Specified Practical: Identifying Unknown Ionic Compounds
Introduction
- A series of chemical tests can be used to identify the ions in unknown ionic compounds
- The different tests will identify the metal and non-metal ions present
Apparatus
- Bunsen burner
- Test tubes and test tube rack
- Bung and delivery tube
- Dropping pipette
- Wooden splints
- Limewater
- Dilute hydrochloric acid
- Silver nitrate solution
- Samples of solid ionic compounds
- De-ionised water
Method
Flame Test
- Dip a damp wooden splint into the sample being tested
- Place the sample into the hottest part of a bunsen flame
- Record the flame colour observed
- Record for the remaining samples
Tests for carbonate ions
- Add the sample into one test tube
- Add 2 cm3 of limewater into a second test tube
- Add 1 cm3 of dilute hydrochloric acid into the sample and attach the bung and delivery tube
- Observe what happens to the limewater
How to test for carbonate ions
Tests for Group 7 ions
- Add the sample to a test tube
- Add de- ionised water to create a solution
- Add a small amount of silver nitrate solution using a pipette
- Record the colour of the precipitate formed
Practical Tip
- Be careful not to overfill the test tube with HCl when you are testing for carbonate ions
- It can bubble over and carry acid into the limewater which would dissolve any cloudy precipitate that might be observed.
Analysis of Results
Record your results for each test carefully in a suitable table like the one below:
| Sample | Observations | Compound | ||
| Flame test | Test for carbonate ions | Test for Group 7 ions | ||
| A | ||||
| B | ||||
| C | ||||
| D | ||||
| E | ||||
Evaluation
- Once you have recorded the results from the tests you performed, use them to identify the ions that the compound contains and name the compound.
- If you are determining the formulae of the unknown compound make sure that you balance the charges on the ions in the formula
- E.g. If 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
A student was given five samples labelled A-E.
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 ionic compound.
| Sample | Observations | Compound | ||
| Flame test | Test for carbonate ions | Test for Group 7 ions | ||
| A | lilac flame | no change | cream precipitate | potassium bromide |
| B | yellow-orange flame | no change | yellow precipitate | sodium iodide |
| C | red flame | no change | white precipitate | lithium chloride |
| D | brick-red flame | limewater turns cloudy | no change | calcium carbonate |
| E | apple green | no change | white precipitate | barium chloride |
To identify the positive ions using a flame test:
- Lithium- red
- Sodium- yellow-orange
- Potassium- lilac
- Calcium- brick red
- Barium- apple green
To identify the Group 7 ions with silver nitrate solution:
- Chloride ions-white precipitate
- Bromide ions- cream precipitate
- Iodide ions- yellow precipitate
To determine whether a carbonate ion is present:
- When hydrochloric acid is added, carbon dioxide is produced which turns limewater cloudy
- Fizzing would also be observed
Exam Tip
Exam questions will ask you to either:
- Name / give the formulae of the compound in the sample
- Name / give the formula of the individual ions
- Name / give the formula for one of the ions
Read the question carefully- you might only be awarded one mark for identifying both ions within the compound- so if you forget one of them you will lose the mark!
