Tests for ions, gases and organic groups (Core Practical 8)

Cations

• Flame tests for Group 1 and 2 ions (Li+, Na+, K+, Ca2+, Sr2+, Ba2+).
• Ammonium: warm with NaOH(aq); ammonia gas turns damp red litmus blue. NH4+ + OH- -> NH3 + H2O.

Anions

• Carbonate: add dilute acid; fizzing, and the gas turns limewater milky (CO2).
• Sulfate: add dilute HCl, then barium chloride solution; white precipitate of BaSO4. The acid removes carbonate first.
• Halides: dilute HNO3 then AgNO3; white, cream or yellow precipitate, then test with ammonia.

Gases

• CO2: limewater turns milky.
• O2: relights a glowing splint.
• H2: squeaky pop with a lit splint.
• NH3: damp red litmus turns blue.
• NO2: brown gas (from heating Group 2 nitrates).
• HCl: steamy fumes; white smoke with ammonia.
• Cl2: bleaches damp litmus paper.

Organic groups

• C=C: decolourises orange bromine water.
• OH (alcohols and acids): PCl5 gives steamy fumes of HCl.
• Primary or secondary alcohol: warming with acidified potassium dichromate(VI) turns it from orange to green. Tertiary alcohols give no change.
• Halogenoalkane: warm with NaOH(aq), acidify with HNO3, add AgNO3 to see the halide precipitate (or use AgNO3 in ethanol).

Worked example

A white solid gives a brick-red flame, and fizzes with dilute HCl giving a gas that turns limewater milky. Identify it.

1. Brick-red flame: Ca2+.
2. Fizzing with CO2: carbonate.

Answer: Calcium carbonate, CaCO3

Check yourself

1. How is the ammonium ion identified?
   • Add barium chloride solution
   • Add silver nitrate solution
   • Add bromine water
   • Warm with sodium hydroxide solution and test the gas with damp red litmus
   Show answer

   Warm with sodium hydroxide solution and test the gas with damp red litmus. NH3 is released, which is alkaline and turns red litmus blue.

2. Which reagents test for sulfate ions?
   • Dilute HNO3 then silver nitrate solution
   • Dilute HCl then barium chloride solution
   • Sodium hydroxide solution, warm
   • Limewater
   Show answer

   Dilute HCl then barium chloride solution. White BaSO4 forms; the acid prevents BaCO3 forming.

3. What is the test for oxygen?
   • It turns limewater milky
   • It gives a squeaky pop
   • It turns damp red litmus blue
   • It relights a glowing splint
   Show answer

   It relights a glowing splint. Oxygen supports combustion.

4. An organic liquid gives steamy fumes with PCl5. Which group is present?
   • An -OH group
   • A C=C bond
   • A C-Br bond
   • A C-H bond only
   Show answer

   An -OH group. PCl5 reacts with -OH to release HCl.

5. Which alcohol does NOT turn acidified potassium dichromate(VI) green when warmed?
   • Propan-1-ol
   • Propan-2-ol
   • 2-methylpropan-2-ol
   • Ethanol
   Show answer

   2-methylpropan-2-ol. It is a tertiary alcohol, which is not oxidised under these conditions.

6. A solution gives a lilac flame and a yellow precipitate with acidified silver nitrate. What is it?
   • Potassium iodide
   • Sodium iodide
   • Potassium bromide
   • Sodium chloride
   Show answer

   Potassium iodide. Lilac = K+; yellow precipitate = I-.

7. Which gas bleaches damp litmus paper?
   • Hydrogen chloride
   • Oxygen
   • Chlorine
   • Carbon dioxide
   Show answer

   Chlorine. Chlorine forms HClO, which bleaches.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page

Last updated 7 November 2026. Spotted a mistake? Tell us.