Qualitative Tests (Edexcel IAL Chemistry)

Reactions & Ionic Equations

Spec point — 8.19

Test Tube Reactions

• Simple test tube reactions can be done to identify the following ions:

  • Ammonium ions (NH₄⁺)
  • Carbonate ions (CO₃^2-) and hydrogencarbonate ions (HCO₃^-)
  • Sulfate ions (SO₄^2-)

• If the sample to be tested is a solid, then it must be dissolved in deionised water and made into an aqueous solution

Testing for Ammonium Ions

• About 10 drops of a solution containing ammonium ions, such as ammonium chloride, should be added to a clean test tube
• About 10 drops of sodium hydroxide should be added using a pipette

Overall equation: NH₄Cl (aq) + NaOH (aq) → NH₃(g) + H₂O (l) + NaCl (aq)

Ionic equation: NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O (l)

• The test tube should be swirled carefully to ensure that it is mixed well
• The test tube of the solution should then be placed in a beaker of water, and the beaker of water should be placed above a Bunsen burner, so that it can become a water bath
• As the solution is heated gently, fumes will be produced
• A pair of tongs should be used to hold a damp piece of red litmus paper near the mouth of the test tube, to test the fumes
• The red litmus paper will change colour and become blue in the presence of ammonia gas

Damp red litmus paper turning blue in the presence of ammonia gas

• Alternatively NH₃ can be tested for as it will form a white smoke if reacted with fumes of hydrogen chloride (from concentrated hydrochloric acid)
• The white smoke formed is ammonium chloride

NH₃(g) + HCl (g) → NH₄Cl (s)

Testing for Carbonate Ions

• A small amount (around 1 cm³) of dilute hydrochloric acid should be added to a test tube using a pipette
• An equal amount of sodium carbonate solution should then be added to the test tube using a clean pipette

Overall equation: 2HCl (aq) + Na₂CO₃(aq) → 2NaCl (aq) + CO₂(g) + H₂O (l)

Ionic equation: 2H⁺(aq) + CO₃^2-(aq) → CO₂(g) + H₂O (l)

• As soon as the sodium carbonate solution is added, a bung with a delivery tube should be attached to the test tube

  • The delivery tube should transfer the gas which is formed into a different test tube which contains a small amount of limewater (calcium hydroxide solution)

• Carbonate ions will react with hydrogen ions from the acid to produce carbon dioxide gas

• Carbon dioxide gas will turn the limewater milky

When carbon dioxide gas is bubbled into limewater it will turn cloudy as calcium carbonate is produced

• A similar reaction is seen with sodium hydrogen carbonate, but the equations are:

Overall equation: HCl (aq) + NaHCO₃(aq) → NaCl (aq) + CO₂(g) + H₂O (l)

Ionic equation: H⁺(aq) + HCO₃^-(aq) → CO₂(g) + H₂O (l)

Testing for Sulfate Ions

Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride If a sulfate is present then a white precipitate of barium sulfate is formed:

Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

A white precipitate of barium sulfate is a positive result for the presence of sulfate ions

Exam Hint HCl is added first to remove any carbonates which may be present and would also produce a precipitate and interfere with the results.