Finding the right ionic compound

Each solid is made up of one cation and one anion from the groups below. None of the solids contains an anion or cation that is used in any of the others.

Introduction

Teachers who have not used the problems before should read the section Using the problems before starting.

Prior knowledge

Tests for the identification of ions including flame tests, precipitation reactions and electrolysis. A detailed knowledge is unnecessary as students are encouraged to consult textbooks and data books during the exercise.

Resources

Data books and inorganic textbooks should be available for reference.

Any four of the possible 13 compounds could be issued, but the following four are recommended the first time the problem is attempted – barium nitrate, anhydrous copper(II) sulphate, sodium carbonate and potassium iodide. The compunds should be supplied at the start of the exercise in unlabelled, numbered bottles.

Students can request apparatus and chemicals during the practical session and these should be issued if they are safe to use. In particular, flame test equipment and electrolysis apparatus may be requested but should not be on view.

Special safety requirements

The hazards associated with some of the chemicals eg barium nitrate should be noted.

Possible solutions

The following procedure is one systematic approach for identifying the compounds.

Stir each substance separately with water in a test tube.

1. Insoluble solids

Any insoluble substance must be barium carbonate, barium sulphate or copper(II) carbonate, but the latter is excluded as students were issued with white solids. Test any insoluble compound for the presence of carbonate by adding a strong acid such as dilute hydrochloric acid:

If the solid effervesces and dissolves, it is barium carbonate; if it does not, it is barium sulphate.

2. Soluble solids

a. If the solid dissolves to form a blue solution it contains copper(II) ions. The anion must be nitrate or sulphate as copper(II) carbonate is insoluble and copper(II) iodide does not exist. Test the solution for sulphate (barium chloride test) or for nitrate (brown ring test).

b. Test other soluble compounds as follows:

(i) for cations

• barium ions (sulphate test or flame test); and
• potassium and sodium ions (flame test).

(ii) for anions

• carbonate ions (strong acid);
• iodide ions (silver nitrate test, in the presence of nitric acid to avoid the precipitation of carbonates) or oxidation to iodine with bromine or chlorine water) ; and
• nitrate ions (brown ring test).

Other things that students could do include:

• adding a metal such as zinc or magnesium;
• electrolysis; and
• heating.

Suggested approach

During trialling the following instructions were given to students and proved to be extremely effective:

You can divide the work amongst yourselves but keep one another informed of your progress.

1. Working as a group, list all the compounds that the solid could be.
2. Again working as a group, devise as many different ways of identifying the solid as you can. Some methods will seem better than others – write down and discuss the advantages and disadvantages of each of the methods that you have devised.

Discussion can play a vital part in working out solutions to problems like this and sufficient time should be spent discussing the different methods and their advantages and disadvantages – perhaps 10 minutes initially with further discussion as required.

3. Devise a systematic procedure to identify the solid. Get this checked for safety. Note – You should ask for the solid at this point.
4. Use your procedure to identify the solid.
5. Working as a group, prepare a short (ca 5-minute maximum) presentation to give to the rest of the class. If possible all group members should take part: any method of presentation (such as a blackboard, overhead projector, etc) can be used.

Outline the problem, explain how you selected possible compounds, explain the systematic procedure you devised to identify the solid, and describe how well it worked in practice. After the presentation, be prepared to accept and answer questions and to discuss what you did with the rest of the class.