3.2.4 AS Qualitative Analysis

Test Tube Reactions

• Simple test tube reactions can be done to identify the following ions:
  • Ammonium ions (NH₄⁺)
  • Halide ions (X^-) - covered in 2.3.4 Halide Ion Reactions
  • 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 

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

Flame tests

• Metal ions produce a colour if heated strongly in a flame
• Ions from different metals produce different colours
• The flame test is thus used to identify metal ions by the colour of the flame they produce
• Dip the loop of an unreactive metal wire such as nichrome or platinum in concentrated acid, and then hold it in the blue flame of a Bunsen burner until there is no colour change
• This cleans the wire loop and avoids contamination
  • This is an important step as the test will only work if there is just one type of ion present
  • Two or more ions means the colours will mix, making identification erroneous

• Dip the loop into the solid sample and place it in the edge of the blue Bunsen flame
• Avoid letting the wire get so hot that it glows red otherwise this can be confused with a flame colour

Diagram showing the technique for carrying out a flame test

Colours Observed in Flame Tests