iodide: yellow ppt
Add ammonia to know which ion is present.
Chloride: dissolves in dilute ammonia
Bromide: dissolves in concentrated ammonia
Iodide: doesn’t dissolve in concentrated ammonia