Aqueous electrolysis: brine and dilute acid
Describe the products and observations in the two required aqueous examples before generalising.
Exam scope and assessment notes
Core and Extended · Chemistry 0971, 2026–2028. Half-equations and detailed charge transfer are taught in the Extended lesson. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Label the anode, cathode and electrolyte.
- Know that an aqueous solution contains water.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What extra substance is present in aqueous NaCl compared with molten NaCl?
What you will learn
- Identify electrode products and observations for concentrated sodium chloride solution and dilute sulfuric acid.
- Distinguish aqueous brine from molten sodium chloride.
Concentrated sodium chloride solution
Concentrated aqueous sodium chloride, commonly called brine, produces hydrogen at the negative cathode and chlorine at the positive anode when inert electrodes are used. Bubbles appear at both electrodes. Hydrogen is colourless and gives a squeaky pop with a lighted splint in an appropriate supervised gas test. Chlorine is pale green and bleaches damp litmus paper. These are recognition observations, not instructions to perform gas tests unsupervised.
Sodium metal is not the cathode product in aqueous brine: water competes and hydrogen is produced instead. The solution remaining becomes alkaline and contains sodium hydroxide. Keep the stated concentration: a dilute chloride solution has a different main anode prediction in the syllabus model, addressed in the Extended lesson. Product names and electrode locations should come before the observations in your answer.
Dilute sulfuric acid
Electrolysis of dilute sulfuric acid with inert electrodes produces hydrogen at the cathode and oxygen at the anode. Both gases are colourless and form bubbles. Hydrogen gives the recognised pop test; oxygen relights a glowing splint in a supervised test. The hydrogen volume is twice the oxygen volume if both gases are collected without loss under the same conditions, consistent with 2H₂O→2H₂+O₂.
The sulfuric acid makes the aqueous solution conduct well; the overall gaseous products correspond to splitting water rather than producing sulfur at the anode. Observations such as colourless bubbles alone cannot distinguish hydrogen from oxygen. Name the electrode and give a specific identifying test result when asked to support the identification.
Pause and explain
Which observation supports oxygen identification in a supervised test?
Worked example
Two inert electrodes are placed in dilute sulfuric acid and current is passed. Identify the gases, give distinguishing observations and predict their volume ratio under equal collection conditions.
Show the worked solution
- Hydrogen forms at the cathode and oxygen at the anode; both appear as colourless bubbles.
- Hydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint in supervised tests.
- The water decomposition equation gives twice as much hydrogen gas as oxygen gas at the same temperature and pressure.
Answer Cathode H₂; anode O₂; hydrogen:oxygen volume ratio 2:1.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Complete an electrode-and-observation table for concentrated aqueous NaCl with inert electrodes.
Give me a hint
Locate hydrogen and chlorine, then add a distinctive observation for each.
Compare my reasoning
- At the negative cathode, hydrogen produces colourless bubbles.
- At the positive anode, chlorine produces pale-green gas that bleaches damp litmus.
- The hydrogen identification uses the recognised squeaky-pop result in a supervised test.
Cathode: hydrogen, colourless bubbles/pop test. Anode: chlorine, pale green/bleaches damp litmus.
Look for these in your work
- Gas identities are tied to the correct electrodes.
- An observation more specific than bubbling is included.
Electrolysis of dilute sulfuric acid collects 18 cm³ oxygen without loss. Predict the hydrogen volume at the same temperature and pressure and identify its electrode.
Give me a hint
Use the water decomposition ratio rather than the acid's formula subscripts.
Compare my reasoning
- 2H₂O→2H₂+O₂ gives H₂:O₂ volume ratio 2:1 under the same conditions.
- Hydrogen volume is 2×18=36 cm³.
- Hydrogen is collected at the cathode.
36 cm³ hydrogen at the cathode.
Look for these in your work
- The ratio is based on the overall water decomposition.
- Matching collection conditions are stated.
A student says molten NaCl and concentrated aqueous NaCl both produce sodium and chlorine. Correct the claim and explain which product changes.
Give me a hint
Compare the cathode when water is absent and present.
Compare my reasoning
- Molten NaCl gives sodium at the cathode and chlorine at the anode.
- Concentrated aqueous NaCl gives hydrogen at the cathode and chlorine at the anode.
- The cathode prediction changes because the aqueous electrolyte contains water.
Only molten NaCl gives sodium; brine gives cathode hydrogen, with anode chlorine in both stated cases.
Look for these in your work
- The two electrolyte states remain distinct.
- The product change is connected with water's presence.
Common mistakes
- Predicting sodium metal from aqueous brine.
- Claiming that colourless bubbles identify a gas uniquely.
- Predicting sulfur from the sulfate-containing acid.
- Ignoring the concentration specified for an aqueous chloride.
What can you explain now?
Name both gases from concentrated aqueous NaCl with inert electrodes and locate them.
Compare with the explanation
Cathode: hydrogen; anode: chlorine.
The aqueous cathode produces hydrogen rather than sodium; concentrated chloride gives the chlorine anode product.
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Make it stick
Write the products, electrode locations and observations for brine and dilute sulfuric acid without looking. Explain why bubbles alone do not establish gas identity.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.