Qualitative Tests (Edexcel International AS Chemistry): Revision Note

Reactions & Ionic Equations

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