2.7.4Inorganic Chemistry
Tests for Anions — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 7 parts Lesson 40 of 52
Part 1 of 5Anions and the tests you need
What an anion is
- What an anion is An anion is a negatively charged ion, often formed from a non-metal or a group of non-metal atoms.
Three families to test
- Three families to test You must be able to test for carbonate ions, CO32–, halide ions (Cl–, Br–, I–) and sulfate ions, SO42–.
- Each test uses a different reagent and gives a different, easily recognised result.
Why acid comes first
- Why acid comes first The halide and sulfate tests begin by adding acid.
- This removes any carbonate that might be present, because carbonate would also form a precipitate and confuse the result.
Part 2 of 5Test for carbonate ions
1
Add a few cm3 of dilute acid, such as hydrochloric acid, to the sample.
2
If a carbonate is present, you will see fizzing (effervescence) as carbon dioxide gas is made.
3
Pass the gas through limewater, calcium hydroxide solution, using a delivery tube.
4
A positive result is limewater turning cloudy or milky, because insoluble white calcium carbonate forms.
5
Equations: CO32–(aq) + 2H+(aq) → CO2(g) + H2O(l), then CO2(g) + Ca(OH)2(aq) → CaCO3(s) + H2O(l).
Part 3 of 5Test for halide ions
1Acidify
- Acidify Add dilute nitric acid to the sample.
- This clears away carbonate ions that would otherwise react with silver ions.
2Add silver nitrate
- Add silver nitrate Add a few drops of silver nitrate solution, AgNO3(aq).
3Silver halide forms
- Silver halide forms Silver ions combine with halide ions to make an insoluble silver halide, shown as (s) in the equation.
4Read the colour
- Read the colour The colour of the solid tells you which halide is present (see the table).
Part 4 of 5Halide and sulfate results
| Ion tested | Reagents added | Positive result | Example equation |
|---|---|---|---|
| Chloride, Cl– | Nitric acid, then silver nitrate | White precipitate of silver chloride | KCl(aq) + AgNO3(aq) → KNO3(aq) + AgCl(s) |
| Bromide, Br– | Nitric acid, then silver nitrate | Cream precipitate of silver bromide | KBr(aq) + AgNO3(aq) → KNO3(aq) + AgBr(s) |
| Iodide, I– | Nitric acid, then silver nitrate | Yellow precipitate of silver iodide | KI(aq) + AgNO3(aq) → KNO3(aq) + AgI(s) |
| Sulfate, SO42– | Dilute hydrochloric acid, then barium chloride | White precipitate of barium sulfate | Ba2+(aq) + SO42–(aq) → BaSO4(s) |
Part 5 of 5Choosing the right acid
Acid used before adding the reagent
Halide test: nitric acid
- Halide test: nitric acid Hydrochloric acid cannot be used,
- because it contains chloride ions that would form silver chloride and give a false positive.
Sulfate test: hydrochloric acid
- Sulfate test: hydrochloric acid Chloride ions do not form an insoluble precipitate with barium ions,
- so hydrochloric acid is safe here and removes carbonate first.
Alternative reagent
- Alternative reagent The sulfate test works equally well with barium nitrate solution in place of barium chloride.
Key terms
| Term | Meaning |
|---|---|
| Anion | A negatively charged ion. |
| Limewater | A solution of calcium hydroxide that turns cloudy when carbon dioxide is bubbled through it. |
| Effervescence | Fizzing caused by a gas being released from a liquid. |
| Halide ion | A negative ion formed by a Group 7 element: chloride, bromide or iodide. |
| Silver nitrate | The reagent used with nitric acid to test for halide ions. |
Avoid these mistakes
1
Describing the carbonate test as 'limewater turns cloudy'. That is only the result. The test is to add acid and bubble the gas through limewater.
2
Using hydrochloric acid to acidify before the silver nitrate test. Its chloride ions interfere, so use nitric acid.
3
Forgetting why the acid is added. It removes carbonate ions that would also give a precipitate.
4
Mixing up the precipitate colourschloride is white, bromide is cream and iodide is yellow.
5
Leaving out state symbols. The silver halides and barium sulfate are all (s).
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