qualitative analysis

• Testing for Negative ions (anions)

  1. Add any dilute acid and observe effervescence

  2. Bubble gas through limewater to test for CO2 – will turn limewater cloudy
• Carbonate


  Fizzing due to CO2 would be observed if a carbonate was present
• Testing for presence of a sulfate

  1. Acidified BaCl2 solution is used as a reagent

  2. If barium chloride is added to a solution that contains sulfate ions a white precipitate forms
• Other anions should give a negative result which is no precipitate forming
• The acid is needed to react with carbonate impurities that are often found in salts which would form a white barium carbonate precipitate and so give a false result
• Sulfuric acid cannot be used to acidify the mixture because it contains sulfate ions which would form a precipitate
• Testing for halide ions with silver nitrate

  1. The test solution is made acidic with nitric acid, and then silver nitrate solution is added dropwise

  2. Fluorides produce no precipitate

  3. Chlorides produce a white precipitate

  4. Bromides produce a cream precipitate

  5. Iodides produce a pale yellow precipitate
• The role of nitric acid is to react with any carbonates present to prevent formation of the precipitate Ag2CO3
• Differentiating silver halide precipitates with ammonia

  1. Silver chloride dissolves in dilute ammonia to form a colourless complex ion

  2. Silver bromide dissolves in concentrated ammonia to form a colourless complex ion

  3. Silver iodide does not react with ammonia
• Hydrochloric acid cannot be used to acidify the mixture because it contains chloride ions which would form a precipitate
• The sequence of tests required is carbonate, sulfate then halide
• Testing for positive ions (cations)


  Test for ammonium ion NH4+ by the reaction with warm NaOH(aq), forming NH3 gas
• Ammonia gas can be identified by its pungent smell or by turning red litmus paper blue