Practical activity - identifying the ions in unknown salts
It is important in this core practical to use apparatus and substances carefully and safely, and to observe chemical changes.
This guide 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. Eye protection must be worn.
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
Question
Carry out a flame test on each solid or its solution.
| Ion present | Flame test colour |
| Lithium, Li+ | Red |
| Sodium, Na+ | Yellow |
| Potassium, K+ | Lilac |
| Calcium, Ca2+ | Orange-red |
| Copper, Cu2+ | Green-blue |
| Ion present | Lithium, Li+ |
|---|---|
| Flame test colour | Red |
| Ion present | Sodium, Na+ |
|---|---|
| Flame test colour | Yellow |
| Ion present | Potassium, K+ |
|---|---|
| Flame test colour | Lilac |
| Ion present | Calcium, Ca2+ |
|---|---|
| Flame test colour | Orange-red |
| Ion present | Copper, Cu2+ |
|---|---|
| Flame test colour | Green-blue |
Hydroxide precipitates tests
Question
Add a few drops of dilute sodium hydroxide solution. Observe and record the colour of any precipitate formed.
| Metal ion | Precipitate colour |
| Iron(II), Fe2+ | Green |
| Iron(III), Fe3+ | Orange-brown |
| Copper(II), Cu2+ | Blue |
| Calcium, Ca2+ | White - no change in excess sodium hydroxide solution |
| Zinc, Zn2+ | White - dissolves in excess sodium hydroxide solution |
| Metal ion | Iron(II), Fe2+ |
|---|---|
| Precipitate colour | Green |
| Metal ion | Iron(III), Fe3+ |
|---|---|
| Precipitate colour | Orange-brown |
| Metal ion | Copper(II), Cu2+ |
|---|---|
| Precipitate colour | Blue |
| Metal ion | Calcium, Ca2+ |
|---|---|
| Precipitate colour | White - no change in excess sodium hydroxide solution |
| Metal ion | Zinc, Zn2+ |
|---|---|
| Precipitate colour | White - dissolves in excess sodium hydroxide solution |
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
Question
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 |
| Halide ion | Chloride, Cl- |
|---|---|
| Precipitate colour | White |
| Halide ion | Bromide, Br- |
|---|---|
| Precipitate colour | Cream |
| Halide ion | Iodide, I- |
|---|---|
| Precipitate colour | Yellow |
Results
Question
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 | White precipitate forms, which does not dissolve in excess alkali |
| Salt | A |
|---|---|
| Test | Flame test |
| Observation | Yellow flame colour |
| Inference |
| Salt | A |
|---|---|
| Test | Dilute nitric acid and silver nitrate solution added |
| Observation | White precipitate forms |
| Inference |
| Salt | B |
|---|---|
| Test | Dilute sodium hydroxide added |
| Observation | Blue precipitate forms |
| Inference |
| Salt | B |
|---|---|
| Test | Dilute hydrochloric acid and barium chloride solution added |
| Observation | White precipitate forms |
| Inference |
| Salt | C |
|---|---|
| Test | Dilute nitric acid added |
| Observation | Brief bubbling, limewater turns milky |
| Inference |
| Salt | C |
|---|---|
| Test | Dilute sodium hydroxide added |
| Observation | White precipitate forms, which does not dissolve in excess alkali |
| Inference |
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.
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.
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.
Salt C is calcium carbonate, ZnCO3. The bubbling and limewater test shows that CO32- ions are present. The test with sodium hydroxide solution shows that Ca2+ ions are present (not Zn2+ ions).
Evaluation
Example
Suggest an explanation for 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
Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:
| Hazard | Possible harm | Possible 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 |
| Hazard | Barium chloride solid |
|---|---|
| Possible harm | Harmful if inhaled and toxic if swallowed |
| Possible precaution | Only use dilute solutions supplied by your teacher |
| Hazard | Silver nitrate solution |
|---|---|
| Possible harm | Causes serious eye irritation, causes skin irritation |
| Possible precaution | Wear eye protection, avoid skin contact by using dropper bottles or by wearing gloves |
Fran Scott demonstrates how to test for various ions and interpret the results