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2.4.7Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols

Qualitative Tests — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 32 of 53 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 5Testing for Ammonium Ions

Ammonium salts release ammonia gas when warmed with an alkali.

1
Add dilute sodium hydroxide solution to the sample and warm the mixture gently in a water bath.
2
The reaction is NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l).
3
Hold damp red litmus paper near the mouth of the tube without letting it touch the liquid.
4
A positive result is that the damp red litmus paper turns blue because ammonia is alkaline.
5
Alternatively, concentrated hydrochloric acid on a rod gives white smoke of ammonium chloride: NH₃(g) + HCl(g) → NH₄Cl(s).
6
Blue litmus is unsuitable for this test, because ammonia is alkaline and would not change the colour of blue paper.

Part 2 of 5Carbonates and Hydrogencarbonates

Carbonates and Hydrogencarbonates
Reagent
  • Reagent Add dilute hydrochloric acid to the solid or solution and collect the gas.
Carbonate
  • Carbonate CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l), producing fizzing and carbon dioxide.
Hydrogencarbonate
  • Hydrogencarbonate HCO₃⁻(aq) + H⁺(aq) → CO₂(g) + H₂O(l), with the same observation.
Confirming the gas
  • Confirming the gas Bubble the gas through limewater, which turns milky because of CaCO₃(s).
Observations
  • Observations Fizzing on adding acid points to a carbonate, but the gas must still be tested because other substances can also fizz.

Part 3 of 5Testing for Sulfates

Acidify first
  • Acidify first Add dilute hydrochloric acid to remove carbonate ions that would also precipitate with barium.
Add barium
  • Add barium Add barium chloride solution (or barium nitrate) to the acidified sample.
Positive result
  • Positive result A white precipitate of barium sulfate forms: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s).
Negative result
  • Negative result If no precipitate forms, then sulfate ions are not present.
Order of tests
  • Order of tests Test for carbonate before sulfate so you know whether acid removal is needed.
Negative result
  • Negative result If limewater stays clear or no litmus change is seen, the ion being tested for is absent from the sample.

Part 4 of 5Worked Example

Question

  • A white solid dissolves in water.
  • Acid gives no fizzing, acidified barium chloride gives a white precipitate, and warming with NaOH turns damp red litmus blue.
  • Name the compound's ions.

  1. No fizzing with acid means carbonate and hydrogencarbonate are absent.
  2. The white precipitate with acidified barium chloride shows sulfate ions.
  3. Red litmus turning blue on warming with alkali shows ammonia, so ammonium ions are present.

Answer The solid contains ammonium (NH₄⁺) and sulfate (SO₄²⁻) ions, so it is ammonium sulfate.

Part 5 of 5Exam Checklist & Tips

2.4.7 – Key Points for the Exam

  • Ammonium ion The NH₄⁺ ion, which forms ammonia gas when warmed with alkali.
  • Limewater Calcium hydroxide solution that turns milky with carbon dioxide.
  • Acidified barium chloride The reagent used to identify sulfate ions.
  • Damp litmus paper Litmus paper moistened with water so that gases such as ammonia can dissolve and change its colour.

Common Mistakes

  • Using dry litmus paper for ammonia, which will not show the colour change.
  • Adding barium chloride without acid, so carbonates give a misleading precipitate.
  • Saying ammonia turns blue litmus red; it turns red litmus blue.
  • Forgetting to warm the mixture when testing for ammonium ions.

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