- Acidify the sample with dilute hydrochloric acid and then add a few drops of aqueous barium chloride.
- If a sulfate is present then a white precipitate of barium sulfate is formed:
Ba2+(aq) + SO42-(aq) → BaSO4(s)
A white precipitate of barium sulfate is a positive result for the presence of sulfate ions
To use chemical tests to identify the ions in unknown single ionic compounds.
The identity of a salt can be determined by analysis of the anion and cation present.
- bunsen burner
- test tubes and test tube rack
- teat pipette
- nichrome wire mounted in handle or cork
- 0.4 mol/dm3 dilute hydrochloric acid
- 0.1 mol/dm3 barium chloride solution
- 0.4 mol/dm3 dilute nitric acid
- 0.05 mol/dm3 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.
- 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.
Record your results for each test carefully in a suitable table.
Once you have tabulated the results from the tests you performed, use them to infer the identity of the ions that the salt contains.
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 -ve charge.
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 identity the salts.
- Salt A contains lithium and bromine so it must be lithium bromide, LiBr.
- Salt B contains iron(II) and sulfate ion so it must be FeSO4.