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Analysis of Calcium Carbonate (CaCO₃)

Class 12 Chemistry Practical

How to do salt analysis · Viva · Quiz

Aim

To analyse the given inorganic salt for one acidic and one basic radical.

Preliminary Test

ExperimentObservationInference
Physical examinationWhite chalky solid; odourlessChalk / limestone / marble type
SolubilityInsoluble in water; dissolves in dilute acid with effervescenceInsoluble carbonate
Dry heatingCO₂ evolved; residue is CaO (lime), alkaline to moist litmusCalcium carbonate
Flame testBrick-red flameCa²⁺ indicated
Dilute H₂SO₄ / HClBrisk effervescence of CO₂; lime water milkyCO₃²⁻ present
Conc. H₂SO₄CO₂; no halide fumesNot a halide

Test of Anion (CO₃²⁻)

ExperimentObservationInference
Add dilute H₂SO₄ (or dilute HCl) to the saltBrisk effervescence; colourless, odourless gasCO₃²⁻ may be present
Confirmatory Test
Lime water
Pass the gas through lime water. Continue passing in excess		Lime water turns milky; milkiness disappears on excess CO₂CO₃²⁻ is confirmed

Ionic equations

  • CO₃²⁻ + 2H⁺ → CO₂ ↑ + H₂O
  • Ca(OH)₂ + CO₂ → CaCO₃ ↓ (milky) + H₂O
  • CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (milkiness disappears)

Test of Cation (Ca²⁺)

ExperimentObservationInference
Groups I-IV absent. Add (NH₄)₂CO₃ in ammoniacal mediumWhite ppt of CaCO₃Group V may be present
Confirmatory Tests
Flame test
Flame test moistened with conc. HCl		Brick-red flameCa²⁺ is confirmed
Ammonium oxalate
Dissolve ppt in acetic acid and add (NH₄)₂C₂O₄		White ppt of calcium oxalateCa²⁺ is confirmed

Ionic equations

  • CaCO₃ → CaO + CO₂ ↑ (heat)
  • Ca²⁺ + C₂O₄²⁻ → CaC₂O₄ ↓

Result

The given salt contains Ca²⁺ as the cation (basic radical) and CO₃²⁻ as the anion (acidic radical). The salt is Calcium Carbonate (CaCO₃).

Precautions

  1. Use small quantities of salt and reagents; do not use excess.
  2. Keep the mouth of the test tube away from the face while heating.
  3. Use freshly prepared FeSO₄ for the brown ring test.
  4. Nessler’s reagent is toxic (mercury); handle with care and do not pipette by mouth.

Viva voce

  1. Why insoluble in water but soluble in acid?

    Acid converts CO₃²⁻ to CO₂ and leaves soluble Ca²⁺.

  2. How is original solution prepared?

    Dissolve in dilute HCl, then proceed with cation groups (CO₂ must be boiled off).

  3. Why brick-red flame?

    Calcium emission.

  4. Why oxalate confirmatory?

    CaC₂O₄ is insoluble in acetic acid.

  5. How from Sr / Ba?

    Flame colours and (NH₄)₂SO₄ / K₂CrO₄ distinctions.

  6. What is the residue after heating?

    CaO; it turns moist red litmus blue.

  7. Why milkiness disappears with excess CO₂?

    Bicarbonate formation.

  8. Is this a Group V salt?

    Yes. Ca²⁺ is Group V. The anion is carbonate, so do the dilute-acid test first.

Related salts