Test for Anions

Question 1

Test for Carbonate, CO₃^2-

Test for Anion and Observations

Answer 1

Test:

• Add dilute acids (e.g HNO₃, HCl, H₂SO₄)

Observation:

1. Effervescence occurs
2. Colourless, odourless gas evolved
3. Gas forms white precipitate in limewater
4. Gas is carbon dioxide

Explanation:
- Acids react with metal carbonates to form salt, water and carbon dioxide gas.
- Carbon dioxide gas, an acidic oxide, reacts with aqueous calcium hydroxide (limewater) to form insoluble calcium carbonate (white precipitate) and water. CO₂(g) +Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

Question 2

Test for Sulfate, SO₄^2- (in solution)

Test for Anion and Observations

Answer 2

Test:

• Add dilute nitric acid, followed by aqueous barium nitrate
• OR add dilute hydrochloric acid, followed by aqueous barium chloride

Observation:

• White precipitate is seen

Explanation:

• Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

Question 3

Test for Chloride, Cl- (in solution)

Test for Anion and Observations

Answer 3

Test:

• Add dilute nitric acid followed by aqueous silver nitrate

Observation:

• White precipitate is seen

Explanation:
- Ag⁺(aq) + Cl^-(aq) → AgCl(s)

Question 4

Test for Iodide, I^- (in solution)

Test for Anion and Observations

Answer 4

Test:

• Add dilute nitric acid followed by aqueous silver nitrate

Observation:

• Pale yellow precipitate is seen
• Ag⁺ (aq) + I^- (aq) → AgI (s)

Question 5

Test for Nitrate, NO₃^-

Test for Anion and Observations

Answer 5

Test:

1. Add aqueous sodium hydroxide followed by aluminium foil and warm
2. Test the gas evolved with damp red litmus paper

Observation:

1. Effervescence occurs (hydrogen)
2. Colourless, pungent gas evolved
3. Gas turns damp red litmus paper blue
4. Gas is ammonia

Initial effervescence seen due to hydrogen gas formed in a side reaction between aluminium foil and aqueous sodium hydroxide

Question 6

What is the purpose of adding dilute nitric acid before adding test reagent in the test for Cl^-, I^- and SO₄^2-?

Answer 6

Dilute nitric acid is added to remove any carbonate ion which may be present in the unknown solution as carbonate ion will also react with the test reagent to form precipitate.

For example, both carbonate ion and sulfate ion will react with aqueous barium nitrate to form white precipitate of insoluble barium carbonate and barium sulfate respectively.

Question 7

What happens if dilute nitric acid is added after adding test reagent in the test for SO₄^2-? (same idea applies for Cl^- and I^-)

Answer 7

After adding aqueous barium nitrate, a white precipitate is seen.
Dilute nitric is then added afterwards.

Observation if the white precipitate is barium carbonate:

• Effervescence occurs.
• White precipitate is soluble in nitric acid to form colourless solution.

Explanation:

• The white precipitate must be barium carbonate as barium carbonate reacts with acid (to produce soluble barium nitrate, water and CO₂ gas). Thus, precipitate dissolves.

Observation if the white precipitate is barium sulfate:

• White precipitate remains insoluble in nitric acid.

Explanation:

• The white precipitate must be barium sulfate as barium sulfate does not react with acid.