Anion tests and reagent order
Key idea: Identify anions from the reagent and its positive result. Acidify chloride and sulfate tests with nitric acid first, so the acid does not add the ion being tested for.
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Guided lessons · coming soon
Guided lesson
Guided steps
- Read each anion test, and why the acid comes first.
- Follow how fizzing and limewater show carbonate.
- Choose the acid, the order, the reagent and the result.
- Decide what the bubbles and the pop each show.
- Correct the learner's conclusion, then compare your answer.
- Choose the acid, the order and the reagent for the chloride test.
- Choose the nitrate test, its gas and the litmus result.
- Read the common mistakes before the topic check.
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ContinueEach anion test adds a reagent that reacts with one ion in a way you can see. Two rules make the tests valid. Bubbles show that a gas has formed, not which gas, so the gas must be tested on its own. And a reagent must never add the ion you are testing for, which decides the choice of acid.
The four anion tests
| Anion | Test | Positive result |
|---|---|---|
| carbonate, CO₃²⁻ | add dilute acid; bubble the gas into limewater | effervescence; the gas turns limewater milky (carbon dioxide) |
| chloride, Cl⁻ | acidify with dilute nitric acid, then add aqueous silver nitrate | white precipitate |
| sulfate, SO₄²⁻ | acidify with dilute nitric acid, then add aqueous barium nitrate | white precipitate |
| nitrate, NO₃⁻ | add aqueous sodium hydroxide, then aluminium foil; warm | ammonia is given off, which turns damp red litmus blue |
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The chloride and sulfate tests both give a white precipitate, so the reagent decides which ion the result is evidence for: silver nitrate for chloride, barium nitrate for sulfate.
Acidify with nitric acid. In the chloride test, hydrochloric acid would add chloride ions; in the sulfate test, sulfuric acid would add sulfate ions. Either way the white precipitate could come from the acid rather than the sample. Nitric acid adds neither ion, so both tests use it. The acid also removes any carbonate ions, which would otherwise form a precipitate with silver or barium ions. Add the dilute nitric acid first, wait for any bubbling to stop, then add the silver nitrate or barium nitrate.
The nitrate test turns nitrate ions into ammonia. Ammonium ions also give ammonia when warmed with sodium hydroxide, even without aluminium, so first check a fresh portion with sodium hydroxide alone.
Worked example 1
Bubbles, then a test
Problem
Show solution step by step
Read the fizzing correctly
Method
Treat effervescence as “a gas is produced”.Reason
Hydrogen, carbon dioxide and other gases all cause fizzing, so bubbles alone identify nothing.Working
Answer
So far: a gas is given off with dilute acid.Identify the gas from its test
Method
Use the limewater result.Reason
Limewater turning milky is the test for carbon dioxide.Working
Answer
The gas is carbon dioxide, CO₂.Infer the anion
Working
Answer
An acid releases carbon dioxide from a carbonate, so W contains carbonate ions, CO₃²⁻.
Quick check
Guided practice 2
Build a valid sulfate test
Problem
You are testing a solution for sulfate ions. Choose the acidification step, when to add the next reagent, the reagent and the positive result.
Choose each step
Hints
Hint 1
contamination
Which acid would add the very ion you are testing for?
Hint 2
reagent
Silver nitrate is the chloride reagent; the sulfate test needs barium ions.
Show solution step by step
Acidify without adding sulfate
Reason
Sulfuric acid would add the sulfate ions you are testing for; nitric acid adds none.
Working
Answer
Add dilute nitric acid to a fresh portion.
Add the sulfate reagent and read the result
Working
Answer
Wait for any bubbling to stop, then add aqueous barium nitrate. A white precipitate shows that sulfate ions are present.
Quick check
If you chose sulfuric acid, the white precipitate could come from the acid’s own sulfate ions, so it would no longer be evidence about the sample.
Use the gas test to check a conclusion
Check your understanding 1 Bubbles and a pop
Dilute sulfuric acid makes a grey solid fizz, and a small sample of the gas gives a squeaky pop with a burning splint. What do the bubbles show, which gas is it, and what can you say about the solid?
Show answer
The bubbles show that a gas forms. The squeaky pop shows the gas is hydrogen, which a reactive metal gives with dilute acid. The results do not tell you which metal it is.
Spot the mistake 3
“It fizzed, so it is carbon dioxide”
Learner response
A learner adds dilute sulfuric acid to a grey solid, sees bubbles and writes: “Carbon dioxide is given off, so the solid is a carbonate.” A burning splint held at the mouth of the tube gives a squeaky pop. Explain the learner’s mistake and write the correct conclusion.
Correct the conclusion
Show solution step by step
Find the mistake
Method
Find where the gas was named.Reason
The learner named the gas from bubbles alone, before any gas test.
Working
Answer
Bubbles only show that a gas forms.Use the positive gas test
Working
Answer
The squeaky pop shows the gas is hydrogen, not carbon dioxide. A reactive metal gives hydrogen with dilute sulfuric acid, so the solid is likely to be a reactive metal, not a carbonate. The test does not say which metal.
Common mistake
Fizzing only shows that a gas forms. Do the gas test before you name the gas.
A white precipitate shows chloride only with acidified silver nitrate, and sulfate only with acidified barium nitrate. Always state the reagent with the result.
Check your understanding 2 Choose a valid chloride test
A solution containing carbonate gave a precipitate when silver nitrate was added straight to it, so the result says nothing about chloride. How should the chloride test be done on a fresh portion?
Show answer
Add dilute nitric acid to a fresh portion and wait for any bubbling to stop, then add aqueous silver nitrate. A white precipitate shows chloride. Adding silver nitrate straight away can precipitate the carbonate, and hydrochloric acid would add chloride itself.
Check your understanding 3 The nitrate test
A fresh portion warmed with sodium hydroxide without aluminium gives no ammonia, so ammonium ions were not detected. Another fresh portion gives a positive nitrate test. What was done in the nitrate test, which gas was given off, and what did it do to the litmus?
Show answer
Add aqueous sodium hydroxide and aluminium foil, then warm. Ammonia is given off and turns damp red litmus blue. Because sodium hydroxide alone gave no ammonia, the ammonia came from nitrate ions. If sodium hydroxide alone had given ammonia, ammonium ions could explain the result. Never smell the gas.
Common mistakes
- Saying effervescence identifies carbonate. It shows a gas formed; limewater identifies carbon dioxide.
- Acidifying with hydrochloric acid before testing for chloride, or with sulfuric acid before testing for sulfate. The acid adds the ion being tested for.
- Recording “white precipitate” without the reagent that produced it.
- Testing for nitrate without warming with aluminium and aqueous sodium hydroxide, or without testing the gas with damp red litmus.
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Syllabus and review details
- SEC G3 Combined Science Chemistry component 2027 · 2027
Chemistry content structure and subject content, PDF pages 26–36
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