12.3.1Experimental Chemistry
Identification of Anions — Combined Science 0653 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 9 of 10 Unit quiz
Part 1 of 5Anions to know
What is an anion?
- What is an anion? An anion is a negatively charged ion, here formed from non-metals or groups of non-metals.
The four tests
- The four tests Carbonate, CO32−
- Halideschloride Cl−, bromide Br− and iodide I−
- Nitrate, NO3−
- Sulfate, SO42−
Part 2 of 5Testing for carbonate ions
1
Add dilute acid to the sample.
2
Look for fizzing, which shows a gas is being released.
3
Bubble the gas through limewater.
4
If the limewater turns cloudy (milky), the gas is carbon dioxide and a carbonate was present.
Part 3 of 5Equations for the carbonate test
| Stage | Ionic or full equation |
|---|---|
| Acid reacts with carbonate | CO32−(aq) + 2H+(aq) → CO2(g) + H2O(l) |
| Gas turns limewater cloudy | CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l) |
Part 4 of 5Testing for halide, sulfate and nitrate ions
| Ion | Test | Positive result |
|---|---|---|
| Chloride | Acidify with dilute nitric acid, then add silver nitrate solution | White precipitate of silver chloride |
| Bromide | Same test | Cream precipitate of silver bromide |
| Iodide | Same test | Yellow precipitate of silver iodide |
| Sulfate | Acidify with dilute nitric acid, then add barium nitrate solution | White precipitate of barium sulfate |
| Nitrate | Add aqueous sodium hydroxide and aluminium foil, then warm gently | Ammonia gas is released, with a sharp smell, and it turns damp red litmus paper blue |
Part 5 of 5Worked example: halide test
KCl + AgNO3
Word equation
- Word equation potassium chloride + silver nitrate → potassium nitrate + silver chloride
Symbol equation
- Symbol equation KCl(aq) + AgNO3(aq) → KNO3(aq) + AgCl(s)
Why nitric acid?
- Why nitric acid? Hydrochloric acid contains chloride ions, which would give a false white precipitate, so nitric acid is used to acidify.
Key terms
| Term | Meaning |
|---|---|
| Anion | A negatively charged ion. |
| Limewater | A solution of calcium hydroxide that turns cloudy with carbon dioxide. |
| Halide ion | A negative ion formed by a Group 7 element, such as Cl−. |
| Precipitate | An insoluble solid formed when two solutions react. |
| Effervescence | Fizzing caused by a gas escaping from a liquid. |
Avoid these mistakes
1
Stating only that limewater turns cloudy. The test itself must include adding acid and bubbling the gas through limewater.
2
Acidifying the halide test with hydrochloric acid, which adds chloride ions. Use nitric acid.
3
Mixing up the colourschloride is white, bromide is cream and iodide is yellow.
4
Using silver nitrate to test for sulfate. Barium nitrate is used for sulfate.
5
Forgetting that the nitrate test needs both sodium hydroxide and aluminium foil, plus warming.
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