3.1.4 Qualitative Analysis

Question 1

Carbonate test

Answer 1

E.g. Na2CO3(aq) + 2HNO3(aq) -> 2NaNO3(aq) + CO2(g) + H2O(l)

1. Add dilute nitric acid to sample (sulfuric acid and hydrochloric acid interfere with results)
2. Effervescence - carbonate ions may be present
3. Test gas product is CO2 by bubbling through limewater (turns cloudy/milky)
   CO2(g) + Ca(OH)2 -> CaCO3(s) + H2O(l)

Question 2

Sulphate test

Answer 2

Sulphate ions react with barium ions to form a insoluble white BaSO4 precipitate
Ba+2(aq) + SO4-2(aq) -> BaSO4(s)

1. Acidify the test solution by adding nitric acid (Sulfuric acid cannot be used because it contains sulfate ions - interferes with test)
2. Add Ba(NO3)2(aq) / BaCl2(aq) (do not use BaCl2 if testing for halide ions)
3. Observe if white precipitate is formed

Reaction used in Barium Meal

Question 3

Halide test

Answer 3

Halide ions react with silver ions to form silver halide precipitates
Ag+(aq) + X-(aq) -> AgX(s)

1. Acidify the halide solution (aq) using a few drops of dilute nitric acid
2. Add AgNO3(aq) to the sample
3. Different coloured silver halide precipitates form, depending on the halide ions present
   Ag+(aq) + Cl-(aq) -> AgCl(s) - white
   Ag+(aq) + Br-(aq) -> AgBr(s) - cream
   Ag+(aq) + I-(aq) -> AgI(s) - yellow
4. Add dilute NH3 (aq) If dissolves - AgCl
   Add conc. NH3 (aq) If dissolves - AgBr If insoluble - AgI

Question 4

Sequence of Anion tests

Answer 4

1. Carbonate test
   Neither sulphate / halide produce bubbles with dilute acid - no possibility of incorrect conclusion
   If no bubbles produced then proceed
2. Sulphate test
   Adding Ba+ to a solution containing carbonate ions produces BaCO3
   BaCO3 is also a white precipitate - false positive result
3. Halide test
   Adding Ag+ to a solution containing carbonate/sulphate ions produces Ag2CO3/Ag2SO4
   Ag2CO3/Ag2SO4 are also white precipitates - false positive result

Question 5

Ammonium Ion test

Answer 5

Ammonium ions react with hydroxide ions to form ammonia(g) when heated
NH4+(aq) + OH-(aq) -> NH3(g) + H2O(l)

1. Add NaOH(aq) to ammonium ion solution
2. Warm mixture and ammonia gas is produced
3. Test gas using damp red litmus paper - turns blue if positive (alkaline)