Test for Cations

Question 1

Sodium (Na⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 1

NaOH(aq):

• No visible change.

NH₃(aq):

• No visible change.

Explanation:

• No reaction between solution containing Na⁺ and NaOH(aq) or NH₃(aq)

Question 2

Potassium (K⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 2

NaOH(aq):

• No visible change.

NH₃(aq):

• No visible change.

Explanation:

• No reaction between solution containing K⁺ and NaOH(aq) or NH₃(aq)

Question 3

Ammonium (NH₄⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 3

NaOH(aq):

• On warming, colourless, pungent gas is evolved
• Gas turns damp red litmus paper blue.
• Gas is ammonia.

NH₃(aq):

• Aqueous ammonia should not be used to test for solution containing ammonium ions
• Upon heating, aqueous ammonia itself will produce ammonia gas. In this case, the production of ammonia cannot be used to confirm the presence of NH4⁺ in the unknown solution.

Explanation:

• Ammonium salts react with aqueous sodium hydroxide (alkali) to form salt, water and ammonia gas.
• NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O(l)

Question 4

Calcium (Ca²⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 4

NaOH(aq):

• White precipitate seen.
• Precipitate insoluble in excess aqueous sodium hydroxide.

NH₃(aq):

• No visible change. (note: slight white precipitate seen if concentration of Ca²⁺ is high)

Explanation:

• Ca²⁺ + 2OH^-(aq) → Ca(OH)₂(s) (white ppt.)
• However, ammonia is a weak alkali that partially dissociates in water to form hydroxide ions. Most ammonia remains as simple molecules in the solution. The low concentration of hydroxide ions in aqueous ammonia results in the formation of a low concentraiton of calcium hydroxide. Since calcium hydroxide is slightly soluble in water, in low concentrations, calcium hydroxide remains soluble. Hence, no white precipitate is formed.

Question 5

Aluminium (Al³⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 5

NaOH(aq):

• White precipitate seen.
• Precipitate dissolves in excess aqueous sodium hydroxide to give a colourless solution.

NH₃(aq):

• White preiciptate seen.
• Precipitate insoluble in excess aqueous ammonia.

Explanation:

• Al³⁺ + 3OH^-(aq) → Al(OH)₃(s) (white ppt.)
• In excess aqueous sodium hydroxide, Al(OH)₃(s) + NaOH(aq) → NaAlO₂(aq) + 2H₂O(l)

Question 6

Lead(II) (Pb²⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 6

NaOH(aq):

• White precipitate seen.
• Precipitate dissolves in excess aqueous sodium hydroxide to give a colourless solution

NH₃(aq):

• White precipitate seen.
• Precipitate insoluble in excess aquous ammonia.

Explanation:

• Pb²⁺ + 2OH^-(aq) → Pb(OH)₂(s) (white ppt.)
• In excess aqueous sodium hydroxide, Pb(OH)₂(s) + 2NaOH(aq) → Na₂PbO₂(aq) + 2H₂O(l)

Question 7

Zinc (Zn²⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 7

NaOH(aq):

• White precipitate seen.
• Precipitate dissolves in excess aqueous sodium hydroxide to give a colourless solution.

NH₃(aq):

• White precipitate seen.
• Precipitate dissolves in excess aqeuous ammonia to give a colourless solution (soluble zinc ammine complex).

Explanation:

• Zn²⁺ + 2OH^-(aq) → Zn(OH)₂(s) (white ppt.)
• In excess aqueous sodium hydroxide, Zn(OH)₂(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + 2H₂O(l)
• In excess aqueous ammonia, Zn(OH)₂(s) + 4NH₃(aq) → Zn(NH₃)₄(OH)₂(aq)

Question 8

Copper(II) (Cu²⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 8

NaOH(aq):

• Blue precipitate seen.
• Precipiate insoluble in excess aqueous sodium hydroxide.

NH₃(aq):

• Blue precipitate seen.
• Precipitate dissolves in excess aquous ammonia to give dark blue solution (soluble copper ammine complexes).

Explanation:

• Cu²⁺ + 2OH^-(aq) → Cu(OH)₂(s) (blue ppt.)
• In excess aqueous ammonia, Cu(OH)₂(s) + 4NH₃(aq) → Cu(NH₃)₄(OH)₂(aq)

Question 9

Iron(II) (Fe²⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 9

NaOH(aq):
- Green precipitate seen.
- Precipitate insoluble in excess aqueous sodium hydroxide.

NH₃(aq):
- Green precipitate seen.
- Precipitate insoluble in excess aqueous ammonia.

Explanation:

• Fe²⁺ + 2OH^-(aq) → Fe(OH)₂(s) (green ppt.)
• After some time, green precipitate turns reddish-brown on standing.
• 4 Fe(OH)₂(s) (green ppt.) + O₂(g) + 2H₂O(l) → 4 Fe(OH)₃(s) (reddish-brown ppt.)

Question 10

Iron(III) (Fe³⁺)

Observation on adding NaOH(aq) and NH₃(aq)

Answer 10

NaOH(aq):

• Reddish-brown precipitate seen.
• Precipitate insoluble in excess aqueous sodium hydroxide.

NH₃(aq):

• Reddish-brown precipitate seen.
• Precipitate insoluble in excess aqueous ammonia.

Explanation:

• Fe³⁺ + 3OH^-(aq) → Fe(OH)₃(s) (reddish-brown ppt.)

Question 11

Why do some precipitates dissolve in excess aqueous sodium hydroxide or aqueous ammonia?

Answer 11

For excess aqueous sodium hydroxide:

• White precipitates Al(OH)₃, Pb(OH)₂ and Zn(OH)₂ are amphoteric hydroxides which react further with excess aqueous sodium hydroxide to form soluble complex salts.

For excess aqueous ammonia:

• White precipitate Zn(OH)₂ and blue precipitate Cu(OH)₂ can react with excess aqueous ammonia to form soluble complex salts

Question 12

Differentiate solutions containing Pb²⁺ and Al³⁺

Reagent added + Observation for Pb²⁺ and Al³⁺

Answer 12

Add aqueous solution containing iodide ions/chloride ions/sulfate ions.

If Pb²⁺ present:

• PbI₂(s) (bright yellow precipitate)
• PbCl₂(s) (white precipitate)
• PbSO₄(s) (white precipitate)

If Al³⁺ present:

• No visible change (AlI₃, AlCl₃, Al₂(SO₄)₃ are soluble in water, hence no reaction occurs)