Identification of Anions

What must be added to a sample of zinc nitrate to confirm the identity of its anion?

• sodium hydroxide

• sulfuric acid

• aluminium

• magnesium

• nitric acid

Answer explanation

When an alkali and aluminium are added to a nitrate, ammonia gas is produced, and this confirms that the original solution contains the nitrate ion.

When an acid is added to magnesium carbonate, the effervescence formed is bubbled into limewater. A white precipitate then forms in the limewater. What is the identity of the precipitate?

• Calcium hydroxide

• Calcium carbonate

• Magnesium hydroxide

• Calcium Oxide

Answer explanation

The following reaction occurs between carbon dioxide and limewater:

CO₂(g) + Ca(OH)₂(aq) -> CaCO₃(s) + H₂O(l)

What is the ionic equation for the formation of the precipitate for when the chloride ion is identified in a sample? (Hint: Consider solubility of salts)

• Ba²⁺(aq) + 2Cl^-(aq) -> BaCl₂(s)

• Ag⁺(aq) + Cl^-(aq) -> AgCl(s)

• Zn²⁺(aq) + 2Cl^-(aq) -> ZnCl₂(s)

• H⁺(aq) + Cl^-(aq) -> HCl(s)

Answer explanation

The test to identify the chloride ion involves adding acidified AgNO₃. The aqueous Ag⁺ then forms a precipitate with Cl^-.

Which substances can be added to a solution to test for the presence of sulfate?

• Acidified barium chloride

• Acidified barium nitrate

• Acidified silver nitrate

• Aqueous sodium hydroxide

Answer explanation

While the notes and PR11 state that acidified barium chloride is used, note that the ionic equation for the precipitation reaction that confirms the presence of sulfate is:

Ba²⁺(aq) + SO₄^2-(aq) -> BaSO₄(s)

Hence, any aqueous barium salt can be used.

Silver nitrate is added to a colourless solution. No acid is added to it.

A white precipitate forms. What anion could the original solution have contained?

• Chloride

• Carbonate

• Sulfate

• Nitrate

Answer explanation

If the anion were chloride, the following reaction would have happened:

Ag⁺(aq) + Cl^-(aq) -> AgCl(s) where the white precipitate is AgCl(s).

If the anion were carbonate, the following reaction would have happened:

2Ag⁺(aq) + CO₃^2-(aq) -> Ag₂CO₃(s) where the white precipitate is Ag₂CO₃(s).

Hence the mere formation of a white precipitate without the addition of an acid cannot confirm that a chloride is present, as the observations for chloride and carbonate are similar when silver nitrate is added without an acid.

This is why that for tests for chloride (and iodide, and sulfate), an additional addition of acid is required to see if carbonate could be present.

If carbonate were present, it would react with the acid to produce an observable change. If it were absent, there would be no observable change upon addition of an acid.