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
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.
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
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
