ChemistryICSE Class 9 · Study of the First Element: Hydrogen

Preparation of Hydrogen: Laboratory Method, Metals with Water, Acids and Alkalis, and the Bosch Process (ICSE Class 9)

Updated 17 October 2026 · Fact-checked

In the laboratory hydrogen is made by granulated zinc and dilute hydrochloric (or sulphuric) acid: Zn + 2HCl → ZnCl₂ + H₂, collected by downward displacement of water. Industrially it is made by the Bosch process: steam over white-hot coke gives water gas (CO + H₂), which with excess steam over an Fe₂O₃ catalyst (Cr₂O₃ promoter) at about 450 °C gives CO₂ + 2H₂.

Understand Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch Process

Sources of hydrogen.

*Cold water:* K, Na, Ca ( $2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$ ; $C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$ ). Violent with K and Na.
*Boiling water:* $M g + 2 H_{2} O \to M g (O H)_{2} + H_{2}$ .
*Steam:* $M g + H_{2} O \to M g O + H_{2}$ ; $2 A l + 3 H_{2} O \to A l_{2} O_{3} + 3 H_{2}$ ; $Z n + H_{2} O \to Z n O + H_{2}$ ; $3 F e + 4 H_{2} O ⇌ F e_{3} O_{4} + 4 H_{2}$ (reversible: a steady stream of steam removes the hydrogen and drives the reaction forward).
*Dilute acids* (metals above hydrogen): $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ ; $F e + H_{2} S O_{4} \to F e S O_{4} + H_{2}$ ; $2 A l + 3 H_{2} S O_{4} \to A l_{2} (S O_{4})_{3} + 3 H_{2}$ .
*Alkalis* (Al, Zn, Pb, warm): $Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$ ; $2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$ . These metals are amphoteric.

Metals not used with dilute acids. K, Na, Ca: too violent. Pb: insoluble coating of PbCl₂ or PbSO₄ stops the reaction. Cu, Hg, Ag, Au: below hydrogen. Dilute nitric acid is an oxidising agent: it oxidises the hydrogen to water and gives nitrogen oxides.

Laboratory preparation. Granulated zinc and dilute HCl (or H₂SO₄) in a flask with a thistle funnel dipping below the acid and a delivery tube. Collected by downward displacement of water because hydrogen is almost insoluble in water; not by displacement of air, since H₂–air mixtures are explosive. Zinc reacts at a moderate rate (impurities in granulated zinc speed it up). Impurities (H₂S, SO₂, CO₂, PH₃, AsH₃, moisture) are removed by lead nitrate (H₂S), silver nitrate (PH₃, AsH₃), potassium hydroxide (SO₂, CO₂) and a drying agent (fused CaCl₂). Test: it burns with a pale blue flame and a pop.

Bosch process.
1. $C + H_{2} O \to C O + H_{2}$ at about 1000 °C (water gas).
2. $C O + H_{2} O \to C O_{2} + H_{2}$ with excess steam, catalyst Fe₂O₃ with Cr₂O₃ promoter, about 450 °C.
3. Separation: CO₂ dissolved in water under pressure or absorbed by KOH ( $C O_{2} + 2 K O H \to K_{2} C O_{3} + H_{2} O$ ); traces of CO in ammoniacal copper(I) chloride.

Overall: $C + 2 H_{2} O \to C O_{2} + 2 H_{2}$ , so 12 kg of carbon gives 4 kg of hydrogen and 44 kg of carbon dioxide.

Mass questions. Balance, take relative masses with coefficients, scale: 65 g Zn → 2 g H₂; 24 g Mg → 2 g H₂; 56 g Fe → 2 g H₂; 54 g Al → 6 g H₂.

Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch Process: formulas and key facts

Laboratory preparation: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$; Granulated zinc; dilute acid.
Zinc with alkali: $Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$; Sodium zincate.
Bosch process: $C + H_{2} O \to C O + H_{2}; C O + H_{2} O \to C O_{2} + H_{2}$; Water gas, then Fe₂O₃/Cr₂O₃ at about 450 °C.
Mass of hydrogen: $65 g Zn \to 2 g H_{2}; 24 g Mg \to 2 g; 56 g Fe \to 2 g$; Zn = 65, Mg = 24, Fe = 56.

How to solve Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch Process questions

Balance first, then scale the masses.

1Choose the correct reactants for the conditions (cold water, steam, acid or alkali).
2Write and balance the equation.
3Take the relative mass of the metal and of the hydrogen with coefficients.
4Scale by (given mass ÷ relative mass of the metal).
5State the observation or reason asked for (pop test, pale blue flame).

Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch 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.

Steam over hot iron

Suppose steam is passed over red-hot iron in a tube.

Why must the steam be passed continuously?

Try it first, then show the solutionSolution

$3 F e + 4 H_{2} O ⇌ F e_{3} O_{4} + 4 H_{2}$ is reversible.
A steady stream of steam carries away the hydrogen, which would otherwise reduce Fe₃O₄ back to iron.

Answer: To remove hydrogen and push the reaction forward.

Memory hook: Remove a product, drive the reaction.

Tricks to remember Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch Process

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

Not too fast, not too slow
Zn is moderate: Na, K, Ca too fast; Cu does not react.
Use it when: Choice of metal.
Amphoteric trio
Al, Zn and Pb react with both acids and alkalis.
Use it when: Alkali reactions.
Water gas then Bosch
C + steam gives CO + H₂; CO + steam gives CO₂ + more H₂.
Use it when: Industrial route.

Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch Process: solved examples

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

Example 1: Collection of hydrogen · 1 mark · MCQBoard level

Hydrogen is collected over water in the laboratory because it is:
(a) heavier than air
(b) almost insoluble in water
(c) very soluble in water
(d) unreactive with metals

Try it first, then show the solutionSolution

Poor solubility allows collection over water; also an air–hydrogen mixture is explosive.

Answer: (b)

Example 2: Two reactions · 2 marks · Short answerBoard level

Write balanced equations for (a) zinc with dilute sulphuric acid and (b) iron with dilute hydrochloric acid, and name the salts.

Try it first, then show the solutionSolution

(a) $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$ (zinc sulphate). (1)
(b) $F e + 2 H C l \to F e C l_{2} + H_{2}$ (iron(II) chloride, ferrous chloride). (1)

Answer: See working

Example 3: Acid and alkali · 3 marks · Short answerHarder board level

13.0 g of zinc (Zn = 65) reacts (a) with excess dilute hydrochloric acid and (b) with excess warm sodium hydroxide solution. Write both equations, find the mass of hydrogen in each case and comment.

Try it first, then show the solutionSolution

(a) $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ : $13.0 \times \frac{2}{65} = 0.4$ g. (1)
(b) $Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$ : also $0.4$ g. (1)
Each atom of zinc displaces one molecule of hydrogen either way; zinc is amphoteric (reacts with acids and alkalis). (1)

Answer: 0.4 g in each case

Example 4: Hydrogen from carbon · 3 marks · Short answerHarder board level

In the Bosch process the two steps are $C + H_{2} O \to C O + H_{2}$ and $C O + H_{2} O \to C O_{2} + H_{2}$ . (a) Add them to get the overall equation. (b) Find the masses of hydrogen and carbon dioxide from 12 kg of carbon (C = 12, O = 16, H = 1).

Try it first, then show the solutionSolution

(a) $C + 2 H_{2} O \to C O_{2} + 2 H_{2}$ . (1)
(b) 12 kg C gives $2 \times 2 = 4$ kg hydrogen. (1)
It also gives 44 kg carbon dioxide. (1)

Answer: C + 2H₂O → CO₂ + 2H₂; 4 kg H₂; 44 kg CO₂

Example 5: Iron and steam · 4 marks · Case-basedHOTS / competency

Steam is passed over red-hot iron: $3 F e + 4 H_{2} O ⇌ F e_{3} O_{4} + 4 H_{2}$ . (Fe = 56, O = 16, H = 1)
(a) Why is the reaction reversible?
(b) How is the forward reaction favoured?
(c) Hydrogen is passed over 23.2 g of hot Fe₃O₄: write the equation and find the mass of iron and of water formed.
(d) In (c), name the substance reduced and the reducing agent.

Try it first, then show the solutionSolution

(a) Hydrogen can reduce Fe₃O₄ back to iron, so the products react to give the reactants. (1)
(b) By passing steam continuously so that hydrogen is carried away. (½)
(c) $F e_{3} O_{4} + 4 H_{2} \to 3 F e + 4 H_{2} O$ ; $23.2 \times \frac{168}{232} = 16.8$ g iron and $23.2 \times \frac{72}{232} = 7.2$ g water. (1½)
(d) Fe₃O₄ is reduced; hydrogen is the reducing agent. (1)

Answer: (c) 16.8 g Fe; 7.2 g H₂O (d) Fe₃O₄ reduced; H₂ reducing agent

Where students lose marks in Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch Process

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

Mistake: Hydrogen collected by upward displacement of air
Hydrogen–air mixtures are explosive.
Wrong working
Collected by upward displacement of air.
Right working
Downward displacement of water.
Do this: Say "over water".
Usually costs: 1 to 2 marks.
Mistake: Sodium suggested for the laboratory method
Too violent with acids.
Wrong working
Sodium with dilute HCl.
Right working
Granulated zinc with dilute HCl.
Do this: Give the reason for the metal chosen.
Usually costs: 1 mark.
Mistake: Alkali equation unbalanced
Water is a reactant in the aluminium reaction.
Wrong working
Al + NaOH → NaAlO₂ + H₂
Right working
2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂
Do this: Balance H and O last.
Usually costs: 1 mark.

Exam tips

Draw the apparatus with the thistle funnel dipping below the acid.
Give a reason for each choice (metal, acid, collection).
Write water gas, Bosch reaction and separation as three numbered steps.
Mention Fe₂O₃ with Cr₂O₃ promoter and 450 °C.

Practice questions on Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch 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 · MCQBoard level
Lead is not used for the laboratory preparation of hydrogen with dilute acids because:
(a) it is too reactive
(b) it forms an insoluble coating that stops the reaction
(c) it is below hydrogen
(d) it is a liquid
Hint
Think of PbCl₂ and PbSO₄.
Answer
(b)
Q2 · 1 mark · Very short answerBoard level
Name the catalyst used in the Bosch reaction.
Hint
Iron compound.
Answer
Iron(III) oxide, Fe₂O₃ (with Cr₂O₃ as promoter).
Q3 · 2 marks · Short answerHarder board level
Balance: Al + H₂SO₄ → Al₂(SO₄)₃ + H₂ and name the salt.
Hint
2 Al and 3 H₂SO₄.
Answer
$2 A l + 3 H_{2} S O_{4} \to A l_{2} (S O_{4})_{3} + 3 H_{2}$ ; aluminium sulphate.
Q4 · 3 marks · Short answerHarder board level
Find the mass of (a) magnesium and (b) iron needed to prepare 0.5 g of hydrogen from dilute acids (Mg = 24, Fe = 56). Write the equations (use HCl for Mg and H₂SO₄ for Fe).
Hint
2 g of H₂ needs 24 g Mg or 56 g Fe.
Answer
(a) $M g + 2 H C l \to M g C l_{2} + H_{2}$ : $0.5 \times \frac{24}{2} = 6$ g. (b) $F e + H_{2} S O_{4} \to F e S O_{4} + H_{2}$ : $0.5 \times \frac{56}{2} = 14$ g.
Q5 · 2 marks · Assertion-reasonHarder board level
Assertion (A): Hydrogen is collected by downward displacement of water.
Reason (R): Hydrogen is almost insoluble in water.
(a) Both true, R explains A.
(b) Both true, R does not explain A.
(c) A true, R false.
(d) A false, R true.
Hint
Which property of a gas allows it to be collected over water?
Answer
(a)
Q6 · 3 marks · Short answerHOTS / competency
Water gas contains CO and H₂. In the Bosch reaction 28 kg of carbon monoxide reacts with excess steam. Find the mass of hydrogen and of carbon dioxide formed and the mass of potassium hydroxide (K = 39) needed to absorb the carbon dioxide ( $C O_{2} + 2 K O H \to K_{2} C O_{3} + H_{2} O$ ).
Hint
CO + H₂O → CO₂ + H₂: 28 → 44 and 2.
Answer
Hydrogen $= 2$ kg; carbon dioxide $= 44$ kg; KOH $= 44 \times \frac{112}{44} = 112$ kg ( $2 \times 56$ ).

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.Laboratory reagents for hydrogen?Show answer
Granulated zinc and dilute HCl (or dilute H₂SO₄).
2.Test for hydrogen?Show answer
Burns with a pale blue flame and a pop.
3.Water gas?Show answer
A mixture of CO and H₂.
4.Which metals react with alkalis to give hydrogen?Show answer
Al, Zn and Pb.
5.Why is dilute nitric acid unsuitable?Show answer
It oxidises the hydrogen to water.

Preparation of Hydrogen: Laboratory Method, Other Sources and the Bosch Process: frequently asked questions

How is hydrogen prepared in the laboratory?

By the action of dilute hydrochloric or sulphuric acid on granulated zinc: Zn + 2HCl → ZnCl₂ + H₂; the gas is collected by downward displacement of water.

What is the Bosch process?

An industrial method: steam over white-hot coke gives water gas, which with excess steam over Fe₂O₃ (Cr₂O₃ promoter) at about 450 °C gives CO₂ and more hydrogen.

Which metals react with both acids and alkalis to liberate hydrogen?

Aluminium, zinc and lead.