Qualitative Analysis

How do you test for carbonate ions (CO₃²⁻)?

Add dilute hydrochloric acid to the solid or solution. If carbonate is present, effervescence occurs and the gas produced turns limewater (Ca(OH)₂ solution) milky/cloudy. This confirms CO₂ has been released.

The equation is: CO₃²⁻ + 2H⁺ → H₂O + CO₂

How do you test for sulfate ions (SO₄²⁻)?

Acidify the solution with dilute hydrochloric acid , then add barium chloride solution (BaCl₂). A white precipitate of barium sulfate (BaSO₄) confirms the presence of sulfate ions. Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

Why must you acidify with dilute HCl before adding BaCl₂ in the sulfate test?

To prevent false positives — barium carbonate (BaCO₃) and barium sulfate (BaSO₃) are also white precipitates that would form in the absence of acid. The HCl dissolves these other possible precipitates, ensuring only the insoluble BaSO₄ remains if sulfate is genuinely present.

How do you test for halide ions (Cl⁻, Br⁻, I⁻)?

A: Acidify with dilute nitric acid, then add silver nitrate solution (AgNO₃)

How do you test for ammonium ions (NH₄⁺)?

Add warm sodium hydroxide solution (NaOH) to the sample and gently heat. If ammonium ions are present, ammonia gas (NH₃) is produced. This is confirmed by: a pungent smell, turning damp red litmus paper blue, or turning damp universal indicator paper blue/purple. Ionic equation: NH₄⁺ + OH⁻ → NH₃ + H₂O

Why must you use warm NaOH for the ammonium test, not just cold?

Heating drives the equilibrium forward and ensures NH₃ gas is produced in sufficient quantity to be detected. At room temperature the reaction is too slow and the gas may not be released.

What is the systematic order for carrying out qualitative analysis on an unknown solution?

• 1 Test for carbonate (dilute HCl — look for effervescence with CO₂).

• 2 Test for sulfate (acidify then BaCl₂).

• 3 Test for halides (acidify with HNO₃, then AgNO₃, then ammonia).