Qualitative analysis refers to the techniques used to identify the ions present in a salt sample without determining their quantities.
What are two physical properties that can provide clues about a salt’s composition?
Colour and solubility in water.
What colour are most copper(II) salts?
Blue or green.
Which two reagents are commonly used to test for cations?
Sodium hydroxide (NaOH) and ammonia solution (NH₃).
What is observed when NaOH is added to a solution containing aluminium ions (Al³⁺)?
A white precipitate of aluminium hydroxide (Al(OH)₃) forms, which is soluble in excess NaOH.
What colour precipitate does Fe²⁺ form with NaOH?
Green.
What happens when copper(II) hydroxide (Cu(OH)₂) reacts with excess ammonia?
It dissolves to form a deep blue solution.
Which reagent is used to confirm the presence of ammonium ions (NH₄⁺)?
Nessler's reagent.
What happens when carbonate ions react with acid?
Carbon dioxide gas is produced, which turns limewater cloudy.
Write the reaction equation for carbonate ions reacting with acid.
2H⁺(aq) + CO₃²⁻(aq) → CO₂(g) + H₂O(l)
Which reagent is used to test for chloride ions?
Silver nitrate (AgNO₃) in the presence of nitric acid (HNO₃).
What is the appearance of silver chloride (AgCl) precipitate?
White.
Which anion produces a white precipitate when reacted with barium chloride in hydrochloric acid?
Sulfate ions (SO₄²⁻).
What is the confirmatory test for nitrate ions?
The brown ring test using iron(II) sulfate and concentrated sulfuric acid.
What is the first step in a systematic approach to qualitative analysis?
Observing the physical properties, such as colour and solubility.
What is the purpose of a flow diagram in qualitative analysis?
To guide the sequential testing of unknown salts for identification.
Which cation forms a yellow precipitate when tested with potassium iodide (KI)?
Lead(II) ions (Pb²⁺).
What colour precipitate is formed when iron(III) ions react with potassium hexacyanoferrate(II)?
Dark blue.
What is the general solubility rule for nitrates?
All nitrates are soluble in water.
Which sulfates are insoluble in water?
Lead(II) sulfate (PbSO₄), barium sulfate (BaSO₄), and calcium sulfate (CaSO₄).
Explanation Questions
Why does aluminium hydroxide dissolve in excess NaOH but not in excess ammonia?
Aluminium hydroxide dissolves in excess NaOH because it forms a soluble aluminate complex, but it does not dissolve in ammonia because no such complex is formed.
Why does ammonia solution dissolve copper(II) hydroxide but not iron(III) hydroxide?
Ammonia forms a soluble complex with Cu²⁺ ions, but Fe³⁺ ions do not form a similar soluble complex with ammonia.
Why is nitric acid added before testing for chloride ions with silver nitrate?
Nitric acid removes interfering ions that might form unwanted precipitates.
Why does carbon dioxide turn limewater cloudy?
Carbon dioxide reacts with calcium hydroxide in limewater to form insoluble calcium carbonate.
Why do sulfates react with barium chloride to form a precipitate?
Barium sulfate (BaSO₄) is highly insoluble in water, so it precipitates out as a solid.
Why do lead(II) ions form a yellow precipitate with potassium iodide?
Lead(II) iodide (PbI₂) is insoluble in water and forms a yellow precipitate.
Why is the brown ring test used to identify nitrate ions?
The reaction forms a brown complex at the interface, which is unique to nitrate ions.
Why is a systematic approach important in qualitative analysis?
It ensures accurate identification by avoiding false positives and interferences between tests.
Why do most carbonates not dissolve in water?
Carbonate ions form strong ionic bonds with metal cations, making them insoluble.
Why are silver chloride, lead(II) chloride, and mercury(I) chloride insoluble?
Their lattice energies are too high for water molecules to overcome, preventing dissolution.