AQA Chemistry (8462) 4.8.3.3–4.8.3.5 · Required practical
Tests for carbonates, halides and sulfates
A white salt with no label. Which tests, in which order and with which acid, prove what it is, and which tests would lie to you?
Triple Science · Foundation tier
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Five salts. Five tubes. Every one said chloride.
A student tested five different white salts for halide ions. She acidified each solution with dilute hydrochloric acid, then added silver nitrate solution. Every tube gave a white precipitate, so she wrote “chloride” five times.
If you had to guess, did the chloride in every tube come from the salts, or from something she added?
Each test looks for one kind of anion (negative ion). Carbonates fizz with dilute acid, and the carbon dioxide turns limewater milky. Halide ions (chloride, bromide, iodide) give a precipitate with silver nitrate solution in the presence of dilute nitric acid; its colour names the halide. Sulfate ions give a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid.
The acid goes in first: carbonate ions would also give a precipitate with either reagent, and the acid removes them as carbon dioxide.
Think again · fizzing
“It fizzed when I added the acid, so it is a carbonate.”
Spot the flaw
ExplainThis tube is fizzing too. What is wrong with the conclusion?
Required practical 7 · chemistry only · the anion tests
Fresh sample. Right acid. Then the reagent.
- Dissolve a small amount of each solid in distilled water.
- Carbonate: add dilute hydrochloric acid in a tube fitted with a delivery tube, and bubble any gas through limewater.
- Halide: add a few drops of dilute nitric acid, then a few drops of silver nitrate solution. Record the colour of any precipitate.
- Sulfate: add a few drops of dilute hydrochloric acid, then a few drops of barium chloride solution. Look for a white precipitate.
- Use a fresh sample for every test and a clean dropper for every reagent.
Barium chloride is toxic if swallowed. Wash your hands afterwards.
Silver nitrate stains skin and is an irritant. Avoid skin contact.
Dilute acids are irritants. Wear eye protection.
Gas in a stoppered tube can blow the bung out. Use a delivery tube.
The test rack · five unknown salts
Salt 1 · call the metal ion first
0 of 5 named
Metal-ion result, already recorded
Flame test: yellow.
Call it first. Which metal ion is in this salt?
Sort it
Fair test or false positive?
Six ways to run a test. Decide what each one can really show.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Equation · the carbonate test
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂
The fizz is the carbon dioxide. Any carbonate does the same: a salt, water and carbon dioxide.
Command words in this lesson
- Describe a test
- Name every reagent in order, acid included, then the result you would see.
- Identify
- Name the ion from the results given.
- Explain
- Here, usually why a step is there: what would go wrong without it.
- Plan
- Tests in a logical order that would tell every sample apart, with each result.
Key fact
Each test has its own acid, the acid always goes in first, and it must never add the ion you are testing for.
Key note · AQA 4.8.3.3–4.8.3.5 · RP7
Tests for carbonates, halides and sulfates
- Carbonate: acid → CO₂ → limewater milky.
- Halides: acidify (HNO₃) + AgNO₃ → Cl=white, Br=cream, I=yellow precipitate.
- Sulfate: acidify (HCl) + BaCl₂ → white BaSO₄ precipitate.
- Always acidify first to remove carbonate ions.
- Never acidify with the acid whose ion you are testing for: no HCl in the halide test, no H₂SO₄ in the sulfate test.
- Naming a salt takes two results: a metal-ion test and an anion test.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
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Cream or yellow? Which acid for which test?
The rack draws each result in one idealised shade and does not model amounts. Real cream and yellow precipitates can look alike in poor light, so compare them side by side; the metal-ion results are given, not simulated.