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12.5 Tests for Negative Ions

GCSE Chemistry · AQA · Chapter 12: Chemical Analysis

A free GCSE Chemistry lesson for AQA (8462) from OBA School: chemical tests for carbonate, halide and sulfate ions, and why acid is added first. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the test for carbonate ions using dilute acid and limewater
  • Describe the test for halide ions using dilute nitric acid and silver nitrate, and recall the colours of the precipitates
  • Describe the test for sulfate ions using dilute hydrochloric acid and barium chloride
  • Explain why the solution is acidified before each precipitate test
  • Write balanced symbol equations, and (Higher tier) ionic equations, for these tests

Key words

  • Negative ion: An ion with a negative charge (an anion), e.g. Cl−, CO₃²−, SO₄²−.
  • Carbonate ion: CO₃²−. Carbonates react with acids to give carbon dioxide.
  • Halide ion: An ion of a Group 7 element: chloride Cl−, bromide Br−, iodide I−.
  • Sulfate ion: SO₄²−.
  • Acidify: Add a little dilute acid to a solution before testing it.

Carbonate ions, CO₃²−

Add dilute acid (e.g. hydrochloric acid) to the sample. If it fizzes, bubble the gas through limewater. Limewater turning milky shows carbon dioxide, so the sample contains carbonate ions.

Most carbonates are insoluble in water, so the acid is usually added to the solid. Group 1 carbonates (e.g. sodium and potassium carbonate) are soluble, so they can also be tested as a solution. In the limewater, the acidic carbon dioxide reacts with the alkaline calcium hydroxide to form a white precipitate of calcium carbonate.

Na₂CO₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

(Higher tier) Ionic equation: CO₃²−(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g)

Halide ions: Cl−, Br−, I−

Add dilute nitric acid, then silver nitrate solution. Silver halides are insoluble, so a precipitate forms. Its colour tells you which halide is present.

  • chloride, Cl−; Precipitate silver chloride, AgCl; Colour white
  • bromide, Br−; Precipitate silver bromide, AgBr; Colour cream
  • iodide, I−; Precipitate silver iodide, AgI; Colour yellow

AgNO₃(aq) + NaBr(aq) → AgBr(s) + NaNO₃(aq)

(Higher tier) Ionic equation: Ag⁺(aq) + Br−(aq) → AgBr(s)

Why acid?: The nitric acid removes any carbonate ions, which would also form a precipitate with silver nitrate. Do not use hydrochloric acid: it contains chloride ions, which would give a white precipitate every time.

Sulfate ions, SO₄²−

Add dilute hydrochloric acid, then barium chloride solution. A white precipitate of barium sulfate shows sulfate ions.

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

(Higher tier) Ionic equation: Ba²⁺(aq) + SO₄²−(aq) → BaSO₄(s)

Why acid?: The hydrochloric acid removes carbonate ions, which would form a white barium carbonate precipitate. Do not use sulfuric acid: it contains sulfate ions. Barium chloride is toxic, so avoid skin contact and wash your hands.

Identifying a whole compound

An ionic compound has a positive ion and a negative ion. Combine the tests from 12.4 and 12.5.

Worked example: naming the compound

A white solid gives a lilac flame. Its solution gives a cream precipitate with dilute nitric acid and silver nitrate.
lilac flame → potassium ion, K⁺; cream precipitate → bromide ion, Br−

Answer: potassium bromide, KBr

  • carbonate; Reagents dilute acid; gas into limewater; Positive result fizzing; limewater turns milky
  • chloride / bromide / iodide; Reagents dilute nitric acid, then silver nitrate solution; Positive result white / cream / yellow precipitate
  • sulfate; Reagents dilute hydrochloric acid, then barium chloride solution; Positive result white precipitate

Common mistakes

  • Adding hydrochloric acid before the silver nitrate test. It adds chloride ions; use nitric acid.
  • Adding sulfuric acid before the barium chloride test. It adds sulfate ions; use hydrochloric acid.
  • Mixing up the halide colours. Remember the order down Group 7: white → cream → yellow.
  • Saying “carbonate fizzes” without testing the gas. Fizzing alone could be another gas; limewater confirms CO₂.
  • Writing BaSO₄ as the reagent. Barium chloride is the reagent; barium sulfate is the precipitate.

Revision questions

Try these, then check your answers below each one.

Question 1. Which reagents are used to test for sulfate ions? (Choose from: Dilute hydrochloric acid, then barium chloride solution, Dilute nitric acid, then silver nitrate solution, Dilute sulfuric acid, then barium chloride solution, Sodium hydroxide solution.)

Answer: Dilute hydrochloric acid, then barium chloride solution

Working: Acidify with dilute HCl, then add barium chloride: a white precipitate of BaSO₄ shows sulfate.

Question 2. Why is dilute nitric acid added before silver nitrate solution in the halide test? (Choose from: To remove carbonate ions, which would also form a precipitate, To add chloride ions, To make the precipitate dissolve, To speed up the reaction.)

Answer: To remove carbonate ions, which would also form a precipitate

Working: Nitric acid reacts with carbonate ions so they cannot give a false positive.

Question 3. True or false? Dilute hydrochloric acid should be used to acidify a solution before testing for chloride ions with silver nitrate.

Answer: False

Working: Hydrochloric acid contains chloride ions, so a white precipitate would always form. Use nitric acid.

Related lessons

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This lesson is section 12.5 (AGCCH1205) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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