QUALITATIVE ANALYSIS — PART 3

Anion analysis and final WAEC qualitative analysis masterclass. This lesson completes the qualitative analysis series by teaching carbonate, chloride, sulphate, nitrate, practical reporting, mixed salt analysis and full WAEC-style solution writing.

Part 3 focuses on anions: CO₃²⁻, SO₄²⁻, Cl⁻, NO₃⁻ and practical interpretation using HNO₃, BaCl₂, AgNO₃, lime water and litmus papers.

1. What Anion Analysis Means

Anion analysis is the identification of negative ions in a salt or solution. WAEC frequently tests anions using simple reagents such as dilute acids, lime water, barium chloride and silver nitrate. The candidate must record the observation first, then infer the anion.

WAEC MARKING WARNING: Do not write “carbonate present” as an observation. Write “effervescence occurs; colourless odourless gas turns lime water milky.” Then infer carbonate.

Carbonate Test

Carbonate reacts with dilute acid to release CO₂, which turns lime water milky.

Sulphate Test

BaCl₂ gives white BaSO₄ precipitate with sulphate. The precipitate is insoluble in dilute HNO₃.

Chloride Test

AgNO₃ gives white AgCl precipitate with chloride after acidifying properly with dilute HNO₃.

2. WAEC Anion Analysis Master Table

Anion Test Observation Inference Equation
CO₃²⁻ Add dilute HNO₃ or HCl, pass gas into lime water Effervescence; colourless gas turns lime water milky Carbonate present; CO₂ evolved CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
SO₄²⁻ Add BaCl₂, then dilute HNO₃ White precipitate insoluble in dilute HNO₃ Sulphate present Ba²⁺ + SO₄²⁻ → BaSO₄
Cl⁻ Acidify with dilute HNO₃, then add AgNO₃ White precipitate Chloride present Ag⁺ + Cl⁻ → AgCl
NO₃⁻ Warm with NaOH and aluminium foil, test gas with moist red litmus Pungent gas turns moist red litmus blue Nitrate present NO₃⁻ is reduced to NH₃ under alkaline conditions
SO₃²⁻ Add dilute acid Choking gas evolved, decolourizes acidified KMnO₄ Sulphite present SO₂ gas formed
For the current WAEC reagent set: CO₃²⁻ is the main positive anion expected because the specimen contains lead(II) carbonate. BaCl₂ and AgNO₃ may give negative results if sulphate and chloride are absent.

3. Practical Class: Carbonate Ion Test

Task

A white solid C contains lead(II) carbonate. Test for the anion present using dilute HNO₃ and lime water.

Procedure

  1. Put a spatulaful of C into a test tube.
  2. Add dilute HNO₃ carefully.
  3. Warm gently if gas evolution is slow.
  4. Pass the evolved gas into lime water.
  5. Record observation and inference.
Expected observation: Effervescence occurs; a colourless odourless gas is evolved; the gas turns lime water milky.

Equation

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

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

Carbon dioxide gas passed into lime water Teacher guide: Label the test tube containing C and dilute HNO3, delivery tube, lime water, gas bubbles and milky lime water. Milky lime water confirms carbon dioxide.

Teacher

Students, this is one of the most repeated WAEC anion tests. Carbonate + acid gives carbon dioxide. The confirmatory test is lime water turning milky.

Student

Sir, why does lime water turn milky?

Teacher

Carbon dioxide reacts with calcium hydroxide in lime water to form calcium carbonate, which is insoluble and appears milky.

WAEC Answer Format

ObservationInference
Effervescence; colourless gas evolved; gas turns lime water milky.CO₂ evolved; CO₃²⁻ present.

4. Practical Class: Sulphate Ion Test Using BaCl₂

Task

To test a solution for sulphate ion using aqueous barium chloride and dilute HNO₃.

Procedure

  1. Place about 2 cm³ of the solution in a test tube.
  2. Add a few drops of BaCl₂(aq).
  3. If white precipitate forms, add dilute HNO₃.
  4. Observe whether the precipitate dissolves or remains insoluble.
Positive result: White precipitate formed and remains insoluble in dilute HNO₃.

Equation

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Barium chloride sulphate confirmation

Show: test solution + BaCl2; white BaSO4 precipitate remains insoluble in dilute acid.

Teacher

BaCl₂ is used because Ba²⁺ reacts with SO₄²⁻ to form BaSO₄, a white insoluble precipitate.

Student

Sir, if no precipitate forms, what should I write?

Teacher

Write exactly what happened: “No white precipitate formed.” Then infer: “SO₄²⁻ absent.” WAEC accepts correct negative observations when the test is properly performed.

In the lead(II) carbonate and starch specimen, sulphate is not the original anion. Therefore, BaCl₂ test may be negative unless sulphate has been introduced.

5. Practical Class: Chloride Ion Test Using AgNO₃

Task

To test a solution for chloride ion using dilute HNO₃ and silver trioxonitrate(V) solution.

Procedure

  1. Place about 2 cm³ of the solution in a test tube.
  2. Acidify with dilute HNO₃.
  3. Add AgNO₃(aq).
  4. Observe any precipitate formed.
Positive result: White precipitate of AgCl is formed.

Equation

Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Silver nitrate chloride confirmation

Show: test solution + AgNO3; white AgCl precipitate confirms chloride.

Teacher

Before AgNO₃ test, we acidify with dilute HNO₃. This removes interfering ions and does not introduce chloride.

Student

Sir, why not use HCl before adding AgNO₃?

Teacher

Excellent question. HCl contains Cl⁻. If you add HCl before AgNO₃, the chloride you detect may come from the acid, not the sample. That gives a false positive.

WAEC TRAP: Never acidify with HCl before testing chloride with AgNO₃. Use dilute HNO₃.

6. Full WAEC-Style Mixed Qualitative Analysis Task

Question

C is a white solid mixture containing an inorganic salt and an organic food substance. Carry out the following tests and complete the observation/inference table.

  1. Add distilled water to C and shake.
  2. Test with red and blue litmus papers.
  3. Filter the mixture.
  4. Add iodine solution to the residue.
  5. Add dilute HNO₃ to another portion of C and warm gently.
  6. Pass any gas evolved into lime water.
  7. To the acid solution, add NaOH in drops and then in excess.
  8. To another portion, add NH₃ in drops and then in excess.
  9. To another portion, add BaCl₂ followed by dilute HNO₃.
  10. To another portion, add AgNO₃ after acidifying with dilute HNO₃.

WAEC Marking Table

TestObservationInference
C + waterWhite suspension; solid does not dissolve completely.Insoluble substance present.
Litmus testNo change on red or blue litmus.Neutral or nearly neutral mixture.
FiltrationColourless filtrate and white residue.Residue retained for further test.
Residue + iodineBlue-black colouration.Starch present.
C + dilute HNO₃Effervescence; colourless gas evolved.Carbonate suspected.
Gas + lime waterLime water turns milky.CO₂ confirmed; CO₃²⁻ present.
Acid solution + NaOH dropsWhite precipitate.Pb²⁺/Zn²⁺/Al³⁺ possible.
Excess NaOHWhite precipitate dissolves.Amphoteric ion present.
Acid solution + NH₃ dropsWhite precipitate.Pb²⁺/Al³⁺/Zn²⁺ possible.
Excess NH₃White precipitate insoluble.Pb²⁺ indicated.
BaCl₂ + dilute HNO₃No white precipitate.SO₄²⁻ absent.
AgNO₃ after dilute HNO₃No white precipitate.Cl⁻ absent.

Final Conclusion

C contains lead(II) carbonate and starch. The carbonate is confirmed by CO₂ gas turning lime water milky, starch is confirmed by iodine, and Pb²⁺ is supported by NaOH/NH₃ behaviour.

7. Final Exam-Ready Summary

Carbonate

Acid gives effervescence. CO₂ turns lime water milky.

Sulphate

BaCl₂ gives white BaSO₄ precipitate insoluble in dilute HNO₃.

Chloride

Acidify with dilute HNO₃, then add AgNO₃. White AgCl confirms chloride.

Starch

Iodine solution gives blue-black colour.

Pb²⁺

White precipitate with NaOH dissolves in excess. White precipitate with NH₃ is insoluble in excess.

Observation vs Inference

Observation: lime water turns milky. Inference: CO₂ is present.

GOLDEN WAEC RULE: A correct negative result is still a result. If no precipitate forms, write “No precipitate formed” and make the correct inference.
FINAL WARNING: Do not invent reagents not supplied. Use only the reagents listed in the practical instruction.