Electrolysis

Q: Arrange Fe2+, Na+, Cu2+ and Mg2+ in increasing order of ease of discharge at the cathode, and give a reason.

Na+ < Mg2+ < Fe2+ < Cu2+. The lower a metal is in the activity series, the less its tendency to stay as an ion, so the more readily its ion accepts electrons.

Q: Why is lead bromide electrolysed in the molten state and in a fume cupboard?

Solid lead bromide has no free ions, so it must be melted to conduct. Bromine vapour formed at the anode is poisonous, so it must be removed safely.

ICSE Class 10 · Chemistry · Chapter 6

What this chapter covers

Electrolytes and non-electrolytes
Substances containing molecules only, ions only, or both
Terms of electrolysis
Migration and selective discharge of ions
Electrolysis of molten lead bromide
Electrolysis of acidified water
Electrolysis of aqueous copper(II) sulphate
Electroplating with nickel and silver
Electrorefining of copper

Key definitions

An electrolyte is a compound which, in the molten state or in aqueous solution, conducts electricity and is decomposed chemically by the current passing through it.

A non-electrolyte is a compound which does not conduct electricity either in the molten state or in aqueous solution, because it does not contain free ions.

Dissociation is the separation of the ions already present in an ionic compound when it is melted or dissolved in water.

Ionisation is the formation of ions from a polar covalent compound, which contains no ions, when it is dissolved in water.

A strong electrolyte is an electrolyte which is almost completely dissociated or ionised in the molten state or in aqueous solution, so that it contains ions only and conducts electricity well.

A weak electrolyte is an electrolyte which is only partially ionised in aqueous solution, so that its solution contains both ions and un-ionised molecules and conducts electricity poorly.

Electrolysis is the process of decomposition of a chemical compound, in the molten state or in aqueous solution, by the passage of a direct electric current through it, the products appearing at the electrodes.

An electrolytic cell is the vessel containing the electrolyte and the two electrodes in which electrolysis is carried out.

An electrode is a rod or plate of a conductor, usually a metal or graphite, through which the current enters or leaves the electrolyte.

The anode is the electrode connected to the positive terminal of the battery; electrons leave the electrolyte through it, and anions move towards it.

The cathode is the electrode connected to the negative terminal of the battery; electrons enter the electrolyte through it, and cations move towards it.

A cation is a positively charged ion, formed when an atom or group of atoms loses one or more electrons, which moves towards the cathode during electrolysis.

Key points

Electrolytes conduct and are decomposed when molten or in aqueous solution; non-electrolytes contain no free ions and do not conduct.
Strong electrolytes contain ions only, weak electrolytes contain ions and molecules, and non-electrolytes contain molecules only.
Dissociation frees ions already present in an ionic compound; ionisation creates ions from a polar covalent compound in water.
The anode is joined to the positive terminal and attracts anions; the cathode is joined to the negative terminal and attracts cations.
Oxidation (loss of electrons) occurs at the anode; reduction (gain of electrons) occurs at the cathode.
Selective discharge depends on the activity series, ion concentration and the nature of the electrodes; for Na, Mg, Fe and Cu, Cu²⁺ is discharged most easily and Na⁺ least easily.
OH^- is discharged in preference to SO₄^2- and NO₃^- from dilute solutions.
Molten lead bromide with graphite electrodes gives lead at the cathode and bromine at the anode.
Acidified water with platinum electrodes gives hydrogen and oxygen in the volume ratio 2 : 1; the acid is not used up.
Copper(II) sulphate with platinum electrodes gives copper and oxygen, and the solution loses its colour and turns acidic; with copper electrodes copper moves from anode to cathode and the solution is unchanged.
In electroplating the article is the cathode, the plating metal is the anode, and a salt of the plating metal is the electrolyte; nickel uses nickel sulphate and silver uses sodium argentocyanide.
Electrorefining of copper uses an impure copper anode, a pure copper cathode and acidified copper(II) sulphate; silver and gold collect as anode mud.

Questions answered

Define an electrolyte and give two examples of strong electrolytes.

An electrolyte is a compound which, in the molten state or in aqueous solution, conducts electricity and is decomposed by it. Examples: sodium chloride solution, dilute sulphuric acid.

Why does solid sodium chloride not conduct electricity, though its aqueous solution does?

In the solid, the ions are held in fixed positions in the crystal lattice and cannot move. In solution they are free to move and carry the current.

Classify into strong electrolyte, weak electrolyte and non-electrolyte: acetic acid, kerosene, potassium hydroxide, ammonium hydroxide, glucose solution, nitric acid.

Strong: potassium hydroxide, nitric acid. Weak: acetic acid, ammonium hydroxide. Non-electrolyte: kerosene, glucose solution.

Distinguish between dissociation and ionisation, with one example of each.

Dissociation separates ions already present in an ionic compound (sodium chloride in water). Ionisation forms ions from a polar covalent compound that had none (hydrogen chloride in water).

Explain, in terms of electrons, why oxidation occurs at the anode.

Anions reach the anode and give up their extra electrons to it, and an active anode's atoms also lose electrons there. Loss of electrons is oxidation.

State three factors that affect the selective discharge of ions.

The position of the ion in the activity (electrochemical) series, the concentration of the ions, and the nature of the electrodes.

Arrange Fe²⁺, Na⁺, Cu²⁺ and Mg²⁺ in increasing order of ease of discharge at the cathode, and give a reason.

Na⁺ < Mg²⁺ < Fe²⁺ < Cu²⁺. The lower a metal is in the activity series, the less its tendency to stay as an ion, so the more readily its ion accepts electrons.

Why is lead bromide electrolysed in the molten state and in a fume cupboard?

Solid lead bromide has no free ions, so it must be melted to conduct. Bromine vapour formed at the anode is poisonous, so it must be removed safely.

Past board exam questions

ICSE 2027 sample paper · 1 mark

Which statement given below is correct with reference to the flow of electric current?

In all aqueous solutions, current is carried by electrons.
In all acidic solutions, current is carried by ions.
In molten electrolytes, current is carried by electrons.
In metal wires, current is carried by ions.

Show answer

In all acidic solutions, current is carried by ions.

ICSE 2027 sample paper · 5 marks

The set-up described below shows the electrolysis of acidulated water. With reference to the set-up, answer the following questions: In the set-up, two inverted test tubes stand in a trough of acidified water, each over an electrode. Electrode A is connected to the positive (+) terminal of the battery and electrode B to the negative (−) terminal. Gas collects at the top of each test tube. (a) Write an equation for the reaction that takes place at A. (b) Give a test to confirm the product formed at A. (c) Which gas is liberated at B? (d) Why is water acidified for electrolysis? (e) What is the ratio by volume of the gases liberated at A and B?

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(a) At the anode A: OH⁻ − e⁻ → OH; 4OH → 2H₂O + O₂ (b) The gas (oxygen) rekindles a glowing splinter / turns alkaline pyrogallol brown. (c) Hydrogen (d) Pure water is a very poor conductor of electricity; dilute sulphuric acid is added to provide ions so that water conducts. (e) A : B = 1 : 2 (oxygen : hydrogen)

ICSE 2027 sample paper · 5 marks

Match Column A with Column B: | | Column A | | Column B | |---|---|---|---| | (a) | CuO + C → Cu + CO | 1. | Oxidation | | (b) | Al³⁺ + 3e⁻ → Al | 2. | Ionization | | (c) | PbBr₂ → Pb²⁺ + 2Br¹⁻ | 3. | Reduction | | (d) | 2O²⁻ − 4e⁻ → O₂ | 4. | Redox | | (e) | HCl → H⁺ + Cl¹⁻ | 5. | Electrolytic dissociation |

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(a) – 4. Redox (b) – 3. Reduction (c) – 5. Electrolytic dissociation (d) – 1. Oxidation (e) – 2. Ionization

ICSE 2026 sample paper · 1 mark

Assertion (A): Aqueous solution of potassium chloride can conduct electricity. Reason (R): Conduction of electric current is due to the presence of free ions.

(A) is true and (R) is false.
(A) is false and (R) is true.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Show answer

Both (A) and (R) are true and (R) is the correct explanation of (A).

ICSE 2026 board paper · 1 mark

With respect to the electrolysis of copper (II) sulphate solution using copper electrodes, which statement is correct?

Copper metal is deposited at the negative electrode.
Oxygen gas is produced at the positive electrode.
The positive electrode increases in mass.
The negative electrode decreases in mass.

Show answer

Copper metal is deposited at the negative electrode.

ICSE 2026 sample paper · 1 mark

In the process of Electrorefining of Copper described below, which of the following statements is correct? (A thick anode connected to the positive terminal and a thin cathode connected to the negative terminal of a battery, both dipping in CuSO₄ solution; anode mud collects at the bottom below the anode.)

The anode is made of pure Copper.
The cathode is made of impure Copper.
Copper is deposited at the anode.
Copper ions from the anode move to the cathode and get deposited as pure Copper.

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Copper ions from the anode move to the cathode and get deposited as pure Copper.

ICSE 2026 board paper · 1 mark

Assertion (A): In the electrolysis of acidified water, the volume of hydrogen liberated is twice the volume of oxygen formed. Reason (R): Water has hydrogen and oxygen in the ratio of 1:2 by volume.

(A) is true but (R) is false.
(A) is false but (R) is true.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Show answer

(A) is true but (R) is false.

ICSE 2026 sample paper · 5 marks

Electroplating steel objects with silver involves a three-step process. (Apparatus for step 1: a copper anode connected to the positive terminal and the steel object (cathode) connected to the negative terminal of a battery, both dipped in aqueous copper(II) sulphate electrolyte in a beaker.) step 1 A coating of copper is applied to the object. step 2 A coating of nickel is applied to the object. step 3 The coating of silver is applied to the object. (a) The apparatus used for step 1 is described above. 1. The chemical process taking place on the surface of the object is Cu²⁺(aq) + 2e⁻ → Cu(s). What is the observation seen on the surface of the object? 2. Explain why the concentration of copper ions in the electrolyte remains constant throughout step 1. (b) Give two changes which would be needed in order to coat nickel onto the object in step 2. (c) Write down the reaction taking place at the positive electrode during step 3.

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(a) 1. A reddish-brown (pink) deposit of copper forms on the surface of the object (its mass increases). 2. The copper anode dissolves, Cu → Cu²⁺ + 2e⁻, releasing as many Cu²⁺ ions into the electrolyte as are discharged at the cathode, so the concentration does not change. (b) The anode must be made of nickel, and the electrolyte must be an aqueous nickel salt such as nickel sulphate solution. (c) At the silver anode: Ag → Ag⁺ + e⁻