Testing for negatively charged ions

Question 1

How are different negatively charged ions in solution identified?

Answer 1

Through the formation of different precipitates

Question 2

Describe testing for carbonate ions

Answer 2

Carbonate ions, CO32- are detected using a dilute acid. Bubbles are given off when an acid, usually dilute hydrochloric acid, is added to the test compound.

The bubbles are caused by carbon dioxide. Limewater is used to confirm that the gas is carbon dioxide. It turns milky when carbon dioxide is bubbled through it.

The test for carbonate ions works whatever acid is added.

CARBONATE + DILUTED ACID = CO2

Question 3

Testing for sulfate ions

Answer 3

Barium ions react with sulfate ions, SO42- to form insoluble white barium sulfate:

Ba2+(aq) + SO42-(aq) → BaSO4(s)

To test for sulfate ions in solution:

• add a few drops of dilute hydrochloric to the sample
• add a few drops of dilute barium chloride solution
• A white precipitate forms if sulfate ions are present.

Question 4

Explain why, in a test for sulfate ions, the sample is acidified with hydrochloric acid first.

Answer 4

Carbonate ions also produce a white precipitate with barium chloride solution. The acid reacts with any carbonate ions present. This removes them, so stopping them giving a false positive result.

Question 5

Testing for halide ions

Answer 5

Silver ions react with halide ions (Cl-, Br- or I- ions) to form insoluble precipitates. The table shows the colours of these silver halide precipitates. To test for halide ions:

• add a few drops of dilute nitric acid to the sample
  add a few drops of dilute silver nitrate solution
  Observe and record the colour of any precipitate that forms.

Question 6

What colour is the precipitate if Chloride,
Bromide and
Iodide are present?

Answer 6

Halide ion Precipitate
colour
Chloride, Cl- White
Bromide, Br- Cream
Iodide, I- Yellow

It may help to remember the silver halide precipitate colours by the mnemonic ‘milk, cream, butter’ for ‘white, cream, yellow’.

Question 7

Explain why, in a test for halide ions, dilute hydrochloric acid cannot be used to acidify the sample.

Answer 7

Hydrochloric acid, HCl(aq), contains chloride ions. These would react with silver ions in the test, giving a false positive result.