8.3 - qualitative analysis

• Carbonate test


  - Carbonates react with acids to form carbon dioxide gas.

  1. Add dilute nitric acid to the solid or solution to be tested.

  2. If you see bubbles, the unknown compound could be a carbonate.

  3. Bubble the gas through lime water (i.e. calcium hydroxide solution).

  4. Carbon dioxide reacts to form a white precipitate of calcium carbonate, which turns the lime water cloudy.
• Sulfate test


  - Barium sulfate is insoluble in water.

  1. Add aqueous barium ions (i.e. barium nitrate) to the solution being tested.

  2. If a white precipitate forms, sulfate ions are present.
• Halide test


  - Silver halides are insoluble in water.

  1. Add aqueous silver nitrate to an aqueous solution of a halide.

  2. The silver halide precipitates are different colours:

  - silver chloride is white.

  - silver bromide is cream.

  - silver iodide is yellow.

  3. Add aqueous ammonia to test the solubility of the precipitate

  - silver chloride will dissolve in dilute ammonia.

  - silver bromide will dissolve in concentrated ammonia.

  - silver iodide is insoluble in concentrated ammonia.
• Sequence of tests for anions

  1. Carbonate.

  2. Sulfate.

  3. Halides.
• Why is the carbonate test carried out first?

  - Neither sulfate nor halide ions produce bubbles with dilute acid.

  - The sulfate and halide tests require carbonates to be removed first.
• Why is the sulfate test carried out second?

  - Barium carbonate is a white precipitate in water.

  - Carbonate test must be carried out first so if a white precipitate forms you know it is the result of barium sulfate and not barium carbonate.
• Why is the halide test carried out last?

  - Silver carbonate and silver sulfate are both white precipitates in water.

  - Carbonate and sulfate tests must be carried out first so you know that the precipitate forming is a silver halide precipitate.
• How to carry out tests on a mixture of ions?

  Carry out the tests in the same sequence on the same solution.

  1. Carbonate test.

  - If bubbles are observed continue adding nitric acid until the bubbling stops.

  2. Sulfate test.

  - Add an excess of barium nitrate, any sulfate ions present will form a precipitate of barium sulfate.

  - Filter the solution to remove the precipitate.

  3. Halide test.

  - Add silver nitrate, different coloured precipitates form for the different silver halides.

  - Add aqueous ammonia to confirm the identity of the silver halide.
• Why is nitric acid used in the carbonate test when carrying out a sequence of tests on the same solution?

  Sulfuric acid and hydrochloric acid cannot be used as they would interfere with the sulfate and halide tests.
• Why is barium sulfate used in the carbonate test when carrying out a sequence of tests on the same solution?

  Barium chloride cannot be used as it would interfere with the halide test.