ChemistryICSE Class 8 · Hydrogen

Preparation of Hydrogen: Electrolysis, Zinc and Dilute Acid, and Bosch's Process (ICSE Class 8 Chemistry)

Updated 17 October 2026 · Fact-checked

In the laboratory, hydrogen is made from granulated zinc and dilute hydrochloric or sulphuric acid (Zn + 2HCl → ZnCl₂ + H₂), collected by downward displacement of water; dilute nitric acid is not used because it oxidises hydrogen. Electrolysis of acidified water gives H₂ at the cathode and O₂ at the anode (2 : 1 by volume); Bosch's process uses steam and red-hot coke.

Understand Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process

Electrolysis of water. Terms: electrolysis is the decomposition of a compound by passing electricity through it (molten or in solution); an electrolyte conducts and is decomposed; electrodes carry current in and out (the anode is joined to the positive terminal, the cathode to the negative).

Pure water conducts poorly, so a little dilute sulphuric acid is added (acidified water). On passing current:

hydrogen collects at the cathode; oxygen at the anode;
volume of hydrogen $=$ twice the volume of oxygen.

$2 H_{2} O electricity 2 H_{2} + O_{2}$

Laboratory preparation. Granulated zinc in a flask with a thistle funnel and delivery tube; dilute acid added through the funnel.

$Z n + 2 H C l \to Z n C l_{2} + H_{2}$ and $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$

Thistle funnel must dip below the acid or the gas escapes.
Apparatus must be airtight: a hydrogen-air mixture explodes on lighting.
Collected by downward displacement of water (sparingly soluble; the jar is seen to fill and the gas is purer).
Let the first bubbles escape (they contain air). Purity test: pure hydrogen burns quietly with a soft pop; a mixture with air gives a loud pop.

Why zinc? Sodium and potassium react violently (even with water); calcium too vigorously and costly; magnesium quickly and costly; iron slowly and impure; lead forms an insoluble coating; copper is below hydrogen and does not react. Zinc reacts at a steady, moderate rate, is cheap and gives fairly pure hydrogen.

Why not dilute nitric acid? It is an oxidising agent: it oxidises the hydrogen formed to water and is itself reduced to oxides of nitrogen, so hardly any hydrogen is obtained. (Concentrated sulphuric acid gives sulphur dioxide.)

Bosch's process (large scale, from steam and coke):
1. Water gas: steam over red-hot coke at about 1000 °C: $C + H_{2} O \to C O + H_{2}$ .
2. Conversion: water gas with more steam over iron(III) oxide catalyst (chromium(III) oxide as promoter) at about 450 °C: $C O + H_{2} + H_{2} O \to C O_{2} + 2 H_{2}$ .
3. Removing CO₂: pass through water under pressure (about 30 atm); CO₂ dissolves, hydrogen does not.
4. Removing CO: pass through ammoniacal copper(I) chloride solution.

Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process: formulas and key facts

Electrolysis: $2 H_{2} O \to 2 H_{2} + O_{2}$; H₂ at the cathode, O₂ at the anode; volumes 2 : 1.
Laboratory method: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$; Or Zn + dilute H₂SO₄; never dilute HNO₃.
Bosch: water gas: $C + H_{2} O \to C O + H_{2}$; About 1000 °C.
Bosch: conversion: $C O + H_{2} + H_{2} O \to C O_{2} + 2 H_{2}$; Fe₂O₃ catalyst, about 450 °C.

How to solve Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process questions

For preparation questions give the reactants, the apparatus reason, the collection method and the test.

1Write the balanced equation.
2State the collection method and why (solubility and density).
3Give precautions: airtight, thistle funnel below acid, first jar discarded.
4Give the purity test (soft pop).
5For electrolysis, name the electrodes and use the 2 : 1 volume ratio.

Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process 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.

Fuelling a rocket

Suppose a space-research unit makes hydrogen by electrolysis and collects 60 cm³ of oxygen in a test.

What volume of hydrogen collects at the other electrode, and what is the total volume of gas?

Try it first, then show the solutionSolution

H₂ is twice the volume of O₂: $2 \times 60 = 120$ cm³.
Total $= 120 + 60 = 180$ cm³.

Answer: 120 cm³ of hydrogen; 180 cm³ in all.

Memory hook: 2 : 1, hydrogen is double.

The first test tube

Imagine a student lights the first test tube of gas collected from a hydrogen generator and hears a loud squeaky pop.

Explain.

Try it first, then show the solutionSolution

The first gas contains air from the apparatus mixed with hydrogen.
A hydrogen-air mixture explodes with a loud pop; pure hydrogen burns with a soft pop.

Answer: The gas is an air-hydrogen mixture; discard the first jar.

Memory hook: First bubbles are impure.

Tricks to remember Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process

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

Cathode = hydrogen
Cathode gets Hydrogen: "C" and "H" together; anode gets oxygen.
Use it when: Electrolysis.
Zinc is Goldilocks
Not too fast (Na, K, Ca, Mg), not too slow (Fe), not inert (Cu).
Use it when: Choice of metal.
Nitric: no
Nitric acid oxidises: it eats the hydrogen.
Use it when: Choice of acid.

Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process: solved examples

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

Example 1: Volumes in electrolysis · 1 mark · MCQBoard level

In the electrolysis of acidified water 40 cm³ of hydrogen is collected. The volume of oxygen collected at the same time is:
(a) 80 cm³
(b) 40 cm³
(c) 20 cm³
(d) 10 cm³

Try it first, then show the solutionSolution

H₂ : O₂ = 2 : 1, so oxygen $= \frac{40}{2} = 20$ cm³.

Answer: (c) 20 cm³

Example 2: Why acidified? · 2 marks · Short answerBoard level

Why is a little dilute sulphuric acid added to water in electrolysis? What is the ratio of the volumes of hydrogen and oxygen?

Try it first, then show the solutionSolution

Pure water conducts electricity very poorly; the acid makes it a better conductor. (1)
Hydrogen : oxygen $= 2 : 1$ . (1)

Answer: To conduct; 2 : 1

Example 3: Why zinc? · 3 marks · Short answerHarder board level

Give reasons for not using each in the laboratory preparation of hydrogen: (a) sodium, (b) copper, (c) lead. State why zinc is suitable.

Try it first, then show the solutionSolution

(a) Reacts violently with water and acid. (¾)
(b) Below hydrogen, so it does not react with dilute acid. (¾)
(c) Forms an insoluble coating that stops the reaction. (¾)
Zinc reacts at a steady rate and is cheap. (¾)

Answer: (a) violent (b) no reaction (c) coating

Example 4: Bosch's process · 4 marks · Long answerHarder board level

Hydrogen is made on a large scale from steam and coke. (a) Write the equations of the first two steps. (b) Name the catalyst of step 2. (c) How are carbon dioxide and carbon monoxide removed afterwards?

Try it first, then show the solutionSolution

(a) $C + H_{2} O \to C O + H_{2}$ (1); $C O + H_{2} + H_{2} O \to C O_{2} + 2 H_{2}$ (1).
(b) Iron(III) oxide (with chromium(III) oxide as promoter). (1)
(c) CO₂ dissolves in water under pressure; CO is absorbed by ammoniacal copper(I) chloride. (1)

Answer: See the working.

Example 5: The generator · 4 marks · Case-basedHOTS / competency

A student sets up granulated zinc and dilute hydrochloric acid with a thistle funnel. (a) Why must the funnel dip below the acid? (b) Why collect over water? (c) What happens if a flame is brought to the first jar? (d) Why is dilute nitric acid unsuitable? (e) Write the equation.

Try it first, then show the solutionSolution

(a) Otherwise the gas escapes through the funnel. (1)
(b) Hydrogen is sparingly soluble; the jar fills visibly and the gas is purer. (½)
(c) A loud squeaky pop, because air is mixed in. (½)
(d) It is an oxidising agent: it oxidises the hydrogen to water. (1)
(e) $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ . (1)

Answer: (a) gas escapes (b) sparingly soluble (c) loud pop (d) oxidises (e) as above

Where students lose marks in Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process

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

Mistake: Swapping the electrodes
Hydrogen at the cathode.
Wrong working
"Hydrogen collects at the anode."
Right working
"Hydrogen collects at the cathode."
Do this: Hydrogen: negative electrode.
Mistake: Wrong volume ratio
H₂ is twice O₂.
Wrong working
"1 : 2 hydrogen to oxygen."
Right working
"2 : 1 hydrogen to oxygen."
Do this: Link to H₂O: 2 H atoms.
Mistake: Using nitric acid
It oxidises the hydrogen.
Wrong working
"Zn + dilute HNO₃ gives hydrogen."
Right working
"Dilute HNO₃ is not used."
Do this: Quote the reason.

Exam tips

Draw the apparatus and label thistle funnel, flask, delivery tube, trough, jar.
Give reasons for every precaution.
Quote the two Bosch equations with conditions.
Remember the first jar is discarded.

Practice questions on Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process

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 · Very short answerBoard level
Name the two electrodes in the electrolysis of water and the gas at each.
Hint
Positive and negative.
Answer
Cathode (negative): hydrogen. Anode (positive): oxygen.
Q2 · 2 marks · Short answerHarder board level
In an electrolysis experiment 30 cm³ of oxygen is collected. Find the volume of hydrogen and the total volume of gas.
Hint
Ratio 2 : 1.
Answer
Hydrogen $= 2 \times 30 = 60$ cm³; total $= 90$ cm³.
Q3 · 3 marks · Short answerHarder board level
Why is dilute nitric acid not used with zinc to make hydrogen? What about concentrated sulphuric acid?
Hint
Oxidising acids.
Answer
Dilute nitric acid oxidises the hydrogen formed to water and is reduced to nitrogen oxides, so little hydrogen is obtained. Concentrated sulphuric acid gives sulphur dioxide instead of hydrogen.
Q4 · 3 marks · Short answerHarder board level
Write the balanced equation for zinc with dilute sulphuric acid and state how the gas is collected, with the reason.
Hint
Solubility.
Answer
$Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$ . Collected by downward displacement of water because hydrogen is sparingly soluble in water.
Q5 · 4 marks · Long answerHOTS / competency
Magnesium and iron both react with dilute acids. Give one reason against using each for the laboratory preparation, and say why hydrogen cannot be made from copper and dilute acid.
Hint
Compare rates, cost and position in the series.
Answer
Magnesium reacts too quickly and is costly. Iron reacts slowly and gives impure hydrogen (it contains sulphur and other impurities). Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from dilute acids.

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 cathode?Show answer
Hydrogen.
2.Volume ratio H₂ : O₂?Show answer
2 : 1.
3.Why is zinc used?Show answer
Steady, moderate rate; cheap.
4.What is water gas?Show answer
CO + H₂.
5.Why is nitric acid avoided?Show answer
It is an oxidising agent.

Preparation of Hydrogen: Electrolysis, Laboratory Method and Bosch's Process: frequently asked questions

How is hydrogen prepared in the laboratory?

By reacting granulated zinc with dilute hydrochloric or sulphuric acid, collecting the gas by downward displacement of water.

Why is dilute nitric acid not used to prepare hydrogen?

It is an oxidising agent that oxidises the hydrogen to water and itself forms oxides of nitrogen.

What is Bosch's process?

A large-scale method: steam over red-hot coke gives water gas (CO and H₂); the CO is converted to CO₂ with steam over a catalyst; CO₂ and remaining CO are removed to leave hydrogen.