Chemistry Preparation
Chemistry Past Papers

Identification of ions and gases

Identification of cations

The identification of cations is done by either adding aqueous sodium hydroxide or aqueous ammonia to solutions with unknown ions that need to be identified. Sodium hydroxide is added slowly and then in excess to solutions of these salts. The colour of the precipitate or solution formed will help you deduce which cation is present in the solution. Initially add a few drops of the solutions, and then the excess of it to produce definitive results.

Here’s a summary of results produced with the following cations:

Identification of anions

Testing for halide ions

The halogens are the elements in group 7 of the Periodic Table, and include chlorine, bromine and iodine. Their ions are called halide ions e.g. chloride, Cl-. You can test for them using silver nitrate solution. But first, nitric acid is added before adding in the silver nitrate solution as the nitric acid will remove any carbonate ions that might be present  – they would produce a white precipitate of silver carbonate, giving a false positive result for chloride ions.

If no precipitate is formed upon adding nitric acid, then add in silver nitrate solution drop wise and observe the colors of the precipitate formed.

Sulphate ions are tested by first adding in dilute nitric acid, and then aqueous barium nitrate solution.

Nitrate ions can be detected by reducing them to ammonia (a pungent gas that turns damp red litmus blue). Identification of nitrate ions is done by adding sodium hydroxide solution, then aluminium powder or foil & heating strongly.

For carbonate ions, add dilute hydrochloric acid & and then test any gas that forms by bubbling it through limewater.

The following table gives a summary of the results:

Test of gases

There are unique tests to detect and identify gases. The results of a test will let you determine which gas is present. When doing these tests in the laboratory, always have splints, litmus paper strips and limewater ready as you need to be quick with the tests!

Here is a summary of the results:

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