Electrolysis ICSE Class 10: Selective Discharge, Electrode Reactions and Electroplating

What Electrolysis is about

Weightage: CISCE publishes no chapter or unit weightage for ICSE Chemistry. Electrolysis is in the 6. Electrolysis part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2027

Key concepts

Electrolytes, non-electrolytes; strong and weak

An electrolyte is a compound which, in the molten state or in aqueous solution, conducts electricity and is decomposed by it. A non-electrolyte does not conduct in either state.

• Ions only → strong electrolyte: almost completely ionised or dissociated. NaCl, NaOH, KOH, dilute HCl, HNO₃, H₂SO₄, CuSO₄, PbBr₂ (molten).
• Molecules and ions → weak electrolyte: only partly ionised. Acetic acid, carbonic acid, ammonium hydroxide (and pure water, very slightly).
• Molecules only → non-electrolyte: sugar solution, alcohol, kerosene, carbon tetrachloride.

Metals conduct by moving electrons and are not decomposed; electrolytes conduct by moving ions and are decomposed.

Worked example

Classify as strong electrolyte, weak electrolyte or non-electrolyte: dilute sulphuric acid, ammonium hydroxide, sugar solution, molten sodium chloride.

Try it first, then show the solutionSolution

1. Dilute H₂SO₄ is almost fully ionised: strong.
2. NH₄OH is partly ionised (molecules + ions): weak.
3. Sugar solution has molecules only: non-electrolyte.
4. Molten NaCl has ions only: strong.

Answer: Strong, weak, non-electrolyte, strong.

Terms: electrodes, ions, oxidation and reduction

• Electrode: a conductor through which current enters or leaves the electrolyte. Anode: connected to the positive terminal. Cathode: connected to the negative terminal.
• Cation: positive ion; moves to the cathode. Anion: negative ion; moves to the anode.
• At the cathode, cations gain electrons: reduction. At the anode, anions lose electrons: oxidation.

$C{u}^{2+}+2e^{-}\to Cu$ (reduction at the cathode)

$2C{l}^{-}-2e^{-}\to C{l}_2$ (oxidation at the anode)

The anode itself does not 'lose electrons' on its own; the anions that reach it lose electrons to it (or, with an active anode such as copper, the anode metal atoms lose electrons and enter the solution).

Worked example

Justify: oxidation always occurs at the anode.

Try it first, then show the solutionSolution

1. The anode is positive, so anions migrate to it.
2. There they give up electrons to the anode (or an active anode's atoms lose electrons).
3. Loss of electrons is oxidation.

Answer: Anions (or anode atoms) lose electrons at the anode, which is oxidation.

Selective discharge of ions

When an electrolyte contains more than one kind of cation (or anion), only one is usually discharged at each electrode. This is selective (preferential) discharge, decided mainly by:

1. Position in the activity series: the lower a metal is in the series, the more easily its ion is discharged. Order of ease of discharge of cations (increasing): K⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺, Zn²⁺, Fe²⁺, Pb²⁺, H⁺, Cu²⁺, Ag⁺. So in water, Cu²⁺ is discharged rather than H⁺, but H⁺ rather than Na⁺.
Anions: SO₄²⁻ and NO₃⁻ are the hardest to discharge; OH⁻ and the halide ions (Cl⁻, Br⁻, I⁻) are discharged far more easily. So OH⁻ is discharged rather than SO₄²⁻ or NO₃⁻, and a halide ion is discharged instead of OH⁻ when it is much more concentrated (factor 2).
2. Concentration: a much more concentrated ion may be discharged even if it is a little higher in the series.
3. Nature of the electrodes: an active anode (copper in CuSO₄) itself dissolves instead of any anion being discharged.

Worked example

Aqueous copper(II) sulphate contains Cu²⁺, H⁺, SO₄²⁻ and OH⁻ ions. With platinum electrodes, which ions are discharged at each electrode?

Try it first, then show the solutionSolution

1. Cations at the cathode: Cu²⁺ is below H⁺ in the activity series, so Cu²⁺ is discharged.
2. Anions at the anode: OH⁻ is discharged more easily than SO₄²⁻.
3. Platinum is inert, so it does not dissolve.

Answer: Cu²⁺ at the cathode; OH⁻ at the anode.

Electrolysis of molten lead bromide

• Electrolyte: lead(II) bromide, heated until molten (solid PbBr₂ does not conduct because its ions are not free).
• Electrodes: graphite (inert; not attacked by bromine). Vessel: silica crucible.
• Ions: $PbB{r}_2\to P{b}^{2+}+2B{r}^{-}$
• Cathode: $P{b}^{2+}+2e^{-}\to Pb$, silvery grey lead metal collects.
• Anode: $B{r}^{-}-e^{-}\to Br$, then $Br+Br\to B{r}_2$: reddish brown vapour of bromine.

The bulb lights only when the bromide melts. The electrolysis should be done in a fume cupboard because bromine vapour is poisonous.

Worked example

Why does the bulb in a lead bromide circuit glow only after the crucible is heated?

Try it first, then show the solutionSolution

1. Solid PbBr₂ is ionic, but its ions are locked in the crystal.
2. On melting, Pb²⁺ and Br⁻ become free to move and carry the current.

Answer: Ions become mobile only in the molten state.

Electrolysis of acidified water (platinum electrodes)

• Electrolyte: water with a little dilute sulphuric acid. Pure water is a very weak electrolyte (it ionises only slightly), so the acid is added to provide enough ions to conduct.
• Electrodes: platinum (inert). Apparatus: Hofmann's voltameter.
• Ions: H⁺, OH⁻ (from water) and H⁺, SO₄²⁻ (from the acid).
• Cathode: $H^{+}+e^{-}\to H$, then $H+H\to H_2$ (or $2H^{+}+2e^{-}\to H_2$).
• Anode: OH⁻ is discharged in preference to SO₄²⁻: $4O{H}^{-}-4e^{-}\to 2H_{2}O+O_2$.
• Observation: hydrogen and oxygen collect in the volume ratio 2 : 1. Hydrogen burns with a pop; oxygen relights a glowing splint.

Overall, only water is decomposed: $2H_{2}O\to 2H_2+O_2$. The acid is not used up.

Worked example

Why is the volume of hydrogen at the cathode twice that of oxygen at the anode?

Try it first, then show the solutionSolution

1. To release one O₂ molecule, 4 electrons are given up by 4 OH⁻ ions.
2. Those 4 electrons discharge 4 H⁺ ions, giving 2 H₂ molecules.
3. By Avogadro's law, 2 molecules : 1 molecule = 2 volumes : 1 volume.

Answer: Equal numbers of electrons give 2 H₂ for every O₂.

Aqueous copper(II) sulphate: platinum versus copper electrodes

Ions present in both cases: $CuS{O}_4\to C{u}^{2+}+S{O}_4^{2-}$ and $H_{2}O\rightleftharpoons H^{+}+O{H}^{-}$.

With platinum (inert) electrodes:

• Cathode: $C{u}^{2+}+2e^{-}\to Cu$, reddish brown copper is deposited.
• Anode: $4O{H}^{-}-4e^{-}\to 2H_{2}O+O_2$, oxygen gas is evolved.
• The blue colour fades as Cu²⁺ ions are removed, and the solution becomes acidic (H⁺ and SO₄²⁻ are left, i.e. sulphuric acid).

With copper (active) electrodes:

• Cathode: $C{u}^{2+}+2e^{-}\to Cu$, the cathode grows thicker.
• Anode: $Cu-2e^{-}\to C{u}^{2+}$, the copper anode dissolves (becomes thinner); no gas is evolved.
• The blue colour stays the same, because each Cu²⁺ removed at the cathode is replaced by one from the anode. This is the basis of refining and plating.

Worked example

Copper(II) sulphate solution is electrolysed with copper electrodes. State what happens to (a) the mass of the anode, (b) the colour of the solution.

Try it first, then show the solutionSolution

1. The anode copper atoms lose electrons and go into solution as Cu²⁺: the anode loses mass.
2. The same number of Cu²⁺ ions is deposited at the cathode, so the concentration of Cu²⁺ is unchanged.

Answer: (a) Decreases (b) Remains blue (no change).

Electroplating with nickel and silver; refining copper

Electroplating is coating a metal object with a thin layer of another metal by electrolysis, to prevent corrosion and to give an attractive finish. Rules: the object to be plated is the cathode; the anode is the plating metal; the electrolyte contains ions of the plating metal; use a low current for a longer time (direct current) and a clean, grease-free object, so the coating is smooth and firmly attached.

	Nickel plating	Silver plating
Electrolyte	nickel sulphate solution	sodium argentocyanide, Na[Ag(CN)₂], solution
Cathode	article	article
Anode	pure nickel	pure silver
Cathode reaction	$N{i}^{2+}+2e^{-}\to Ni$	$A{g}^{+}+e^{-}\to Ag$
Anode reaction	$Ni-2e^{-}\to N{i}^{2+}$	$Ag-e^{-}\to A{g}^{+}$

Sodium argentocyanide is preferred to silver nitrate because it releases Ag⁺ ions slowly and steadily, giving an even, adherent coat; with silver nitrate, deposition is too fast and the coating is uneven and flaky.

Electro-refining of copper: electrolyte is acidified copper(II) sulphate solution; anode: impure copper (thick block); cathode: thin sheet of pure copper. Anode: $Cu-2e^{-}\to C{u}^{2+}$; cathode: $C{u}^{2+}+2e^{-}\to Cu$. Pure copper builds up on the cathode; insoluble impurities fall below the anode as anode mud.

Worked example

An iron spoon is to be nickel plated. Name the electrolyte, the cathode and the anode, and give the cathode reaction.

Try it first, then show the solutionSolution

1. Electrolyte must contain Ni²⁺: nickel sulphate solution.
2. Object to be plated = cathode: the iron spoon.
3. Anode = pure nickel block.
4. Cathode: $N{i}^{2+}+2e^{-}\to Ni$.

Answer: Nickel sulphate solution; spoon as cathode; nickel anode; Ni²⁺ + 2e⁻ → Ni.

Chemistry in real life: Electrolysis

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.

Topics in this chapter

• Electroplating and Electrorefining
• Selective Discharge of Ions
• Electrolysis of Acidified Water and Copper(II) Sulphate

Electrolysis formula sheet

Molten lead bromide

$PbB{r}_2\to P{b}^{2+}+2B{r}^{-}$

Cathode: Pb²⁺ + 2e⁻ → Pb; anode: 2Br⁻ - 2e⁻ → Br₂.

Acidified water: cathode

$2H^{+}+2e^{-}\to H_2$

Hydrogen at the cathode.

Acidified water / CuSO₄ (Pt): anode

$4O{H}^{-}-4e^{-}\to 2H_{2}O+O_2$

OH⁻ discharged in preference to SO₄²⁻.

Copper deposition

$C{u}^{2+}+2e^{-}\to Cu$

Cathode with either electrode type.

Copper anode dissolving

$Cu-2e^{-}\to C{u}^{2+}$

Active copper anode; colour unchanged.

Overall decomposition of water

$2H_{2}O\to 2H_2+O_2$

H₂ : O₂ = 2 : 1 by volume.

Electrolysis: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the board asks Electrolysis

The question patterns to expect, and the method that works for each.

Tricks to remember Electrolysis

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

• OIL RIG at AN OX and RED CAT

  Oxidation Is Loss, Reduction Is Gain; ANode = OXidation, REDuction = CAThode.

  Use it when: Assigning oxidation and reduction.

• PANIC

  Positive Anode, Negative Is Cathode.

  Use it when: Electrode signs.

• Plate what you place at the cathode

  The thing that gets coated always hangs at the cathode; the metal that gives up atoms is the anode.

  Use it when: Electroplating and refining set-ups.

• Copper anode keeps the blue

  Copper electrodes: blue stays. Platinum electrodes: blue fades (and the solution turns acidic).

  Use it when: Colour change MCQs.

Electrolysis: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Electrolysis

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

• ✗Mistake: Placing the article at the cathode without a reason

  CISCE examiners' comments, 2024: in a nickel-plating diagram question, candidates said the object goes at the cathode without saying that Ni²⁺ ions are reduced and deposited there, and some wrote the cathode equation in reverse or with the wrong charge.

  Wrong working

  Ni → Ni²⁺ + 2e⁻ at the cathode

  Right working

  Cathode: $N{i}^{2+}+2e^{-}\to Ni$ (reduction, metal deposited on the article)

  ✓Do this: Cathode = gain of electrons = metal deposited; write it that way round.

  Usually costs: 1-2 marks.

• ✗Mistake: Unsimplified electrode equation, no gas test, no reason for the acid

  CISCE examiners' comments, 2025: for acidulated water, candidates wrote 4H⁺ + 4e⁻ → 2H₂ instead of the expected 2H⁺ + 2e⁻ → H₂, gave no test for oxygen, and only said water is covalent without explaining why acid is added.

  Wrong working

  Water is acidified because it is covalent.

  Right working

  Pure water has too few ions to conduct; the acid supplies ions.

  ✓Do this: Write the simplest balanced electrode equation and always give a gas test.

  Usually costs: 1 mark each.

• ✗Mistake: Saying the anode loses electrons

  CISCE examiners' comments, 2025: candidates said the anode loses electrons, rather than anions migrating to it and losing electrons, and defined oxidation by oxygen instead of electrons.

  Wrong working

  Oxidation occurs at the anode because the anode loses electrons.

  Right working

  Anions migrate to the anode and lose electrons there; loss of electrons is oxidation.

  ✓Do this: Use the electron definition of oxidation and reduction.

  Usually costs: 1 mark.

• ✗Mistake: Thinking copper ions are used up with copper electrodes

  CISCE examiners' comments, 2025: candidates forgot that the Cu²⁺ concentration stays constant with copper electrodes, and overlooked 'NOT' in the question.

  Wrong working

  The blue colour fades with copper electrodes.

  Right working

  The blue colour stays: the copper anode replaces the ions deposited.

  ✓Do this: Circle 'NOT' in MCQs; copper anode keeps the blue.

  Usually costs: 1 mark.

• ✗Mistake: Wrong plating conditions

  CISCE examiners' comments, 2024: candidates wrote 'large current' or 'no change at the anode' for electroplating.

  Wrong working

  Use a high current so plating is quick.

  Right working

  Use a low current for a longer time; the anode dissolves to replace the ions.

  ✓Do this: Low current, long time, clean object.

  Usually costs: 1 mark.

Practice questions on Electrolysis

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

1. Q1 · 1 mark · MCQBoard level

   Which of these contains only molecules?
   (a) Dilute hydrochloric acid
   (b) Ammonium hydroxide solution
   (c) Carbon tetrachloride
   (d) Molten sodium chloride

   Hint

   Non-electrolyte.

   Answer

   (c)

2. Q2 · 1 mark · MCQBoard level

   During electrolysis, reduction takes place at the:
   (a) anode, by loss of electrons
   (b) cathode, by gain of electrons
   (c) anode, by gain of electrons
   (d) cathode, by loss of electrons

   Hint

   RED CAT.

   Answer

   (b)

3. Q3 · 1 mark · MCQHarder board level

   Statement I: In the electrolysis of acidified water, the volume of gas at the anode is twice that at the cathode.
   Statement II: Oxygen is liberated at the anode.
   (a) Both true
   (b) Both false
   (c) I true, II false
   (d) I false, II true

   Hint

   Which gas is 2 volumes?

   Answer

   (d): the cathode (hydrogen) has twice the volume.

4. Q4 · 1 mark · Assertion-reasonHarder board level

   Assertion (A): In silver plating, the article is made the anode.
   Reason (R): Oxidation takes place at the anode.
   (a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
   (b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
   (c) (A) is true and (R) is false.
   (d) (A) is false and (R) is true.

   Hint

   Where is silver deposited?

   Answer

   (d)

5. Q5 · 2 marks · Short answerBoard level

   Give reasons: (a) Graphite electrodes are used in the electrolysis of molten lead bromide. (b) A low current is used for a long time in electroplating.

   Hint

   Inertness; smooth deposit.

   Answer

   (a) Graphite is inert and is not attacked by bromine. (b) Slow deposition gives an even, firmly adhering coat.

6. Q6 · 3 marks · Short answerHarder board level

   Write the electrode reactions for the electrolysis of aqueous copper(II) sulphate with copper electrodes, and state one observation at each electrode.

   Hint

   Anode dissolves.

   Answer

   Cathode: $C{u}^{2+}+2e^{-}\to Cu$, reddish brown copper deposits (cathode thickens). Anode: $Cu-2e^{-}\to C{u}^{2+}$, anode becomes thinner. No gas evolved.

7. Q7 · 3 marks · Short answerHarder board level

   Arrange in increasing ease of discharge: (a) Cu²⁺, Na⁺, H⁺, Fe²⁺ (b) OH⁻, SO₄²⁻, NO₃⁻. Use (a) to explain why sodium cannot be obtained by electrolysing aqueous sodium chloride.

   Hint

   Activity series.

   Answer

   (a) Na⁺ < Fe²⁺ < H⁺ < Cu²⁺ (b) SO₄²⁻ < NO₃⁻ < OH⁻. H⁺ (from water) is discharged before Na⁺, so hydrogen forms at the cathode instead of sodium.

8. Q8 · 2 marks · Short answerHOTS / competency

   In an electro-refining cell, the cathode gains 6.35 g while the anode loses 7.0 g. Explain the difference and state where the extra material goes.

   Hint

   Impure anode.

   Answer

   The impure anode also loses impurities. Insoluble ones fall as anode mud; some soluble metal impurities stay in solution. Only copper is deposited on the cathode.

9. Q9 · 3 marks · Short answerHOTS / competency

   A student plates an iron nail with copper using copper(II) sulphate solution but connects the nail to the positive terminal. Predict what happens and correct the set-up.

   Hint

   The nail becomes the anode.

   Answer

   The nail is the anode, so iron atoms lose electrons and dissolve (Fe → Fe²⁺ + 2e⁻); no copper coating forms on it and the nail corrodes. Connect the nail to the negative terminal (cathode) and a copper plate to the positive terminal.

10. Q10 · 2 marks · Very short answerBoard level

    Name: (a) a liquid that is a non-electrolyte, (b) the ion discharged at the anode in electrolysis of CuSO₄ with Pt electrodes, (c) the electrolyte used in electro-refining of copper, (d) the gas at the cathode with acidified water.

    Hint

    Half a mark each.

    Answer

    (a) Kerosene or CCl₄ (b) OH⁻ (c) Acidified copper(II) sulphate solution (d) Hydrogen

11. Q11 · 2 marks · Short answerHarder board level

    Explain the difference between conduction in copper metal and in molten lead bromide, stating what moves and whether a chemical change occurs.

    Hint

    Electrons versus ions.

    Answer

    Copper conducts by movement of free electrons with no chemical change. Molten PbBr₂ conducts by movement of ions, which are discharged at the electrodes: the compound decomposes into lead and bromine.

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. 1.Define electrolysis.Show answer

   The decomposition of an electrolyte, in the molten state or in solution, by passing an electric current through it.

2. 2.Which ions move to the cathode?Show answer

   Cations (positive ions).

3. 3.Oxidation in terms of electrons?Show answer

   Loss of electrons.

4. 4.Electrolyte for nickel plating?Show answer

   Nickel sulphate solution.

5. 5.Electrolyte for silver plating?Show answer

   Sodium argentocyanide solution.

6. 6.Product at the anode, molten PbBr₂?Show answer

   Bromine (reddish brown vapour).

7. 7.Volume ratio H₂ : O₂ from acidified water?Show answer

   2 : 1.

8. 8.Anode in electro-refining of copper?Show answer

   Impure copper.

9. 9.Example of a substance with molecules only?Show answer

   Sugar solution, alcohol, kerosene or CCl₄.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give one example each of a strong electrolyte, a weak electrolyte and a non-electrolyte?
• ☐Can you write both electrode reactions for molten PbBr₂, acidified water and CuSO₄ with Pt and with Cu electrodes?
• ☐Can you explain why OH⁻ is discharged rather than SO₄²⁻, and Cu²⁺ rather than H⁺?
• ☐Can you set up nickel and silver plating and give the electrolyte, electrodes and reactions?
• ☐Can you explain the colour change (or none) in CuSO₄ electrolysis?

Electrolysis: frequently asked questions