Tests for anions — carbonate, sulphate, chloride, nitrate identification

Question

How do we systematically test for common anions — carbonate, sulphate, chloride, and nitrate — in a salt sample? What confirmatory tests distinguish each?

(JEE Main, CBSE 12 — anion detection is tested in both practical exams and theory papers)

Solution — Step by Step

Test: Add dilute HCl to the salt.

Na_2CO_3 + 2HCl \rightarrow 2NaCl + H_2O + CO_2\uparrow

Observation: Brisk effervescence. Pass the gas through lime water — it turns milky.

CO_2 + Ca(OH)_2 \rightarrow CaCO_3\downarrow + H_2O

Excess $CO_2$ clears the milkiness: $CaCO_3 + CO_2 + H_2O \rightarrow Ca(HCO_3)_2$ (soluble).

Distinction: Carbonates react with dilute acid immediately. Bicarbonates also effervesce but dissolve more readily in cold water.

Test: Dissolve the salt in water, add dilute HCl first (to prevent false positives from carbonates/sulphites), then add $BaCl_2$ solution.

Na_2SO_4 + BaCl_2 \rightarrow BaSO_4\downarrow + 2NaCl

Observation: White precipitate of $BaSO_4$ that is insoluble in concentrated HCl — this is the key confirmatory point. ( $BaCO_3$ and $BaSO_3$ dissolve in HCl, but $BaSO_4$ does not.)

$K_{sp}$ of $BaSO_4 = 1.1 \times 10^{-10}$ — extremely insoluble.

Test: Dissolve salt in water, add dilute $HNO_3$ (not HCl, obviously), then add $AgNO_3$ .

NaCl + AgNO_3 \rightarrow AgCl\downarrow + NaNO_3

Observation: Curdy white precipitate of $AgCl$ that is soluble in ammonium hydroxide (forming the soluble complex $[Ag(NH_3)_2]^+$ ).

AgCl + 2NH_3 \rightarrow [Ag(NH_3)_2]Cl

This distinguishes $Cl^-$ from $Br^-$ (pale yellow $AgBr$ , partially soluble in $NH_3$ ) and $I^-$ (yellow $AgI$ , insoluble in $NH_3$ ).

Test: Dissolve salt in water. Add freshly prepared $FeSO_4$ solution, then carefully pour concentrated $H_2SO_4$ along the side of the test tube (do not mix).

Observation: A brown ring forms at the junction of the two layers.

NO_3^- + 3Fe^{2+} + 4H^+ \rightarrow 3Fe^{3+} + NO\uparrow + 2H_2O

Fe^{2+} + NO \rightarrow [Fe(H_2O)_5(NO)]^{2+}

The brown colour is due to the nitroso-ferrous complex $[Fe(H_2O)_5(NO)]^{2+}$ .

flowchart TD
    A["Salt sample"] --> B["Add dil. HCl"]
    B --> C{"Effervescence?"}
    C -->|"Yes: gas turns lime water milky"| D["CO₃²⁻ confirmed"]
    C -->|"No"| E["Dissolve in water + dil. HCl + BaCl₂"]
    E --> F{"White ppt insoluble in conc. HCl?"}
    F -->|"Yes"| G["SO₄²⁻ confirmed"]
    F -->|"No"| H["Add dil. HNO₃ + AgNO₃"]
    H --> I{"Curdy white ppt soluble in NH₃?"}
    I -->|"Yes"| J["Cl⁻ confirmed"]
    I -->|"No"| K["FeSO₄ + conc. H₂SO₄ along side"]
    K --> L{"Brown ring at junction?"}
    L -->|"Yes"| M["NO₃⁻ confirmed"]

Why This Works

Each anion test relies on forming a characteristic insoluble compound or a coloured complex. The key is specificity — adding dilute HCl before the $BaCl_2$ test eliminates interference from $CO_3^{2-}$ and $SO_3^{2-}$ (which also form white barium precipitates but dissolve in acid). Similarly, using $HNO_3$ (not HCl) before the $AgNO_3$ test ensures you are not introducing extra $Cl^-$ ions.

The brown ring test works because NO is a neutral ligand that coordinates with $Fe^{2+}$ to form a deeply coloured complex — a beautiful example of coordination chemistry in action.

Common Mistake

The most common error in the sulphate test: forgetting to add dilute HCl before $BaCl_2$ . Without it, carbonates and sulphites also give white precipitates with barium, leading to a false positive. The confirmatory step is that $BaSO_4$ is insoluble even in concentrated HCl. CBSE board examiners specifically check for this in practical viva.

For the chloride test, never use HCl as the acidifying agent — you would be adding the very ion you are testing for. Use dilute $HNO_3$ instead. This sounds obvious but under exam pressure, students mix it up regularly.

Question

Solution — Step by Step

Why This Works

Common Mistake

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