Required practical
Using chemical tests
It is important in this core practical to use the appropriate 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 anions.
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.
| Ion present | Flame test colour |
| Lithium, Li+ | Crimson |
| Sodium, Na+ | Yellow |
| Potassium, K+ | Lilac |
| Calcium, Ca2+ | Orange-red |
| Copper, Cu2+ | Green |
| Ion present | Lithium, Li+ |
|---|---|
| Flame test colour | Crimson |
| 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 |
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 |
| Magnesium, Mg2+ | White - no change in excess sodium hydroxide solution |
| Copper, Cu2+ | Blue |
| Iron(II), Fe2+ | Green |
| Iron(III), Fe3+ | Brown |
| Metal ion | Aluminium, Al3+ |
|---|---|
| Colour | White - dissolves in excess sodium hydroxide solution |
| Metal ion | Calcium, Ca2+ |
|---|---|
| Colour | White - no change in excess sodium hydroxide solution |
| Metal ion | Magnesium, Mg2+ |
|---|---|
| Colour | White - no change in excess sodium hydroxide solution |
| Metal ion | Copper, Cu2+ |
|---|---|
| Colour | Blue |
| Metal ion | Iron(II), Fe2+ |
|---|---|
| Colour | Green |
| Metal ion | Iron(III), Fe3+ |
|---|---|
| Colour | Brown |
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/cloudy.
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 |
| Halide ion | Chloride, Cl- |
|---|---|
| Precipitate colour | White |
| Halide ion | Bromide, Br- |
|---|---|
| Precipitate colour | Cream |
| Halide ion | Iodide, I- |
|---|---|
| Precipitate colour | 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 needed | Bried bubbling, limewater turns milky | |
| B | Dilute sodium hydroxide added | Blue precipitate forms | |
| B | Dilute hydrochloric acid and barium chloride solution added | White precipitate forms | |
| C | Dilute nitric acid and silver nitrate solution added | White precipitate forms | |
| C | Flame test | Green flame colour |
| Salt | A |
|---|---|
| Test | Flame test |
| Observation | Yellow flame colour |
| Inference |
| Salt | A |
|---|---|
| Test | Dilute nitric acid needed |
| Observation | Bried bubbling, limewater turns milky |
| 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 and silver nitrate solution added |
| Observation | White precipitate forms |
| Inference |
| Salt | C |
|---|---|
| Test | Flame test |
| Observation | Green flame colour |
| Inference |
Analysis
Use the results to identify the ions present in each salt, and then to name the salts.
Question
Identify solution A using the results in the table.
Solution A is sodium carbonate, Na2CO3. The yellow flame colour shows that Na+ ions are present, and the bubbles of gas which turn limewater cloudy are carbon dioxide from the carbonate ion.
Question
Identify solution 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 copper chloride. The green flame shows that Cu2+ ions are present, and the white precipitate of silver chloride shows that Cl- ions are present.
Evaluation
Worked 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
