ChemistryICSE Class 10 · Electrolysis

Electrolysis of Acidified Water and Copper(II) Sulphate Solution (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

Acidified water electrolysed with platinum electrodes gives hydrogen at the cathode and oxygen at the anode in a 2 : 1 volume ratio. Copper(II) sulphate solution gives copper at the cathode; with platinum electrodes oxygen forms at the anode and the blue colour fades, but with copper electrodes the anode dissolves and the colour stays.

Understand Electrolysis of Acidified Water and Copper(II) Sulphate

Acidified water (platinum electrodes). Pure water is a very weak electrolyte: it ionises only slightly, so it conducts poorly. A few drops of dilute sulphuric acid are added to provide plenty of ions so that a current flows. The apparatus is usually Hofmann's voltameter, with platinum electrodes, which are inert.

Ions present: H⁺ and SO₄²⁻ (from the acid), H⁺ and OH⁻ (from water).

Cathode: H⁺ ions gain electrons: $2 H^{+} + 2 e^{-} \to H_{2}$ . A colourless gas collects that burns with a 'pop' (hydrogen).
Anode: OH⁻ is discharged in preference to SO₄²⁻: $4 O H^{-} - 4 e^{-} \to 2 H_{2} O + O_{2}$ . A colourless gas collects that relights a glowing splint (oxygen).
For every 4 electrons that flow, 2 molecules of H₂ and 1 molecule of O₂ form, so the volumes of hydrogen and oxygen are in the ratio 2 : 1. The overall change is the decomposition of water, $2 H_{2} O \to 2 H_{2} + O_{2}$ ; the sulphuric acid is not used up.

Copper(II) sulphate solution. Ions present: Cu²⁺ and SO₄²⁻ (from the salt), H⁺ and OH⁻ (from water). At the cathode, Cu²⁺ is discharged in preference to H⁺ because copper is below hydrogen in the activity series: $C u^{2 +} + 2 e^{-} \to C u$ , and a reddish-brown layer of copper forms. What happens at the anode depends on the electrode.

Platinum (inert) anode. OH⁻ is discharged in preference to SO₄²⁻, so oxygen is evolved. Cu²⁺ ions are removed and not replaced, so the blue colour fades. H⁺ and SO₄²⁻ ions are left behind, so the solution becomes acidic (dilute sulphuric acid).
Copper (active) anode. Copper atoms lose electrons more readily than any anion, so the anode itself dissolves: $C u - 2 e^{-} \to C u^{2 +}$ . No gas is evolved. Each Cu²⁺ ion deposited at the cathode is replaced by one from the anode, so the blue colour and the concentration stay the same. The anode gets thinner, the cathode thicker, by the same mass. This set-up is the basis of electrorefining and electroplating.

In every case the current in the solution is carried by ions, while electrons flow in the external wires; this is the difference between electrolytic and metallic conduction.

Electrolysis of Acidified Water and Copper(II) Sulphate: formulas and key facts

Acidified water: cathode: $2 H^{+} + 2 e^{-} \to H_{2}$
Acidified water or CuSO₄ with platinum: anode: $4 O H^{-} - 4 e^{-} \to 2 H_{2} O + O_{2}$
Overall: acidified water: $2 H_{2} O \to 2 H_{2} + O_{2}$; Volume ratio H₂ : O₂ = 2 : 1.
CuSO₄: cathode (any electrodes): $C u^{2 +} + 2 e^{-} \to C u$
CuSO₄ with copper: anode: $C u - 2 e^{-} \to C u^{2 +}$
Copper(II) sulphate: platinum versus copper electrodes: Anode product oxygen with platinum; none, the anode dissolves, with copper
Blue colour fades with platinum; unchanged with copper
Solution becomes acidic with platinum; stays the same with copper
Anode mass unchanged with platinum; decreases with copper
Tests for the gases: Hydrogen at the cathode: burns with a pop | Oxygen at the anode: relights a glowing splint

Anode product	oxygen with platinum; none, the anode dissolves, with copper
Blue colour	fades with platinum; unchanged with copper
Solution	becomes acidic with platinum; stays the same with copper
Anode mass	unchanged with platinum; decreases with copper

How to solve Electrolysis of Acidified Water and Copper(II) Sulphate questions

These are describe-and-explain questions. Use the same five headings every time: electrolyte, electrodes, ions, electrode reactions, observations.

1Name the electrolyte and why it is used (acid added to make water conduct).
2State the electrodes and whether they are inert (platinum) or active (copper).
3List the ions from the compound and from water.
4Apply selective discharge: Cu²⁺ before H⁺ at the cathode; OH⁻ before SO₄²⁻ at an inert anode; an active copper anode dissolves instead.
5Write both electrode equations and state what is seen at each electrode and in the solution (colour, gas, mass change).
6For gas volumes, use the electron count: 4e⁻ give 2H₂ and 1O₂.

Electrolysis of Acidified Water and Copper(II) Sulphate in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Volume ratio and gas tests

Suppose a hobbyist electrolyses water acidified with a little dilute sulphuric acid, using platinum wires, and collects 100 cm³ of gas over the cathode.

What volume of gas collects over the anode in the same time, what is it, and how could each gas be identified safely in small test-tube samples?

Try it first, then show the solutionSolution

Cathode gas is hydrogen: $2 H^{+} + 2 e^{-} \to H_{2}$ ; anode gas is oxygen: $4 O H^{-} - 4 e^{-} \to 2 H_{2} O + O_{2}$ .
For 4 electrons, 2 H₂ and 1 O₂ form, so the volume of oxygen is half that of hydrogen: 50 cm³.
A burning splint held to a small sample of hydrogen gives a pop; a glowing splint relights in oxygen.

Answer: 50 cm³ of oxygen; hydrogen pops with a burning splint, oxygen relights a glowing splint.

Memory hook: Water's formula is H₂O, and its gases come out H₂ : O₂ = 2 : 1.

Tricks to remember Electrolysis of Acidified Water and Copper(II) Sulphate

Short memory aids. Say each one aloud once, then use it on a question the same day.

Pt fades, Cu stays
Platinum electrodes: blue fades and the solution turns acidic. Copper electrodes: blue stays because the anode refills the ions.
Use it when: Comparison questions on copper sulphate.
Two to one, like H₂O
Hydrogen at the cathode is double the oxygen at the anode, just as the formula H₂O has two H for one O.
Use it when: Volume ratio in Hofmann's voltameter.
Pop and glow
Hydrogen pops a burning splint; oxygen makes a glowing splint glow into flame.
Use it when: Confirmatory tests for the gases.

Electrolysis of Acidified Water and Copper(II) Sulphate: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Volume ratio · 1 mark · MCQBoard level

In the electrolysis of acidified water using platinum electrodes, the ratio of the volume of gas at the cathode to that at the anode is:
(a) 1 : 1
(b) 1 : 2
(c) 2 : 1
(d) 4 : 1

Try it first, then show the solutionSolution

Cathode: hydrogen; anode: oxygen.
4 electrons give 2H₂ and 1O₂.

Answer: (c) 2 : 1.

Example 2: Why acidify the water? · 2 marks · Short answerBoard level

A student passes a current through distilled water between platinum electrodes and sees hardly any bubbles. After she adds a few drops of dilute sulphuric acid, gas bubbles steadily at both electrodes. (a) Explain the difference. (b) How could she show that the gas at the positive electrode is oxygen?

Try it first, then show the solutionSolution

(a) Distilled water is a very weak electrolyte: it ionises only slightly and conducts poorly. The acid supplies ions, so a current flows and water is electrolysed.
(b) Collect the anode gas in a test tube and insert a glowing splint: it relights.

Answer: (a) The acid provides ions, so the water conducts. (b) The gas relights a glowing splint.

Example 3: Copper sulphate with platinum electrodes · 3 marks · Short answerHarder board level

Aqueous copper(II) sulphate is electrolysed using platinum electrodes. State what is observed (a) at the cathode, (b) at the anode, (c) in the electrolyte after some time. Give the cathode equation.

Try it first, then show the solutionSolution

(a) A reddish-brown deposit of copper: $C u^{2 +} + 2 e^{-} \to C u$ .
(b) Bubbles of a colourless gas (oxygen), from discharge of OH⁻.
(c) The blue colour fades as Cu²⁺ ions are removed, and the solution becomes acidic.

Answer: Copper deposited at the cathode; oxygen at the anode; blue colour fades and the solution turns acidic.

Example 4: Copper sulphate with copper electrodes · 3 marks · Short answerHarder board level

Aqueous copper(II) sulphate is electrolysed with copper electrodes. Write the reaction at each electrode and explain why the colour of the solution does not change.

Try it first, then show the solutionSolution

Cathode: $C u^{2 +} + 2 e^{-} \to C u$ (cathode becomes thicker).
Anode: $C u - 2 e^{-} \to C u^{2 +}$ (anode dissolves and becomes thinner).
Cu²⁺ ions removed at the cathode are replaced at the same rate by ions from the anode, so their concentration, and the blue colour, stay constant.

Answer: Copper deposits at the cathode and dissolves from the anode at the same rate, so the Cu²⁺ concentration and colour are unchanged.

Example 5: Comparing the two electrolyses · 4 marks · Long answerHOTS / competency

Compare the electrolysis of aqueous copper(II) sulphate using (i) platinum electrodes and (ii) copper electrodes under these heads: (a) anode reaction, (b) product at the anode, (c) change in colour of the electrolyte, (d) change in mass of the anode.

Try it first, then show the solutionSolution

(a) Pt: $4 O H^{-} - 4 e^{-} \to 2 H_{2} O + O_{2}$ . Cu: $C u - 2 e^{-} \to C u^{2 +}$ .
(b) Pt: oxygen gas. Cu: no product; copper goes into solution.
(c) Pt: blue colour fades. Cu: no change.
(d) Pt: no change (inert). Cu: anode loses mass.

Answer: Platinum: OH⁻ discharged, O₂ evolved, colour fades, anode mass unchanged. Copper: anode dissolves, no gas, colour unchanged, anode loses mass.

Example 6: Oxygen from the copper deposited · 3 marks · Short answerHOTS / competency

Copper(II) sulphate solution is electrolysed using platinum electrodes until 0.635 g of copper has been deposited. Using the electrode equations, calculate the volume of oxygen released at the anode at S.T.P. (Cu = 63.5)

Try it first, then show the solutionSolution

Moles of Cu $= 0.635/63.5 = 0.01$ mol; each needs 2 electrons, so 0.02 mol of electrons flowed.
At the anode, 4 mol of electrons release 1 mol of O₂: moles of O₂ $= 0.02/4 = 0.005$ mol.
Volume $= 0.005 \times 22400 = 112$ cm³ at S.T.P.

Answer: 112 cm³ of oxygen at S.T.P.

Where students lose marks in Electrolysis of Acidified Water and Copper(II) Sulphate

The slips examiners see most often, and what to write instead.

Mistake: Not explaining why acid is added
In 2025 (Q7(iv)) examiners noted answers that only said water is covalent or polar, without saying the acid provides ions.
Do this: Say: 'Pure water is a poor conductor (very weak electrolyte); dilute sulphuric acid provides ions so that it conducts.'
Usually costs: 1 mark.
Mistake: Unsimplified cathode equation
The same report noted $4 H^{+} + 4 e^{-} \to 2 H_{2}$ written instead of $2 H^{+} + 2 e^{-} \to H_{2}$ .
Wrong working
4H⁺ + 4e⁻ → 2H₂
Right working
2H⁺ + 2e⁻ → H₂
Do this: Write the electrode equation for one molecule of product.
Usually costs: 1 mark.
Mistake: Observation without a test
Candidates wrote only 'O₂ evolved' at the anode with no confirmatory test (2025 Q7(iv)).
Do this: Give what is seen and the test: 'colourless gas bubbles that relight a glowing splint'.
Usually costs: 1 mark.
Mistake: Thinking Cu²⁺ falls with copper electrodes
In 2025 (Q1(xiii)) many forgot that the Cu²⁺ concentration stays constant with copper electrodes, and overlooked the word 'not'.
Do this: Copper anode in, copper cathode out: ion concentration unchanged. Underline 'not' in the stem.
Usually costs: 1 mark.

Exam tips

Learn the four-way comparison (anode reaction, anode product, colour, anode mass) for platinum versus copper electrodes; it is a favourite Section B item.
For observations, mention colour: reddish-brown copper, blue solution fading, colourless gas bubbles.
Say 'platinum electrodes are inert' when asked why they are used.
If a question says 'acidulated water', it means acidified water.

Practice questions on Electrolysis of Acidified Water and Copper(II) Sulphate

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
A student passes a current for an hour through copper(II) sulphate solution between two copper plates. Which observation would she NOT expect?
(a) A fresh reddish-brown layer on the cathode
(b) The anode plate becoming thinner
(c) Bubbles of a colourless gas at the anode
(d) The solution keeping the same shade of blue
Hint
Which is easier to oxidise at a copper anode: OH⁻ ions or the copper itself?
Answer
(c). The copper anode dissolves ( $C u - 2 e^{-} \to C u^{2 +}$ ) instead of OH⁻ being discharged, so no gas is evolved; the anode ions replace those deposited, so the blue colour stays.
Q2 · 1 mark · Very short answerBoard level
Name the apparatus commonly used for the electrolysis of acidified water.
Hint
It has three limbs.
Answer
Hofmann's voltameter.
Q3 · 1 mark · Very short answerBoard level
Why is pure water a poor conductor of electricity?
Hint
How much does it ionise?
Answer
It ionises only very slightly, so it contains very few ions.
Q4 · 2 marks · Short answerHarder board level
Write the cathode reaction in the electrolysis of acidified water, and explain why the volume of hydrogen is twice that of oxygen.
Hint
Count electrons per molecule of each gas.
Answer
$2 H^{+} + 2 e^{-} \to H_{2}$ . Each H₂ needs 2 electrons and each O₂ releases 4, so the same charge gives twice as many H₂ molecules as O₂ molecules, hence twice the volume (Avogadro's law).
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The amount of sulphuric acid does not decrease during the electrolysis of acidified water.
Reason (R): Only water is decomposed; H⁺ ions discharged at the cathode are replaced as OH⁻ ions are discharged at the anode.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Overall equation: 2H₂O → 2H₂ + O₂.
Answer
(a).
Q6 · 2 marks · Short answerHarder board level
During electrolysis of acidified water, 30 cm³ of hydrogen collects at the cathode. What volume of gas collects at the anode at the same temperature and pressure, and what test confirms it?
Hint
Ratio 2 : 1.
Answer
15 cm³ of oxygen; it relights a glowing splint.
Q7 · 2 marks · Short answerHarder board level
Which of the two set-ups, CuSO₄ with platinum electrodes or CuSO₄ with copper electrodes, is the basis of electroplating and refining? Give two reasons.
Hint
Think about the concentration of the bath.
Answer
Copper electrodes. (1) The anode dissolves to replace the ions deposited, so the bath concentration stays constant and can be used for a long time. (2) Metal is transferred from anode to cathode with no gas evolved, so pure metal builds up on the cathode.
Q8 · 3 marks · Short answerHOTS / competency
After electrolysing copper(II) sulphate solution with platinum electrodes, a student finds that the cathode is coated reddish-brown and the remaining solution turns blue litmus red. Explain both observations and name the substance responsible for the second.
Hint
Which ions are left in solution?
Answer
Cu²⁺ ions are discharged at the cathode ( $C u^{2 +} + 2 e^{-} \to C u$ ), giving the reddish-brown copper coat. OH⁻ ions are discharged at the anode, so H⁺ and SO₄²⁻ ions accumulate: the solution now contains sulphuric acid, which turns blue litmus red.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Gas at the anode in acidified water electrolysis?Show answer
Oxygen.
2.Why does the blue colour of CuSO₄ fade with platinum electrodes?Show answer
Cu²⁺ ions are deposited as copper and not replaced.
3.Anode reaction with a copper anode in CuSO₄?Show answer
$C u - 2 e^{-} \to C u^{2 +}$ .
4.Volume of O₂ if 40 cm³ of H₂ is collected?Show answer
20 cm³.
5.Which acid accumulates when CuSO₄ is electrolysed with platinum electrodes?Show answer
Sulphuric acid.

Electrolysis of Acidified Water and Copper(II) Sulphate: frequently asked questions

Why is dilute sulphuric acid added to water for electrolysis?

Pure water contains very few ions and conducts poorly. The acid provides ions so that a current can pass; it is not itself used up.

Why are platinum electrodes used for acidified water?

Platinum is inert: it conducts electricity without reacting with the electrolyte or the oxygen formed.

What is the difference between electrolysing CuSO₄ with platinum and with copper electrodes?

With platinum, OH⁻ is discharged at the anode (oxygen), and the solution loses its colour and becomes acidic. With copper, the anode dissolves, no gas forms, and the colour stays the same.

Why is the volume of hydrogen double that of oxygen?

Two electrons make one H₂ molecule but four electrons are released for each O₂ molecule, so the same current gives twice as many hydrogen molecules.