Identifying Unknown Ionic Compounds (WJEC GCSE Chemistry)
Revision Note
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
Examiner Tips and Tricks
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!
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?