Identifying Anions & Cations (AQA A Level Chemistry): Revision Note

Identifying Anions & Cations

REQUIRED PRACTICAL 4

Test Tube Reactions

• Group 2 ions (M²⁺)

• Ammonium ions (NH₄⁺)

• Halide ions (X^-)

• Hydroxide ions (OH^-)

• Carbonate ions (CO₃^2-)

• 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 Group 2 Metals

• Four test tubes should be placed in a test tube rack

• Around 10 drops of 0.1 mol dm^-3 barium chloride solution should be added to the first test tube

• Around 10 drops of dilute sodium hydroxide solution (NaOH) should be added to the same test tube

• Swirl the test tube carefully to mix well

• Continue to add sodium hydroxide dropwise to the test tube until it is in excess

• This should then be repeated in the other test tubes, for calcium bromide solution, magnesium chloride solution, and strontium chloride solution

• Any observations should be noted down in a suitable results table

• The same test as above can also be done using ammonia solution and sulfuric acid solution

The positive results of testing for the presence of Group 2 ions 

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

• 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

• The ionic equation is:

NH₄⁺ + OH^- → NH₃ + H₂O

Testing for Halide Ions

• The sample being tested should be added using a pipette to a test tube

• The test tube should be placed into a test tube rack

• A small amount of dilute nitric acid should be added to the sample using a pipette, followed by a small amount of silver nitrate solution

• Acidifying removes carbonate and hydroxide ions that would otherwise form their own silver precipitates and give a false positive

• A precipitate will form, either white, cream, or yellow, if a halide ion is present in the sample

• The ionic equations are:

Ag⁺(aq) + Cl^-(aq) $\to$AgCl(s)

Ag⁺(aq) + Br^-(aq) $\to$AgBr(s)

Ag⁺(aq) + l^-(aq) $\to$Agl(s)

• The white precipitate will form if chloride ions are present in the sample. The white precipitate is AgCl

• The cream precipitate will form if bromide ions are present in the sample. The cream precipitate is AgBr

• The yellow precipitate will form if iodide ions are present in the sample. The yellow precipitate is AgI

Further Test using Ammonia

• To distinguish between the three silver halide precipitates, further tests can be carried out using ammonia solution

• Silver chloride dissolves in dilute ammonia solution

• Silver bromide dissolves in concentrated ammonia solution

• Silver iodide dissolves in both

Testing for Hydroxide Ions

• A small amount (around 1 cm³) of the solution should be added to a test tube using a pipette

• Test the pH of the solution using red litmus paper or universal indicator paper

  • The presence of hydroxide ions will turn the red litmus paper blue, and the pH will be clearly alkaline on the universal indicator paper if hydroxide ions are present

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

• 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 that 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

Testing for Sulfate Ions

• Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride

• If a sulfate is present, then a white precipitate of barium sulfate is formed:

Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s)

Order of Testing

• The order in which the tests must be carried out on an unknown sample is carbonate, then sulfate, then halide

• Carbonate gives a white precipitate with both barium chloride and silver nitrate (and silver sulfate is only sparingly soluble), so carbonate has to be removed first and sulfate tested before the halides

Related topics

• Group 2 elements

• Group 7 (the halogens)