Core Practical: Identifying Ions

Aim:

To use chemical tests to identify the ions in unknown binary ionic compounds

Materials:

Bunsen burner
Test tubes and test tube rack
Teat pipette
Nichrome wire mounted in handle or cork
Limewater
0.4 mol dm^-3 dilute hydrochloric acid
0.1 mol dm^-3 barium chloride solution
0.4 mol dm^-3 dilute nitric acid
0.05 mol dm^-3 silver nitrate solution
Various samples of salts and salt solutions

Practical Tip:

Key to this practical is your level of organisation. You will have many containers, solutions and samples so your work space and results table must be neat and tidy

Method:

There are a number of strategies you could choose in order to identify the ions in unknown salts
Common analysis strategies include flame tests, and tests for sulphate, carbonate and halide ions
They can be carried out in any particular order, and you will probably not need to carry them all out on any one sample
Only small amounts of each sample and reagent are needed
You may need to dissolve a sample of salt in a little distilled water if the salt you are given is in the solid state
Record your observations carefully in a table of results as you work through the tests
Repeat any tests that do not provide a clear result i.e. a colour change that was difficult to identify

Results:

Record your results for each test carefully in a suitable table

Evaluation:

Once you have tabulated the results from the tests you performed, use them to infer the identity of the ions that the salt containsConclusion:

You can then identify the salt from the cation and anion present. When inferring the formulae and names of unknown salts, make sure that you balance the charges on the ions in the formula. E.g. the test for B was positive for the Fe²⁺ cation, therefore the anion must be an ion with a -2 charge or two ions each with a single -1 charge

Worked example

Example

A student was given two salts labelled A and B. The following set of results were obtained from a series of qualitative tests performed by the student on the samples. Use the results to identify the salts.

Required Practical 7 Results Table, downloadable IGCSE & GCSE Chemistry revision notes

Conclusion:

Salt A contains lithium and bromide ions, so it must be lithium bromide, LiBr
Salt B contains iron(II) and sulfate ions, so it must be FeSO₄

Hazards, risks and precautions

harful-toxic

Hazard symbols to show substances that are harmful to health and toxic

Limewater, dilute nitric acid and sodium hydroxide are harmful to health as they are irritating to the eyes and skin
Dilute hydrochloric acid is not classified as hazardous at the concentrations typically used in this practical, however it may still cause harm to the eyes or the skin
Barium chloride and silver nitrate solutions are not classified as hazardous at the concentrations typically used in this practical (at higher concentrations, barium chloride can be classified as toxic)
For limewater, dilute nitric acid and dilute hydrochloric acid, avoid contact with the skin and use safety goggles
NOTE: During the test for ammonium ions, ammonia gas is evolved which is toxic if inhaled
As the quantities produced are small, then a fume cupboard is not required, but for reactions where larger amounts of ammonia gas are produced, a fume cupboard should be used

Core Practical: Identifying Ions (Edexcel GCSE Chemistry)

Revision Note