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GCSE Chemistry - Qualitative Analysis

Updated on November 19, 2012

Cations and Anions Definitions

Cation: A positive ion (an atom that has lost electrons).
Anion: A negative ion (an atom that has gained electrons).

Analysing Cations:

Flame Test results:

A flame test is a test that is used to determine what an unknown metal ion is. The heat of the flame 'excites' the metals ions, causing them to emit visible light. The colour of the visible light is what shows us what metal ion is present.

Calcium - Yellow-red flame.
Lithium – Red flame.
Sodium – Orange flame.
Copper - Green-blue flame.
Potassium – Lilac flame.
Barium - Pale green flame.

Add sodium hydroxide to the 'mystery solution', metal hydroxides are insoluble and will precipitate out of the solution. Some of these metal hydroxides have characteristic colours, then you can tell which metal is in the compound.

Calcium – White precipitate of calcium hydroxide is formed.
Magnesium – White precipitate of magnesium hydroxide is formed.
Aluminium – White precipitate of aluminium hydroxide is formed.
Iron(II) – Green precipitate of iron(II) hydroxide is formed.
Iron(III) - Brown precipitate of iron(III) hydroxide is formed.
Copper – Pale blue precipitate of copper (II) Hydroxide is formed.

Analysing Anions:

The 'Silver Nitrate Test' for halide ions (chloride, bromide and iodide):

Add a few drops of silver nitrate to your 'mystery compound', this is go get rid of any carbonate or sulphite ions before the test. Then add a few drops of silver nitrate solution. Precipitates of silver chloride, silver bromide and silver iodide form. You can tell which ion is present because the different precipitates have the following characteristic colours:

Chloride – White precipitate
Bromide – Cream precipitate
Iodide – Yellow precipitate

To test for sulphate ions, put some of a solution of the compound that you are testing into a test tube and then add few drops of dilute hydrochloric acid (again, this is to get rid of any sulphite or carbonate ions). Then add a few drops of barium chloride solution. If a sulphate ion is present a white precipitate of barium sulphate will form.

To test for sulphites SO32-- Add hydrochloric acid to solution that you want to test. The sulphites then react with the hydrogen to give sulphur dioxide and water. Sulphur dioxide turnsdamp potassium dichromate paper (VI) from orange to green.

To test for carbonate ions
add a few drops of dilute hydrochloric acid to the test substance. Bubbles of carbon dioxide gas are given off if carbonate ions are present.


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