QUALITATIVE ANALYSIS — PART 2

Cation analysis masterclass for WAEC Chemistry Practical. This lesson trains candidates to identify metallic ions using NaOH, NH₃, HCl and H₂SO₄ with accurate observations, inferences, equations and examiner-style explanations.

Part 2 focuses on cations: Pb²⁺, Zn²⁺, Al³⁺, Cu²⁺, Fe²⁺, Fe³⁺, Ca²⁺ and NH₄⁺.

1. What Cation Analysis Means

Cation analysis is the identification of positive ions in a salt solution. In WAEC practical chemistry, the common method is to add reagents such as sodium hydroxide solution and aqueous ammonia in drops, then in excess. The colour of precipitate and its behaviour in excess reagent help identify the ion.

WAEC MARKING WARNING: Do not write “solution changed” without stating the exact observation. You must write “white precipitate formed”, “blue precipitate formed”, “precipitate dissolved in excess” or “precipitate insoluble in excess.”

NaOH Test

NaOH supplies OH⁻ ions which form metal hydroxide precipitates.

NH₃ Test

Aqueous ammonia also supplies OH⁻ ions, but some precipitates dissolve due to complex formation.

Precipitate Colour

Colour is a powerful clue: blue for Cu²⁺, green for Fe²⁺, brown for Fe³⁺ and white for Pb²⁺/Zn²⁺/Al³⁺.

2. Reagents Used in Cation Analysis

ReagentMain FunctionWAEC Skill Tested
NaOH(aq)Forms metal hydroxide precipitatesColour and solubility in excess
NH₃(aq)Forms hydroxides and complex ionsDistinguishes Zn²⁺, Cu²⁺ and Pb²⁺
Dilute HClForms insoluble chlorides with some ionsPb²⁺ confirmation by white PbCl₂
Dilute H₂SO₄Forms sulphatesPb²⁺ confirmation by white PbSO₄
Red litmus paperTests alkaline gasNH₃ turns moist red litmus blue
HeatDrives off ammonia from ammonium saltsNH₄⁺ confirmation
In this WAEC practical series, Pb²⁺ is especially important because the specimen contains lead(II) carbonate. After dissolving it in dilute HNO₃, the solution contains Pb²⁺ ions.

3. WAEC Cation Analysis Master Table

Cation With NaOH in Drops With Excess NaOH With NH₃ in Drops With Excess NH₃ Inference
Pb²⁺ White precipitate Dissolves White precipitate Insoluble Lead(II) ion present
Zn²⁺ White precipitate Dissolves White precipitate Dissolves Zinc ion present
Al³⁺ White gelatinous precipitate Dissolves White precipitate Insoluble Aluminium ion possible
Cu²⁺ Blue precipitate Insoluble Light blue precipitate Dissolves to deep blue solution Copper(II) ion present
Fe²⁺ Dirty green precipitate Insoluble Dirty green precipitate Insoluble Iron(II) ion present
Fe³⁺ Reddish-brown precipitate Insoluble Reddish-brown precipitate Insoluble Iron(III) ion present
Ca²⁺ White precipitate Sparingly soluble/insoluble No precipitate or slight white precipitate No significant change Calcium ion possible

4. Practical Class: Identifying Pb²⁺ from Lead(II) Carbonate

Task

A white solid contains lead(II) carbonate. Add dilute HNO₃, warm gently and use the resulting solution for cation tests.

Procedure

  1. Put a little PbCO₃-containing sample in a test tube.
  2. Add dilute HNO₃ and warm gently.
  3. Observe effervescence.
  4. Use the clear acid solution for Pb²⁺ tests.
  5. Add NaOH in drops and then in excess.
  6. Add NH₃ in drops and then in excess to another portion.
  7. Add dilute H₂SO₄ to another portion.
  8. Add dilute HCl to another portion.
Lead(II) cation test flow

Show: Pb2+ + NaOH gives white precipitate soluble in excess; Pb2+ + NH3 gives white precipitate insoluble in excess.

Teacher

Students, once PbCO₃ reacts with dilute nitric acid, carbon dioxide escapes and soluble lead(II) nitrate remains in solution. That solution now contains Pb²⁺.

Expected Results

TestObservationInference
Solution + NaOH in dropsWhite precipitate formedPb²⁺, Al³⁺ or Zn²⁺ possible
Excess NaOHPrecipitate dissolvesAmphoteric ion present
Solution + NH₃ in dropsWhite precipitate formedPb²⁺, Al³⁺ or Zn²⁺ possible
Excess NH₃White precipitate remains insolublePb²⁺ indicated
Solution + H₂SO₄White precipitate formedPb²⁺ confirmed as PbSO₄
Solution + HClWhite precipitate formedPb²⁺ confirmed as PbCl₂

Equations

PbCO₃(s) + 2HNO₃(aq) → Pb(NO₃)₂(aq) + CO₂(g) + H₂O(l)

Pb²⁺(aq) + 2OH⁻(aq) → Pb(OH)₂(s)

Pb(OH)₂(s) + 2OH⁻(aq) → [Pb(OH)₄]²⁻(aq)

Pb²⁺(aq) + SO₄²⁻(aq) → PbSO₄(s)

Pb²⁺(aq) + 2Cl⁻(aq) → PbCl₂(s)

Student

Sir, why is Pb²⁺ confused with Al³⁺ sometimes?

Teacher

Because both may give white precipitate with NaOH which dissolves in excess NaOH. The difference is that Pb²⁺ can be confirmed with dilute H₂SO₄ to form white PbSO₄ and with dilute HCl to form white PbCl₂.

5. Practical Class: Testing for NH₄⁺

Although the supplied specimen may not contain ammonium salt, WAEC frequently tests ammonium ion knowledge. This is important for Question 3 practical theory.

Procedure

  1. Put a little ammonium salt solution in a test tube.
  2. Add NaOH(aq).
  3. Warm gently.
  4. Test the gas with moist red litmus paper.
Expected observation: Pungent gas evolved; gas turns moist red litmus paper blue.

Inference

NH₃ gas evolved; NH₄⁺ is present.

NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

Teacher

Ammonia gas is alkaline. That is why it turns moist red litmus paper blue. The word “moist” is important because gases act better on damp litmus paper.

Student

Sir, can I write “ammonia produced” as observation?

Teacher

No. Write the observation first: “pungent gas evolved; moist red litmus paper turns blue.” Then infer ammonia or ammonium ion.

6. WAEC-Style Mixed Practical Task

Question

D is a colourless solution obtained after treating a white solid with dilute HNO₃. Carry out the following tests and complete the table.

  1. To the first portion, add NaOH in drops and then in excess.
  2. To the second portion, add NH₃ in drops and then in excess.
  3. To the third portion, add dilute H₂SO₄.
  4. To the fourth portion, add dilute HCl.

WAEC Marking Table

TestObservationInference
D + NaOH dropsWhite precipitatePb²⁺/Zn²⁺/Al³⁺ possible
Excess NaOHPrecipitate dissolvesAmphoteric ion
D + NH₃ dropsWhite precipitatePb²⁺/Al³⁺/Zn²⁺ possible
Excess NH₃Precipitate insolublePb²⁺ indicated
D + H₂SO₄White precipitatePb²⁺ confirmed
D + HClWhite precipitatePb²⁺ confirmed

Teacher Summary

This question follows a common WAEC pattern: first identify the possible ion using NaOH and NH₃, then confirm it using a more specific reagent.

7. Part 2 Exam-Ready Summary

Pb²⁺

White ppt with NaOH; soluble in excess NaOH. White ppt with NH₃; insoluble in excess. White ppt with H₂SO₄/HCl.

Zn²⁺

White ppt with NaOH and NH₃; dissolves in excess of both.

Cu²⁺

Blue ppt with NaOH; light blue ppt with NH₃ dissolves in excess to deep blue solution.

Fe²⁺

Dirty green precipitate, insoluble in excess.

Fe³⁺

Reddish-brown precipitate, insoluble in excess.

NH₄⁺

With NaOH and warming, pungent NH₃ gas turns moist red litmus blue.

FINAL WARNING: Always state whether the precipitate dissolves or remains insoluble in excess reagent. This is often the difference between a partial mark and full mark.