Experimental techniques and chemical analysis - Identification of ions and gases (3)
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1.
(c) Describe the tests to identify the nitrate, NO3-, ion by reduction with aluminium foil and aqueous sodium hydroxide, then testing for ammonia gas.
Test 1: Reduction with Aluminium and Sodium Hydroxide
- A small amount of the unknown solution is mixed with a piece of aluminium foil in dilute aqueous sodium hydroxide (NaOH).
- The mixture is allowed to react.
- The aluminium reduces the nitrate ions (NO3-) to ammonia (NH3).
- The reaction is as follows: 4NO3-(aq) + 3Al(s) + 6H2O(l) → 3NH3(g) + 4Al(OH)3(aq)
Test 2: Testing for Ammonia Gas
- The gas evolved in the first test is passed into damp litmus paper.
- If ammonia gas is present, the damp litmus paper will turn blue.
- Alternatively, the gas can be passed into lime water (calcium hydroxide solution, Ca(OH)2). If ammonia is present, the lime water will turn milky due to the formation of calcium ammonium hydroxide.
- The balanced chemical equation for the reaction with lime water is: NH3(g) + Ca(OH)2(aq) → Ca(OH)2(aq) + NH3(aq)
Conclusion: If the aluminium foil reacts vigorously with sodium hydroxide and ammonia gas is produced, it confirms the presence of the nitrate ion.
2.
(e) Describe the tests to identify the sulfite, SO32-, ion by reaction with acidified aqueous potassium permanganate(VII).
Test: Reaction with Acidified Potassium Permanganate(VII)
- To the unknown solution, add a few drops of dilute nitric acid (HNO3) to acidify the solution.
- Then, add a few drops of acidified potassium permanganate (KMnO4) solution.
- Observe the solution for a colour change.
Observation and Conclusion:
- If the purple colour of the acidified potassium permanganate disappears, it confirms the presence of the sulfite ion. Reaction: 2SO32-(aq) + 2KMnO4(aq) + 2H+(aq) → 2Mn2+(aq) + 2K+(aq) + 2SO42-(aq) + 2H2O(l)
- If the purple colour remains, the sulfite ion is not present.
3.
(a) Describe the tests to identify the carbonate, CO32-, ion by reaction with dilute acid and then testing for carbon dioxide gas.
Test 1: Reaction with Dilute Acid
- A small amount of the unknown salt is added to dilute hydrochloric acid (HCl) or dilute sulphuric acid (H2SO4).
- The reaction is observed for effervescence (the production of bubbles).
- If carbon dioxide gas is produced, the solution will fizz. This is because the carbonate ion reacts with the acid to form carbonic acid (H2CO3), which is unstable and decomposes into carbon dioxide and water.
- The balanced chemical equation for the reaction is: CO32-(aq) + 2H+(aq) → H2CO3(aq) → H2O(l) + CO2(g)
Test 2: Testing for Carbon Dioxide Gas
- The gas evolved in the first test is passed into limewater (calcium hydroxide solution, Ca(OH)2).
- If carbon dioxide is present, the limewater will turn milky due to the formation of calcium carbonate (CaCO3).
- The balanced chemical equation for the reaction is: CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l)
Conclusion: If both tests show effervescence in the first test and the limewater turns milky in the second test, it confirms the presence of the carbonate ion.