4.6.3 Anion tests
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What is an anion?
A negative ion.
How do you test for halides?
Using the silver nitrate test. - Dissolve the sample in water - Add nitric acid (HNO3) - Add silver nitrate (AgNO3) solution. - A ppt should form.
What is the test for Cl-?
White ppt (AgCl) forms in the silver nitrate test. (nitric acid and silver nitrate).
What’s the test for Br-?
Cream ppt (AgBr) forms in the silver nitrate test. (nitric acid and silver nitrate).
What’s the test for I-?
Yellow ppt (AgI) forms in the silver nitrate test. (nitric acid and silver nitrate).
Why is it important to add HNO3 in the silver nitrate test?
Ag+ ions also form ppts with other anions. The HNO3 reacts with these anions, preventing false positives.
What is the test for carbonates?
• Add HCl and look for effervescence. • If there is fizzing, bubble the gas through the limewater. • The limewater should turn cloudy.
What is the test for sulphates?
• Test for carbonates first. • If there is no CO3^2- ions then add barium chloride (BaCl2) solution. • A white ppt (BaSO4) should form.
Why is it important to test for carbonates first when testing for sulphates?
Carbonate ions also form a white ppt with Ba2+ ions, this prevents a false positive.