9.1 Qualitative Analysis: Tests for Ions
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What is the colour of a flame when a compound is lit based on
The metal ion in the compound
Lithium ion flame test
Red
Sodium ion flame test
Yellow
Potassium ion flame test
Lilac
Calcium ion flame test
Orange-red
Copper ion flame test
Blue-green
Experiment to carry out a flame test
1- Set up a Bunsen burner to a blue flame 2- Clean a nichrome wire loop by dipping it into hydrochloric acid and then rinsing it it distilled water 3- Dip the wire loop into a sample of the metal compound and put the loop into the clear blue part of the flame 4- Record the colour the flame goes
Why does the sodium hydroxide test work
Many metal hydroxides are insoluble in water and so will precipitate out of the solution
Experiment to identify a metal ion using sodium hydroxide
1- Add a few drops of sodium hydroxide solution to a solution of your compound 2- Record the colour of the precipitate
Aluminium ion sodium hydroxide precipitate
White at first before turning the solution colourless
Explain why the aluminium ion’s precipitate is different during the sodium hydroxide precipitate experiment
It redissolves in excess sodium hydroxide
Calcium ion sodium hydroxide precipitate
White
Copper ion sodium hydroxide precipitate
Blue
Iron(II) ion sodium hydroxide precipitate
Green
Iron(III) ion sodium hydroxide precipitate
Brown
Name the tests for metal ions
-Flame test -Sodium hydroxide precipitate
Test for ammonium ions
Add sodium hydroxide solution to a solution of your substance and gently heat it and see if it produces ammonia gas
Test for ammonia gas
Hold a piece of damp red litmus paper over it and see if it turns blue
How else may ammonia be detected
It has a strong, distinctive smell
Why shouldn’t you smell a gas to see if it is ammonia
Ammonia is an irritant and is toxic
Test for halide ions
Add some nitric acid, then add a few drops of silver nitrate solution and see if a silver halide precipitate is formed
Explain why nitric acid is first added when testing for halide ions
It gets rid of carbonate ions which produce a pale precipitate and would confuse the results
Why can’t you use hydrochloric acid instead of nitric acid during the halide ion test
You would be adding chloride ions
Silver chloride precipitate
White