Testing for Gases and Identifying Ions

This lesson covers the chemical tests for gases and methods for identifying ions that you need to know for the AQA GCSE Combined Science Trilogy (8464) specification. Being able to carry out and describe these tests is essential for both practical work and written exams.

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Tests for Gases

You must know the tests for four gases: hydrogen, oxygen, carbon dioxide, and chlorine.

Gas	Test	Positive Result
Hydrogen (H₂)	Hold a burning splint near the gas	Burns with a squeaky pop
Oxygen (O₂)	Insert a glowing splint into the gas	Splint relights
Carbon dioxide (CO₂)	Bubble through limewater (calcium hydroxide solution)	Limewater turns milky (cloudy white)
Chlorine (Cl₂)	Hold damp litmus paper in the gas	Litmus paper is bleached white

Explaining the Limewater Test

$\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$Ca(OH)2​(aq)+CO2​(g)→CaCO3​(s)+H2​O(l)

The milky appearance is caused by tiny particles of insoluble calcium carbonate (CaCO₃) suspended in the solution.

Exam Tip: Learn these four gas tests thoroughly. They come up almost every year in AQA GCSE papers. Make sure you describe the test AND the observation separately — both are needed for full marks.

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Flame Tests

Flame tests are used to identify metal ions in a compound. A sample is placed on a nichrome wire loop (cleaned with hydrochloric acid), then held in a Bunsen flame. The colour of the flame indicates which metal ion is present.

Metal Ion	Flame Colour
Lithium (Li⁺)	Crimson (red)
Sodium (Na⁺)	Yellow
Potassium (K⁺)	Lilac
Calcium (Ca²⁺)	Orange-red
Copper (Cu²⁺)	Green

Exam Tip: The sodium flame test produces a very strong yellow colour that can mask other colours. If testing a mixture, the yellow sodium flame may dominate. This is a common limitation to mention in evaluations.

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Testing for Negative Ions (Anions)

Test for Carbonates (CO₃²⁻)

Add dilute hydrochloric acid to the solid or solution. If a carbonate is present:

• Fizzing (effervescence) occurs.
• The gas produced turns limewater milky (confirming it is CO₂).

$\text{Na}_2\text{CO}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{CO}_2$Na2​CO3​+2HCl→2NaCl+H2​O+CO2​

Test for Halide Ions (Cl⁻, Br⁻, I⁻)

Add dilute nitric acid followed by silver nitrate solution (AgNO₃).

Halide Ion	Precipitate Colour
Chloride (Cl⁻)	White precipitate (AgCl)
Bromide (Br⁻)	Cream precipitate (AgBr)
Iodide (I⁻)	Yellow precipitate (AgI)

Key Point: Nitric acid is added first to remove any carbonate ions that might interfere with the test by also forming a white precipitate.

Test for Sulfate Ions (SO₄²⁻)

Add dilute hydrochloric acid followed by barium chloride solution (BaCl₂).

• A white precipitate of barium sulfate (BaSO₄) confirms the presence of sulfate ions.

$\text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)}$Ba2+(aq)+SO42−​(aq)→BaSO4​(s)

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Testing for Metal Ions Using Sodium Hydroxide

Adding a few drops of sodium hydroxide solution (NaOH) to a solution containing metal ions produces characteristic coloured precipitates (insoluble metal hydroxides).

Metal Ion	Precipitate Colour	Formula
Calcium (Ca²⁺)	White	Ca(OH)₂
Magnesium (Mg²⁺)	White	Mg(OH)₂
Aluminium (Al³⁺)	White (dissolves in excess NaOH)	Al(OH)₃
Copper (Cu²⁺)	Blue	Cu(OH)₂
Iron(II) (Fe²⁺)	Green	Fe(OH)₂
Iron(III) (Fe³⁺)	Brown	Fe(OH)₃

Key Point: Aluminium hydroxide dissolves in excess NaOH to form a colourless solution. This distinguishes it from calcium and magnesium hydroxides, which remain as white precipitates.

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Worked Example: Identifying an Unknown Solution

Question: An unknown solution gives a green precipitate when sodium hydroxide is added. What metal ion is present?

Answer: The green precipitate indicates the presence of iron(II) ions (Fe²⁺), because Fe(OH)₂ is green.

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Common Mistakes

• Confusing the precipitate colours (green = Fe²⁺, brown = Fe³⁺ — not the other way around).
• Forgetting to add nitric acid before silver nitrate in the halide test.
• Not specifying "dilute" when naming acids in test procedures.
• Describing the limewater test result as "turns white" instead of "turns milky/cloudy."

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Summary

• Gas tests: hydrogen (squeaky pop), oxygen (relights splint), CO₂ (limewater goes milky), chlorine (bleaches litmus)
• Flame tests identify metal ions by the colour of the flame
• Carbonates fizz with acid and produce CO₂ (confirmed by limewater)
• Halides tested with nitric acid then silver nitrate: white (Cl⁻), cream (Br⁻), yellow (I⁻)
• Sulfates tested with HCl then barium chloride: white precipitate
• NaOH produces coloured precipitates with metal ions: blue (Cu²⁺), green (Fe²⁺), brown (Fe³⁺)

Exam Tip: These chemical tests are frequently examined. Practise writing out the full test procedure AND expected observation. Questions often ask "describe a test to show that..." — you need to say what you add AND what you would see.

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Extended Worked Examples and Exam Technique

Worked Example 1: Identifying Gas Evolved From a Reaction (Trilogy-core)

Question: A student added dilute hydrochloric acid to an unknown white solid. A gas was given off which turned limewater milky. Identify the gas and suggest what the solid might be.

Solution: Limewater (calcium hydroxide solution) turning milky is the positive test for carbon dioxide (CO₂). An acid producing CO₂ from a white solid suggests the solid contained a carbonate ion (CO₃²⁻). Likely candidates include calcium carbonate, sodium carbonate or magnesium carbonate.

Ionic equation reminder (Trilogy-friendly): $2\text{HCl(aq)} + \text{Na}_2\text{CO}_3\text{(aq)} \rightarrow 2\text{NaCl(aq)} + \text{H}_2\text{O(l)} + \text{CO}_2\text{(g)}$2HCl(aq)+Na2​CO3​(aq)→2NaCl(aq)+H2​O(l)+CO2​(g)

Worked Example 2: Choosing the Right Gas Test (Trilogy-core)

Question: A test tube contains an unknown gas. Describe the tests you would do, in order, to identify it as oxygen, hydrogen, carbon dioxide or chlorine.

Model answer:

1. Colour and smell: pale green with a sharp, bleaching smell → suspect chlorine. Confirm with damp blue litmus paper — it turns red then bleaches white.
2. If colourless, insert a lit splint: a loud squeaky pop indicates hydrogen.
3. If no pop, insert a glowing splint: it relights in oxygen.
4. If no relight, bubble the gas through limewater: a milky appearance indicates carbon dioxide.

Summary Table: The Four Core Gas Tests (Trilogy)