4.6.3 Anion tests

Question 1

What is an anion?

Answer 1

A negative ion.

Question 2

How do you test for halides?

Answer 2

Using the silver nitrate test.
- Dissolve the sample in water
- Add nitric acid (HNO3)
- Add silver nitrate (AgNO3) solution.
- A ppt should form.

Question 3

What is the test for Cl-?

Answer 3

White ppt (AgCl) forms in the silver nitrate test. (nitric acid and silver nitrate).

Question 4

What’s the test for Br-?

Answer 4

Cream ppt (AgBr) forms in the silver nitrate test. (nitric acid and silver nitrate).

Question 5

What’s the test for I-?

Answer 5

Yellow ppt (AgI) forms in the silver nitrate test. (nitric acid and silver nitrate).

Question 6

Why is it important to add HNO3 in the silver nitrate test?

Answer 6

Ag+ ions also form ppts with otehr anions. The HNO3 reacts with these anions, preventing false positives.

Question 7

What is the test for carbonates?

Answer 7

• Add HCl and look for effervescence.
• If there is fizzing, bubble the gas through the limewater.
• The limewater should turn cloudy.

Question 8

What is the test for sulphates?

Answer 8

• Test for carbonates first.
• If there is no CO3^2- ions then add barium chloride (BaCl2) solution.
• A white ppt (BaSO4) should form.

Question 9

Why is it important to test for carbonates first when testing for sulphates?

Answer 9

Carbonate ions also form a white ppt with Ba2+ ions, this prevents a false positive.