Flex learner’s investigative muscles in this practical where they can design their own tests

 Learners can devise an experimentation plan to identify the chloride within a set of substances

Introduction

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

Prior knowledge

Properties of carbonates, flame tests, electrolysis, test for chloride ions and properties of concentrated sulphuric acid. 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.

Numbered, solid samples of barium carbonate, calcium chloride, copper(II) carbonate, potassium carbonate and anhydrous sodium carbonate should be provided at the start of the exercise. Students can request apparatus and chemicals during the practical session, and these should be issued if they are safe to use. In particular, electrolysis and flame test equipment will probably be required but should not be on view.

Special safety requirements

The use of concentrated sulphuric acid and electrolysis to produce chlorine gas requires teacher supervision.

Possible methods

  1. Add dilute acid – the one that does not effervesce is the chloride.
  2. Flame tests.
  3. Make solutions of the three soluble salts.

a) Test them with pH paper: two are strongly alkaline: the third salt – calcium chloride – is neutral.

b) Mix the salts together in pairs. The one that gives a white precipitate twice (calcium carbonate) is the calcium chloride.

c) Silver nitrate test.

If the students know the test in the form ‘make the solution acidic by adding dilute nitric acid and then add silver nitrate solution’, then it can be applied directly.

However, if they have not met this test for halide ions before they are likely to need help because of the precipitation of silver carbonate. They should add silver nitrate solution to the three solutions; all give a precipitate; reference to solubility tables show that silver carbonate and silver chloride are both insoluble. Addition of dilute nitric acid dissolves silver carbonate but not silver chloride and thus identifies the calcium chloride.

  1. Add to water and electrolyse – chloride gives chlorine at the anode.
  2. Add a drop of concentrated sulphuric acid to the solid – the chloride gives acidic steamy fumes of hydrogen chloride.

 

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Additional information

This resource is part of our Creative problem-solving in chemistry collection.