Core Practical: Investigating Electrolysis of Aqueous Solutions

This lesson covers the Core Practical for investigating the electrolysis of aqueous solutions as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand the practical setup, how to identify gases produced at each electrode, how to record results systematically, and how to explain the products formed.

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Aim

To investigate the electrolysis of a range of aqueous solutions and identify the products formed at each electrode.

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Background

When an aqueous solution of an ionic compound is electrolysed, there are four types of ion present:

1. Positive ions from the dissolved compound (e.g. Cu²⁺, Na⁺)
2. Negative ions from the dissolved compound (e.g. Cl⁻, SO₄²⁻)
3. H⁺ ions from the partial dissociation of water
4. OH⁻ ions from the partial dissociation of water

The products at each electrode depend on the reactivity of the ions and on the rules for predicting products.

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Apparatus

Item	Purpose
Beaker (250 cm³)	To hold the electrolyte
Two carbon (graphite) electrodes	Inert electrodes — do not react
Power supply (DC) or battery pack (6 V)	Provides direct current
Connecting wires with crocodile clips	To connect the circuit
Test tubes	To collect gases
Splints	For gas tests
Limewater	To test for CO₂ (though usually not needed here)
Litmus paper (blue and red)	To test for chlorine
Stopwatch	To time the experiment if comparing rates
Safety goggles	Eye protection

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Method

graph TD
    A["1. Pour 200 cm³ of<br/>the aqueous solution<br/>into a beaker"] --> B["2. Place two graphite<br/>electrodes into the<br/>solution"]
    B --> C["3. Connect the electrodes<br/>to the DC power supply<br/>using wires and clips"]
    C --> D["4. Turn on the power<br/>supply and observe<br/>both electrodes"]
    D --> E["5. Collect any gas<br/>produced in inverted<br/>test tubes over each<br/>electrode"]
    E --> F["6. Test the collected<br/>gases to identify them"]
    F --> G["7. Record all<br/>observations in<br/>a results table"]
    G --> H["8. Repeat with<br/>different aqueous<br/>solutions"]

    style A fill:#2980b9,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#27ae60,color:#fff
    style E fill:#e67e22,color:#fff
    style F fill:#e67e22,color:#fff
    style G fill:#8e44ad,color:#fff
    style H fill:#8e44ad,color:#fff

Detailed Steps

1. Prepare the electrolyte: Pour approximately 200 cm³ of the aqueous solution (e.g. copper sulfate solution) into a clean beaker.

2. Set up electrodes: Insert two graphite (carbon) electrodes into the solution. Graphite is used because it is inert — it does not react with the electrolyte or the products.

3. Connect the circuit: Attach connecting wires with crocodile clips from each electrode to the terminals of a DC power supply (approximately 6 V). Ensure you know which electrode is connected to the positive terminal (anode) and which to the negative terminal (cathode).

4. Switch on: Turn on the power supply and observe what happens at each electrode. Look for:

   • Bubbles of gas forming at one or both electrodes
   • Colour changes in the solution
   • Solid deposits forming on the cathode (e.g. a layer of metal)

5. Collect gases: If bubbles are produced, you can collect the gas by placing an inverted test tube filled with the electrolyte solution over each electrode. The gas displaces the solution in the test tube.

6. Test the gases: Use the appropriate gas test to identify what has been produced (see gas tests section below).

7. Record: Write down all observations in a results table, noting the product at the cathode and the product at the anode for each solution.

8. Repeat: Wash the beaker and electrodes, then repeat with different aqueous solutions (e.g. sodium chloride, copper chloride, dilute sulfuric acid, potassium nitrate).

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Gas Tests

Gas	Test	Positive Result
Hydrogen (H₂)	Hold a burning splint near the mouth of the test tube	Burns with a squeaky pop
Oxygen (O₂)	Insert a glowing splint into the test tube	The splint relights
Chlorine (Cl₂)	Hold damp blue litmus paper near the mouth of the tube	The paper turns red then white (bleaches)

Exam Tip: The chlorine test is distinctive: damp blue litmus turns red (because chlorine is acidic) and then turns white (because chlorine bleaches it). If the litmus only turns red, the gas is likely just an acid gas — the bleaching confirms chlorine.

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Predicted Results

Solutions to Test

Solution	Ions Present	Product at Cathode (−)	Product at Anode (+)
Copper sulfate (CuSO₄)	Cu²⁺, H⁺, SO₄²⁻, OH⁻	Copper (brown/pink coating) — Cu is less reactive than H	Oxygen (bubbles) — no halide present
Sodium chloride (NaCl)	Na⁺, H⁺, Cl⁻, OH⁻	Hydrogen (bubbles) — Na is more reactive than H	Chlorine (bubbles, pungent smell) — halide present
Copper chloride (CuCl₂)	Cu²⁺, H⁺, Cl⁻, OH⁻	Copper (brown/pink coating) — Cu is less reactive than H	Chlorine (bubbles) — halide present
Dilute sulfuric acid (H₂SO₄)	H⁺, SO₄²⁻, OH⁻	Hydrogen (bubbles)	Oxygen (bubbles) — no halide present
Potassium nitrate (KNO₃)	K⁺, H⁺, NO₃⁻, OH⁻	Hydrogen (bubbles) — K is more reactive than H	Oxygen (bubbles) — no halide present
Sodium bromide (NaBr)	Na⁺, H⁺, Br⁻, OH⁻	Hydrogen (bubbles)	Bromine (brown solution near anode) — halide present

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Key Observations in Detail

Copper Sulfate Solution

• Cathode: A brown/pink layer of copper metal is deposited on the electrode. The blue colour of the solution fades over time as Cu²⁺ ions are removed from the solution.
• Anode: Bubbles of oxygen gas form. A glowing splint relights.

Sodium Chloride Solution

• Cathode: Bubbles of hydrogen gas form. A burning splint gives a squeaky pop.
• Anode: Bubbles of chlorine gas form. There is a distinctive bleach-like smell. Damp blue litmus paper turns red then white.

Dilute Sulfuric Acid

• Cathode: Bubbles of hydrogen gas form (squeaky pop with a burning splint).
• Anode: Bubbles of oxygen gas form (relights a glowing splint).
• Note: Electrolysis of dilute sulfuric acid is effectively the decomposition of water (2H₂O → 2H₂ + O₂).

Exam Tip: When describing observations at the cathode during the electrolysis of copper sulfate, say a "brown/pink solid is deposited on the cathode" — this is copper metal. Do NOT say "the cathode turns copper" — describe what you see.

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Recording Results

Use a clear results table:

Solution	Product at Cathode (−)	Gas Test Result (Cathode)	Product at Anode (+)	Gas Test Result (Anode)
CuSO₄ (aq)	Copper (brown coating)	n/a — solid deposited	Oxygen (bubbles)	Glowing splint relights
NaCl (aq)	Hydrogen (bubbles)	Squeaky pop	Chlorine (bubbles)	Litmus: red → white
H₂SO₄ (aq)	Hydrogen (bubbles)	Squeaky pop	Oxygen (bubbles)	Glowing splint relights

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Explaining the Results Using Half Equations (Higher Tier)

Copper Sulfate Solution

At the cathode: Cu²⁺ + 2e⁻ → Cu

At the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻

Sodium Chloride Solution

At the cathode: 2H⁺ + 2e⁻ → H₂

At the anode: 2Cl⁻ → Cl₂ + 2e⁻

Dilute Sulfuric Acid

At the cathode: 2H⁺ + 2e⁻ → H₂

At the anode: 4OH⁻ → O₂ + 2H₂O + 4e⁻

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Risk Assessment

Hazard	Risk	Control Measure
Chlorine gas	Toxic if inhaled	Perform in a well-ventilated room or near a fume cupboard; use small volumes
Bromine vapour	Toxic and corrosive	Use a fume cupboard for NaBr electrolysis
Electrical equipment near liquids	Electric shock	Ensure no water splashes on the power supply; use low voltage (6 V DC)
Acids (H₂SO₄)	Irritant/corrosive	Wear safety goggles; wash skin if contact occurs

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Evaluating the Practical

Common evaluation points: