6.11 Qualitative Analysis

6.11 Qualitative Analysis - Chemistry Form 4

CHEMISTRY • Form 4 • Chapter 6: Acid, Base and Salt

6.11Qualitative Analysis

Qualitative analysis of a salt is a systematic chemical technique used to identify the cation and anion present in an unknown salt by analysing its physical and chemical properties. Through a step-by-step procedure—observing physical appearance, testing solubility, observing the effect of heat, and conducting specific anion, cation, and confirmatory tests—the exact identity of the salt can be deduced.

Learning Objectives

Identify the cation and anion present in a salt through experiment.
Describe confirmatory tests to identify cations and anions.

Flowchart of Qualitative Salt Analysis Workflow

Guide students step-by-step through the identification of an unknown salt from solid observation to confirmation of cations and anions.

Physical Properties

Colour of solid salt and solubility in water

Effect of Heat

Colour of residue and testing of gases released

Anion & Cation Tests

Specific reagents in aqueous solution

Confirmatory Tests

Distinguish ions with identical initial observations

Anion Tests

CO₃²⁻ → dilute acid → CO₂ gas
Cl⁻ → HNO₃ + AgNO₃ → white ppt
SO₄²⁻ → HCl + BaCl₂ → white ppt
NO₃⁻ → brown ring test

Cation Tests

NaOH / NH₃ → precipitate colour and solubility
Zn²⁺, Al³⁺, Pb²⁺ dissolve in excess NaOH
Only Zn²⁺ and Cu²⁺ dissolve in excess NH₃
KI confirms Pb²⁺ vs Al³⁺

Brief Explanation

A logical process of elimination identifies the ions in an unknown salt.

Qualitative analysis uses a logical process of elimination:

1. Check the Colour

Blue, green and brown immediately point to transition metal ions: Cu²⁺, Fe²⁺ and Fe³⁺.

2. Heat the Salt

Heating releases gases such as CO₂ from carbonates and NO₂ from nitrates, providing evidence for the corresponding anions. Residue colours such as ZnO, yellow when hot and white when cold, also help identify cations.

3. Add Aqueous Alkalis

Adding aqueous NaOH and NH₃ sorts cations by whether precipitates form, their colour, and their solubility in excess alkali.

4. Confirm Identical Behaviour

When cations show identical behaviour, such as Al³⁺ and Pb²⁺, a confirmatory reagent such as KI is added to give a definitive identification.

Anion	Reagents Added	Observation	Inference
CO₃²⁻	Add dilute acid	Effervescence; gas turns limewater cloudy	CO₃²⁻ present
Cl⁻	Excess dilute HNO₃, then AgNO₃	White precipitate formed	Cl⁻ present
SO₄²⁻	Excess dilute HCl, then BaCl₂	White precipitate formed	SO₄²⁻ present
NO₃⁻	Dilute H₂SO₄ + FeSO₄, then concentrated H₂SO₄	Brown ring at junction of two liquid layers	NO₃⁻ present

Cation	Small amount of NaOH	Excess NaOH	Small amount of NH₃	Excess NH₃
Ca²⁺	White precipitate	Insoluble	No precipitate	No change
Mg²⁺	White precipitate	Insoluble	White precipitate	Insoluble
Al³⁺	White precipitate	Soluble	White precipitate	Insoluble
Pb²⁺	White precipitate	Soluble	White precipitate	Insoluble
Zn²⁺	White precipitate	Soluble	White precipitate	Soluble
Cu²⁺	Blue precipitate	Insoluble	Blue precipitate	Soluble, dark blue solution
Fe²⁺	Green precipitate	Insoluble	Green precipitate	Insoluble
Fe³⁺	Brown precipitate	Insoluble	Brown precipitate	Insoluble
NH₄⁺	No precipitate; on heating, pungent gas turns moist red litmus blue	—	No precipitate	—

Confirmatory Test for Pb²⁺ and Al³⁺

Add potassium iodide solution, KI. Pb²⁺ forms a yellow precipitate of PbI₂ that dissolves upon heating and reforms upon cooling. Al³⁺ forms no precipitate.

Confirmatory Tests for Fe²⁺ and Fe³⁺

K₃Fe(CN)₆ gives a dark blue precipitate with Fe²⁺, while K₄Fe(CN)₆ gives a dark blue precipitate with Fe³⁺. KSCN gives a blood-red solution with Fe³⁺.

Try to Answer First

Answer mentally, then check the explanation.

An unknown solid salt is heated strongly. A brown gas is released that turns moist blue litmus paper red, and a glowing wooden splinter rekindles. The residue remaining is yellow when hot and white when cold. Identify the salt.

Answer: Zinc nitrate, Zn(NO₃)₂. The brown acidic gas is nitrogen dioxide, NO₂, and the gas that rekindles a glowing splinter is oxygen, O₂, indicating a nitrate ion, NO₃⁻. The residue that is yellow when hot and white when cold is zinc oxide, ZnO, indicating the zinc ion, Zn²⁺.

When sodium hydroxide solution is added dropwise to a solution of salt Z, a white precipitate forms which remains insoluble in excess sodium hydroxide. However, when aqueous ammonia is added to salt Z, no precipitate is observed. Identify the cation present in salt Z.

Answer: Calcium ion, Ca²⁺. Calcium hydroxide forms a white precipitate that is insoluble in excess NaOH, while Ca²⁺ forms no precipitate with aqueous ammonia.

Why must excess dilute nitric acid be added to an unknown salt solution before adding silver nitrate solution when testing for chloride ions?

Answer: To eliminate any carbonate ions, CO₃²⁻, present in the solution. Carbonate ions react with silver ions to form silver carbonate, Ag₂CO₃, which is a pale yellow precipitate that could interfere with the test. Adding acid decomposes the carbonate into carbon dioxide and water, preventing a false positive result.

Common Mistakes

Avoid these errors during qualitative analysis.

Forgetting to add excess dilute acid before adding BaCl₂ or AgNO₃, leading to false positive tests for sulphate or chloride when carbonate is present.

Confusing the confirmatory reagents for Fe²⁺ and Fe³⁺: Fe²⁺ needs potassium hexacyanoferrate(III) to give a dark blue precipitate, while Fe³⁺ needs potassium hexacyanoferrate(II) to give a dark blue precipitate.

Forgetting that lead(II) iodide, PbI₂, is soluble in hot water; students must heat and cool the mixture to confirm the precipitate dissolves and recrystallises.

Concept Misunderstandings

Compare each misconception with the correct fact.

Misconception

All white precipitates are identical chemical compounds.

Correct Fact

White precipitates are different metal hydroxides such as Zn(OH)₂, Al(OH)₃, Pb(OH)₂, Mg(OH)₂ and Ca(OH)₂, which have distinct amphoteric and complexing properties in excess alkalis.

Misconception

A colourless solution means pure water without any salt dissolved.

Correct Fact

All white soluble salts, such as sodium, potassium, calcium, magnesium, zinc, lead and aluminium salts, dissolve in water to produce completely colourless solutions.

Summary

Key facts to remember.

Qualitative analysis identifies cations and anions using systematic observations and chemical tests.
Residues on heating: CuO is black, Fe₂O₃ is brown, ZnO is yellow when hot and white when cool, PbO is brown when hot and yellow when cool.
Anion identification: CO₃²⁻ gives CO₂ gas; Cl⁻ gives white AgCl; SO₄²⁻ gives white BaSO₄; NO₃⁻ gives a brown ring.
Cations with NaOH: Zn²⁺, Al³⁺ and Pb²⁺ dissolve in excess; Mg²⁺, Ca²⁺, Cu²⁺, Fe²⁺ and Fe³⁺ are insoluble in excess; NH₄⁺ gives no precipitate but releases NH₃ upon heating.
Cations with NH₃: only Zn²⁺ forms a colourless solution and Cu²⁺ forms a dark blue solution in excess; Ca²⁺ and NH₄⁺ form no precipitate.
Distinguishing Al³⁺ and Pb²⁺: add KI; Pb²⁺ forms a yellow precipitate that dissolves upon heating and recrystallises when cooled.

Short Activity

Complete all parts before checking your score.

A. Objective Quiz

Which reagent is used to confirm and differentiate between aluminium ion, Al³⁺, and lead(II) ion, Pb²⁺, in aqueous solution?

Nessler's reagent Potassium iodide solution Potassium thiocyanate solution Barium chloride solution

When dilute hydrochloric acid is added to solid salt W, effervescence occurs. The gas released turns limewater cloudy. Which anion is present in salt W?

Chloride ion Nitrate ion Carbonate ion Sulphate ion

B. Fill in the Blanks

In the confirmatory test for the nitrate ion, a concentrated drop of sulphuric acid added slowly down the test tube forms a distinctive _______ ring at the junction of the liquids.

A cation that forms a blue precipitate with aqueous ammonia and dissolves in excess aqueous ammonia to produce a dark blue solution is _______.

C. Match the Ion or Test with the Observation

Drag each choice on desktop. On mobile, tap a choice and then tap the matching box.

Ion / Reagent Test

Lead(II) ion + KI solution Ammonium ion + Nessler's reagent Iron(III) ion + KSCN solution Sulphate ion + acidified BaCl₂

Observation / Result

1Yellow precipitate that dissolves when heated and reforms on cooling

2Brown precipitate

3Blood red solution

4White precipitate of BaSO₄

Complete every answer before checking.

Keywords

Tap or hover over a term to read its meaning.

Qualitative analysis Precipitate Residue Effervescence Limewater Confirmatory test Nessler's reagent Brown ring test

Quick Notes