Chemical tests: anions, gases and organic compounds

What you'll learn

Chemical tests form a critical practical component of Edexcel GCSE Chemistry, allowing you to identify unknown substances through characteristic reactions. This topic covers systematic tests for negative ions (anions), identification of common gases, and simple tests for organic functional groups. Expect both written questions and practical-based exam questions worth 4-6 marks.

Key terms and definitions

Anion — a negatively charged ion formed when atoms gain electrons, such as carbonate (CO₃²⁻), sulfate (SO₄²⁻) or halides (Cl⁻, Br⁻, I⁻).

Precipitate — an insoluble solid formed when two solutions react, appearing as cloudiness or solid particles suspended in liquid.

Test tube — standard laboratory glassware used to hold small volumes of chemicals during identification tests.

Limewater — an aqueous solution of calcium hydroxide used to test for carbon dioxide gas, turning milky/cloudy when positive.

Flame test — a procedure using a clean platinum or nichrome wire to identify metal ions by the colour they produce in a Bunsen flame.

Functional group — a specific group of atoms within an organic molecule that determines its chemical properties and reactions.

Silver nitrate solution — an aqueous reagent (AgNO₃) used with dilute nitric acid to test for halide ions, producing characteristic coloured precipitates.

Hydrochloric acid — a strong acid (HCl) used in chemical tests to distinguish between carbonate and sulfate anions.

Core concepts

Testing for carbonate ions (CO₃²⁻)

The test for carbonate ions uses dilute acid and produces carbon dioxide gas. This two-stage test appears frequently in Edexcel GCSE Chemistry practicals.

Method:

1. Add dilute hydrochloric acid or dilute nitric acid to the solid or solution
2. Bubble any gas produced through limewater
3. Observe for a positive result

Positive result: Effervescence (fizzing) occurs, and limewater turns milky or cloudy white.

Chemical equation:

• Carbonate + acid → salt + water + carbon dioxide
• Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)
• CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

The cloudiness forms because calcium carbonate is insoluble, creating a white precipitate in the limewater.

Testing for sulfate ions (SO₄²⁻)

The sulfate test produces a distinctive white precipitate of barium sulfate, one of the most insoluble compounds you'll encounter at GCSE.

Method:

1. Add dilute hydrochloric acid to the test solution (this removes any carbonate ions that would interfere)
2. Add barium chloride solution (BaCl₂) or barium nitrate solution
3. Observe for precipitate formation

Positive result: A white precipitate forms immediately.

Chemical equation:

• Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

The addition of dilute hydrochloric acid first prevents false positives from carbonate or sulfite ions, which would also form white precipitates with barium ions.

Testing for halide ions (Cl⁻, Br⁻, I⁻)

The halide test distinguishes between chloride, bromide and iodide ions using silver nitrate solution. This test requires careful observation of precipitate colour.

Method:

1. Add dilute nitric acid to the test solution (removes carbonate ions)
2. Add silver nitrate solution (AgNO₃)
3. Observe the colour of any precipitate formed

Positive results:

• Chloride ions (Cl⁻): white precipitate of silver chloride (AgCl)
• Bromide ions (Br⁻): cream precipitate of silver bromide (AgBr)
• Iodide ions (I⁻): yellow precipitate of silver iodide (AgI)

Chemical equations:

• Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
• Ag⁺(aq) + Br⁻(aq) → AgBr(s)
• Ag⁺(aq) + I⁻(aq) → AgI(s)

Dilute nitric acid is used specifically (not sulfuric or hydrochloric) because it doesn't introduce interfering ions. Using hydrochloric acid would add chloride ions and give a false positive.

Testing for common gases

Gas identification tests appear in both Paper 1 and Paper 2 contexts for Edexcel GCSE Chemistry.

Hydrogen (H₂):

• Test: Insert a lit splint into the gas
• Positive result: Burns with a squeaky pop sound
• Source: Reaction of metals with acids, or electrolysis of water

Oxygen (O₂):

• Test: Insert a glowing (not flaming) splint into the gas
• Positive result: Splint relights and burns brightly
• Source: Thermal decomposition of metal oxides, or electrolysis

Carbon dioxide (CO₂):

• Test: Bubble gas through limewater
• Positive result: Limewater turns milky/cloudy
• Source: Combustion, respiration, reaction of carbonates with acids

Chlorine (Cl₂):

• Test: Hold damp blue litmus paper in the gas
• Positive result: Litmus paper is bleached white (colour removed)
• Source: Electrolysis of sodium chloride solution, reaction of hydrochloric acid with bleach

Ammonia (NH₃):

• Test: Hold damp red litmus paper in the gas
• Positive result: Litmus paper turns blue (ammonia is alkaline)
• Source: Heating ammonium salts with alkali

Testing for water

While not strictly an anion or organic test, water identification appears in the same exam context.

Test for presence of water:

• Method: Add anhydrous copper(II) sulfate (white powder)
• Positive result: Turns blue
• Chemical change: CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s)

Test for pure water:

• Method: Measure boiling point with thermometer
• Result for pure water: Boils at exactly 100°C at standard atmospheric pressure
• Impure water boils above 100°C

Testing organic compounds

Organic chemistry tests identify functional groups in carbon-containing compounds.

Testing for alkenes (C=C double bond):

• Method: Add bromine water to the organic compound
• Positive result: Bromine water decolourises (changes from orange/brown to colourless)
• Explanation: The C=C double bond reacts with bromine in an addition reaction
• Example: Ethene (C₂H₄) decolourises bromine water; ethane (C₂H₆) does not

Testing for alcohols:

• Method: Add phosphorus pentachloride (PCl₅) to the alcohol
• Positive result: Steamy fumes of hydrogen chloride gas produced (turns damp blue litmus paper red)
• Alternative oxidation test: Acidified potassium dichromate(VI) turns from orange to green when heated with primary or secondary alcohols

Testing for carboxylic acids:

• Method: Add sodium carbonate solution
• Positive result: Effervescence (fizzing) as carbon dioxide is produced
• Observation: Similar to the carbonate test but identifies the organic acid
• Example: Ethanoic acid (CH₃COOH) + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂

Worked examples

Example 1: Identifying an unknown salt

Question: A student has an unknown white solid. They carry out the following tests:

• Test 1: Adding dilute hydrochloric acid produces effervescence. The gas produced turns limewater milky.
• Test 2: After filtering, barium chloride solution is added to the filtrate but no precipitate forms.

(a) What ion was identified in Test 1? [1 mark] (b) Write the ionic equation for the reaction in Test 1 with the limewater. [2 marks] (c) What does Test 2 tell you about the original solid? [1 mark]

Answers: (a) Carbonate ion / CO₃²⁻ [1]

(b) CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l) [1 for correct equation] OR ionic: CO₂(g) + Ca²⁺(aq) + 2OH⁻(aq) → CaCO₃(s) + H₂O(l) [1] [Award 2 marks for fully correct balanced equation with state symbols]

(c) The original solid did not contain sulfate ions / was not a sulfate [1]

Example 2: Gas identification

Question: During electrolysis of dilute sulfuric acid, two gases are produced.

• Gas A collects at the positive electrode
• Gas B collects at the negative electrode

(a) Describe a test for Gas A and state the positive result. [2 marks] (b) Describe a test for Gas B and state the positive result. [2 marks] (c) Name Gas A and Gas B. [2 marks]

Answers: (a) Insert a glowing splint [1] The splint relights / burns brightly [1]

(b) Insert a lit/burning splint [1] Burns with a squeaky pop / makes a popping sound [1]

(c) Gas A: oxygen / O₂ [1] Gas B: hydrogen / H₂ [1]

Example 3: Halide testing

Question: A chemist tests three solutions X, Y and Z. Each contains a different halide ion. After adding dilute nitric acid and silver nitrate solution, the following observations are made:

• Solution X forms a white precipitate
• Solution Y forms a cream precipitate
• Solution Z forms a yellow precipitate

(a) Identify the halide ion in solution Y. [1 mark] (b) Write the ionic equation for the reaction in solution Z. [2 marks] (c) Explain why dilute nitric acid is added before the silver nitrate solution. [2 marks]

Answers: (a) Bromide / Br⁻ [1]

(b) Ag⁺(aq) + I⁻(aq) → AgI(s) [1 for correct formula] [Award 1 mark for state symbols if all correct]

(c) To remove carbonate ions [1] Which would form a precipitate with silver ions / interfere with the test / give a false positive [1]

Common mistakes and how to avoid them

Mistake: Confusing the precipitate colours in halide tests — describing silver chloride as cream or silver bromide as yellow. Correction: Learn the sequence: chloride = white, bromide = cream, iodide = yellow. The colour gets darker down Group 7.

Mistake: Using hydrochloric acid before testing for chloride ions with silver nitrate. Correction: Always use dilute nitric acid before the halide test. Hydrochloric acid contains chloride ions that would contaminate the test and produce a white precipitate even if the original sample contained no chlorides.

Mistake: Stating that limewater "turns white" when testing for carbon dioxide. Correction: Limewater turns milky, cloudy or forms a white precipitate. The solution itself becomes cloudy rather than the liquid turning white.

Mistake: Not understanding why tests require two stages (e.g., acid first, then the reagent). Correction: The preliminary acid addition removes interfering ions. For sulfate tests, it removes carbonates; for halide tests, it removes carbonates and sulfites.

Mistake: Describing the hydrogen test as a "pop" without the qualifier "squeaky". Correction: The mark scheme specifically requires "squeaky pop" or similar description. A simple "pop" may not earn the mark.

Mistake: Confusing the splint test for oxygen and hydrogen — using a glowing splint for hydrogen. Correction: Oxygen requires a glowing (not flaming) splint which relights. Hydrogen requires a lit (burning) splint which produces a squeaky pop.

Exam technique for "Chemical tests: anions, gases and organic compounds"

Command word patterns: "Describe the test for..." typically requires both the method (what you add/do) and the positive observation. This is usually worth 2 marks — 1 for procedure, 1 for result. Questions asking you to "identify" need only the name of the ion or gas.

Ionic equations: When asked to write equations for precipitate reactions, ionic equations are often preferred at GCSE. Include state symbols for full marks: (s) for precipitate, (aq) for aqueous ions. The mark scheme usually awards 1 mark for correct formulae and 1 mark for balancing/state symbols.

Practical questions: Extended 6-mark questions may ask you to design an identification procedure for an unknown substance. Structure your answer chronologically: test 1 with method and result, test 2 with method and result, conclusion. Reference specific observations (colours, effervescence, precipitates) rather than vague terms.

Quality of written communication: In longer answers about chemical tests, marks are awarded for logical sequencing and correct use of technical terms. Always write "add dilute [acid]" not just "add acid", specify "silver nitrate solution" not "silver nitrate", and state the observation clearly (e.g., "white precipitate forms" not "goes cloudy").

Quick revision summary

Test carbonate ions with dilute acid — look for fizzing and limewater turning milky from CO₂. Test sulfates with hydrochloric acid then barium chloride — white precipitate confirms sulfate. Test halides with nitric acid then silver nitrate: chloride gives white, bromide gives cream, iodide gives yellow. Gas tests: hydrogen burns with squeaky pop, oxygen relights glowing splint, carbon dioxide turns limewater milky, chlorine bleaches damp litmus. Alkenes decolourise bromine water; alcohols produce steamy fumes with PCl₅. Always add acid first to remove interfering ions.