Tests for Ions

Testing for Cations (Positive Ions)

Flame Tests

Flame tests are useful for identifying certain metal ions (cations).
Flame test colours can be used to identify different ions: For example, calcium ions give a brick-red flame, sodium ions give a yellow flame, and copper ions give a green flame.
Flame tests are primarily qualitative and are more useful when a sample contains only one type of ion.

Ammonia Solution Tests

Some metal ions (like copper, iron, and aluminium) can also be identified using sodium hydroxide or ammonia solution tests.
Copper(II) ions react with sodium hydroxide to form a blue precipitate (copper hydroxide).
Iron(II) ions produce a green precipitate and iron(III) ions produce a brown or reddish-brown precipitate when reacted with sodium hydroxide.
Aluminium ions form a white precipitate with sodium hydroxide which dissolves when excess sodium hydroxide is added.

Testing for Anions (Negative Ions)

Carbonate Ions

Carbonate ions can be identified by adding a dilute acid; if effervescence (or bubbling) is observed, it means carbon dioxide is being produced.
To confirm the presence of carbon dioxide, collect the gas produced and bubble it through limewater; if the limewater turns cloudy, carbonate ions were present.

Sulphate Ions

To confirm the presence of sulphate ions, add barium chloride solution; if a white barium sulphate precipitate is formed, sulphate ions were present.
This test should be carried out in acidic conditions to remove any other ions that may interfere with the test. Addition of dilute hydrochloric acid beforehand can provide these conditions.

Halide Ions

Halide ions can be identified using silver nitrate solution, which is used to form a precipitate with halides. Each halide produces a different coloured precipitate: chloride ions form a white precipitate, bromide ions form a cream precipitate and iodide ions form a yellow precipitate.
To further distill the result, nitric acid and ammonia can also be used. With nitric acid added first to remove any interfering ions, follow this with an ammonia solution to notice a varying degree of solubility for each distinctive precipitate.

Note: Always remember safety precautions when conducting tests. Some substances can be hazardous in certain forms or concentrations, and should be handled with care.