How to identify the ions in an ionic compound - BBC Bitesize (2025)

• An contains a positive cation (cation) and a negative (anion). These both need to be identified to work out the identity of an unknown ionic compound.

• Cations can be identified with flame tests or precipitate tests using sodium hydroxide and ammonia solutions.

• Anions (halide, sulfate, and carbonate ions) can be identified using precipitate tests or reaction with dilute acid.

Ionic compounds contain a positive cation and a negative anion.

To fully identify an unknown , chemical tests must be carried out to identify both the cation and the anion.

Some metal cations have distinctive colours when held in a blue Bunsen burner flame.

A nichrome wire is used to hold the test solid – nichrome has no flame colour of its own and it has a high melting point.

• Bunsen burner, heatproof mat.

• Nichrome wire (in holder).

Ionic compounds to test

• Sodium chloride, calcium chloride, lithium chloride, potassium chloride, copper(II) sulfate (1 g of each salt)

• Concentrated hydrochloric acid

Safety: Follow the safety advice given by a teacher.

1. Take a piece of nichrome wire with a loop at one end.

2. Firstly clean the nichrome wire by dipping the loop in the concentrated hydrochloric acid and placing it in the hot blue flame of the Bunsen burner.

3. Repeat until the flame is no longer coloured, i.e. impurities are removed.

4. Dip the clean nichrome wire in concentrated hydrochloric acid and then into the solid ionic compound which is sitting in a watch glass.

5. Hold the loop with the solid ionic compound in the hottest part of the flame and record the characteristic colour.

6. Repeat for all the solid ionic compounds available, cleaning the wire loop each time.

Table of results

When some metal ions are mixed with an alkali solution a coloured precipitate can form.

A precipitate is an insoluble solid formed when two solutions are mixed.

Cu²⁺, Al³⁺, Fe²⁺, Fe³⁺, Zn²⁺ and Mg²⁺ ions can be identified using sodium hydroxide solution and ammonia solution.

• Copper(II) sulfate solution (0.2 mol/dm³)

• Iron(II) sulfate solution (0.2 mol/dm³)

• Iron(III) nitrate solution (0.2 mol/dm³)

• Aluminium nitrate solution (0.2 mol/dm³)

• Zinc nitrate solution (0.2 mol/dm³)

• Magnesium chloride solution (0.2 mol/dm³)

• Sodium hydroxide solution (0.5 mol/dm³)

• Ammonia solution (2 mol/dm³)

• Test tubes and stoppers (x6)

• Test-tube rack

• Small beaker (100 cm³)

• Disposable pipettes/droppers

Safety: Follow the general safety advice of a teacher

1. Add approximately 5 cm³ of the solution to be tested to a test-tube.

2. Add sodium hydroxide solution slowly drop wise.

3. Note your observations in the table.

4. Continue adding sodium hydroxide drop wise to any precipitates until a large excess is present (about 2/3 full).

5. Stopper and shake the test tube thoroughly.

6. Note your observations in the following table.

Repeat the tests above using ammonia solution in place of sodium hydroxide solution.

Record your results in the table below:

There are many anions, however, you are required to know the tests for just a few.

These are:

• Chloride, Cl^-

• Bromide, Br^-

• Iodide, I^-

• Sulfate, SO₄^2-

• Carbonate, CO₃^2-

• Test tubes (x 5)

• Test-tube rack

• Small beakers (3 x 100 cm³)

• Disposable pipettes/droppers

• Sodium sulfate solution (0.2 mol/dm³)

• Sodium carbonate solution (0.2 mol/dm³)

• Sodium chloride solution (0.2 mol/dm³)

• Potassium bromide solution (0.2 mol/dm³)

• Potassium iodide solution (0.2 mol/dm³)

• Barium chloride solution (0.2 mol/dm³)

• Silver nitrate solution (0.2 mol/dm³)

• Nitric Acid (0.2 mol/dm³)

• Limewater (calcium hydroxide solution)

• Hydrochloric acid solution (2 mol/dm³)

Follow the general safety, please follow that advice of a teacher.

Note: In all the following reactions the ions are present in solutions.

If a solid is supplied, simply dissolve 1 spatula of the solid in deionised water.

The carbonate can be tested as a solid.

• Add sodium sulfate solution to the test tube until the tube is \(\frac{1}{3}\) full.

• Add barium chloride solution dropwise.

• A white precipitate indicates a sulfate ion.

• Add each of the 3 solutions to separate test tubes until the tubes are each \(\frac{1}{3}\) full each.

• Add four drops of nitric acid, shake gently.

• Add silver nitrate solution dropwise.

• A white precipitate indicates the presence of the chloride ion.

• A cream precipitate indicates the presence of the bromide ion.

• A yellow precipitate indicates the presence of the iodide ion.

• Take a small amount of solid or solution.

• Add a little dilute hydrochloric acid.

• Collect any gas formed, this is easily done by opening and closing an empty disposable pipette/dropper above the reaction. The dropper contents are then bubbled through 1cm³ of limewater.

• A colourless gas is given off that turns limewater cloudy/milky, this indicates the presence of a carbonate ion.