Identifying the ions in unknown salts
There are a number of ways that you could identify the ions in unknown salts in Chemistry. This is an outline of the required steps to undertake one of these methods.
This includes a summary of all the tests needed to carry out the practical. The tests can be carried out in any order, and you may not need to carry them all out on a particular substance.
Aims
To identify the ions in unknown salts, using the tests for the specified positive and negative ions.
Method
- Carry out one or more tests on each salt. You may need to dissolve a sample of salt in a little distilled water if you are given solids, rather than solutions.
- Record your observations carefully. Repeat any tests that do not get clear results.
Flame tests
Carry out a flame test as described earlier:
- dip a clean wire loop into a solid sample of the compound being tested
- put the loop into the edge of the blue flame from a Bunsen burner
- observe and record the flame colour produced
| Ion present | Flame test colour |
| Lithium, Li+ | Red |
| Sodium, Na+ | Yellow |
| Potassium, K+ | Lilac |
| Calcium, Ca2+ | Orange-red |
| Copper, Cu2+ | Blue-green |
Hydroxide precipitates tests
Add a few drops of dilute sodium hydroxide solution. Observe and record the colour of any precipitate formed.
| Metal ion | Colour |
| Aluminium, Al3+ | White – dissolves in excess sodium hydroxide solution |
| Calcium, Ca2+ | White – no change in excess sodium hydroxide solution |
| Copper, Cu2+ | Blue |
| Iron(II), Fe2+ | Green |
| Iron(III), Fe3+ | Brown |
Test for ammonium ions
Add a few drops of dilute sodium hydroxide solution, then warm gently. Ammonia gas is produced if ammonium ions are present. Confirm that the gas is ammonia – damp red litmus paper turns blue.
Test for carbonate ions
Add a few drops of dilute hydrochloric acid. Bubbles are produced if carbonate ions are present. Confirm that the gas is carbon dioxide – limewater turns milky.
Test for sulfate ions
Add a few drops of dilute hydrochloric acid, then a few drops of barium chloride solution. A white precipitate forms if sulfate ions are present.
Test for halide ions
Add a few drops of dilute nitric acid, then a few drops of silver nitrate solution. Observe and record the colour of any precipitate formed.
| Halide ion | Precipitate colour |
| Chloride, Cl– | White |
| Bromide, Br– | Cream |
| Iodide, I– | Yellow |
Results
Record the results in a suitable table. The table here gives some example results. An inference is what the results of a test mean.
| Salt | Test | Observation | Inference |
| A | Flame test | Yellow flame colour | |
| A | Dilute nitric acid and silver nitrate solution added | White precipitate forms | |
| B | Dilute sodium hydroxide added | Blue precipitate forms | |
| B | Dilute hydrochloric acid and barium chloride solution added | White precipitate forms | |
| C | Dilute nitric acid added | Brief bubbling,limewater turns milky | |
| C | Dilute sodium hydroxide added, then warmed | Smelly gas produced that turns damp red litmus paper blue |
Analysis
Use the results to identify the ions present in each salt, and then to name the salts.
Question
Identify salt A using the results in the table.
answer
Salt A is sodium chloride, NaCl. The yellow flame colour shows that Na+ ions are present, and the white silver halide precipitate shows that Cl– ions are present.
Question
Identify salt B using the results in the table.
answer
Salt B is copper sulfate, CuSO4. The blue hydroxide precipitate colour shows that Cu2+ ions are present, and the white barium sulfate precipitate shows that SO42- ions are present.
Question
Identify salt C using the results in the table.
answer
Salt C is ammonium carbonate, (NH4)2CO3. The bubbling and limewater test shows that CO32- ions are present. The test with sodium hydroxide solution and litmus paper shows that NH4+ ions are present.
Evaluation
Example
Suggest an explanation for a reason why it may be difficult to distinguish between very dilute solutions of chloride ions, bromide and iodide ions using silver nitrate solution.
Very dilute solutions give very faint precipitates. This makes it difficult to tell whether a precipitate is really white, or just cream or yellow that is too pale to be sure.
Hazards, risks and precautions
It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances.
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
| Hazard | Harm | Precaution |
| Barium chloride solid | Harmful if inhaled and toxic if swallowed. | Only use dilute solutions supplied by your teacher. |
| Silver nitrate solution | Causes serious eye irritation.Causes skin irritation. | Wear eye protection.Avoid skin contact by using dropper bottles or by wearing gloves. |
