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

  1. 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.
  2. 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

Hydroxide precipitates tests

Question

Add a few drops of dilute sodium hydroxide solution. Observe and record the colour of any precipitate formed.

Metal ionPrecipitate 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

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 ionPrecipitate colour
Chloride, Cl-White
Bromide, Br-Cream
Iodide, I-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.

SaltTestObservationInference
AFlame testYellow flame colour
ADilute nitric acid and silver nitrate solution addedWhite precipitate forms
BDilute sodium hydroxide addedBlue precipitate forms
BDilute hydrochloric acid and barium chloride solution addedWhite precipitate forms
CDilute nitric acid addedBrief bubbling, limewater turns milky
CDilute sodium hydroxide addedWhite precipitate forms, which does not dissolve in excess alkali

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:

HazardPossible harmPossible precaution
Barium chloride solidHarmful if inhaled and toxic if swallowedOnly use dilute solutions supplied by your teacher
Silver nitrate solutionCauses serious eye irritation, causes skin irritationWear 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