ChemistryICSE Class 9 · Chapter 6

Study of the First Element, Hydrogen: ICSE Class 9 Preparation, Bosch Process and Redox

Q: Does magnesium react with hot water to give MgO or Mg(OH)₂?

With hot (boiling) water it gives magnesium hydroxide: Mg + 2H_2O \rightarrow Mg(OH)_2 + H_2. With steam it gives magnesium oxide: Mg + H_2O \rightarrow MgO + H_2. Some books write MgO for hot water too; give the hydroxide for water and the oxide for steam.

Q: Why is impure zinc better than pure zinc?

Impurities such as copper in contact with zinc speed up the reaction with the acid (they form tiny cells). Very pure zinc reacts very slowly with dilute acid.

Q: Does the syllabus need the uses of hydrogen?

The 2028 Class IX syllabus does not list uses of hydrogen. It does ask for position, preparation (lab, from water, acids, alkalis), the Bosch process and redox.

Q: Is every reaction that gives hydrogen a redox reaction?

Yes, in this chapter: the metal (or carbon) is oxidised by losing electrons, and hydrogen in water, acid or alkali is reduced to H₂ by gaining electrons.

Updated 15 October 2026 · Fact-checked

Hydrogen is made in the laboratory by the action of dilute hydrochloric or sulphuric acid on granulated zinc, and collected over water. Active metals release it from water, acids and alkalis. Industrially it comes from the Bosch process. Oxidation is loss of electrons (or gain of oxygen); reduction is gain of electrons.

What Study of the First Element: Hydrogen is about

Hydrogen, H, is the first and lightest element: one proton and one electron. It is a colourless, odourless gas that burns with a pale blue flame and gives a 'pop' with a lighted splint.

The CISCE 2028 syllabus (Class IX, topic 6) covers: hydrogen's position in the periodic table (valence electron, burning, ion formation); hydrogen from water (cold water on Na, K, Ca; hot water on Mg; steam on Al, Zn and red-hot iron, with the reversible iron-steam reaction; steam on carbon) with the activity series; from dilute acids (Mg, Al, Zn, Fe) and why other metals and dilute nitric acid are not used; from alkalis (Al, Zn, Pb); the laboratory preparation from zinc, removal of impurities and precautions; the Bosch process with separation of CO₂ and CO; and oxidation and reduction in terms of oxygen, hydrogen and electrons, oxidising and reducing agents and redox reactions.

Weightage: CISCE publishes no chapter weightage for ICSE Chemistry. Study of the First Element: Hydrogen is topic 6 of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2028 (Class IX)

Key concepts

Position of hydrogen in the periodic table

Hydrogen (configuration 1) is placed at the top of group 1, but it also resembles the halogens (group 17).

Like alkali metals	Like halogens
one valence electron	needs one electron to complete its shell (duplet)
loses it to form H⁺ (like Na⁺)	gains one to form the hydride ion H⁻ (like Cl⁻), e.g. NaH
valency 1; reducing agent	diatomic molecule H₂ (like Cl₂); combines with metals
forms oxide H₂O (like Na₂O)	is a non-metal and a gas

Burning: hydrogen burns in air or oxygen with a pale blue flame, forming water: $2 H_{2} + O_{2} \to 2 H_{2} O$ . A hydrogen-air mixture explodes when lit.

Worked example

Give one property in which hydrogen resembles sodium and one in which it resembles chlorine.

Try it first, then show the solutionSolution

Like sodium: one valence electron, forms a positive ion H⁺.
Like chlorine: forms a diatomic molecule H₂ and a negative ion H⁻ with active metals.

Answer: As above.

Hydrogen from water and the activity series

Activity series: K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au. The higher the metal, the more readily it displaces hydrogen from water.

Cold water (very active metals):

$2 K + 2 H_{2} O \to 2 K O H + H_{2}$ : violent; hydrogen catches fire with a lilac flame.
$2 N a + 2 H_{2} O \to 2 N a O H + H_{2}$ : sodium melts into a silvery ball, darts about with a hissing sound; may catch fire (golden yellow flame).
$C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$ : calcium sinks, reacts steadily; solution turns milky (slightly soluble Ca(OH)₂).

Hot (boiling) water: $M g + 2 H_{2} O \to M g (O H)_{2} + H_{2}$ (slow).

Steam (heated metal):

$M g + H_{2} O \to M g O + H_{2}$ (burns brilliantly)
$2 A l + 3 H_{2} O \to A l_{2} O_{3} + 3 H_{2}$
$Z n + H_{2} O \to Z n O + H_{2}$ (ZnO yellow when hot, white when cold)
$3 F e + 4 H_{2} O ⇌ F e_{3} O_{4} + 4 H_{2}$ (red-hot iron; reversible: if hydrogen is not removed, it reduces Fe₃O₄ back to iron. Passing a steady stream of steam carries the hydrogen away and drives the reaction forward.)

Steam on a non-metal: $C + H_{2} O \to C O + H_{2}$ (white-hot coke, about 1000 °C; the mixture is water gas).

Metals below iron (Pb, Cu, Hg, Ag, Au) do not give hydrogen with water or steam.

Worked example

Why does sodium float and form a ball on water while calcium sinks?

Try it first, then show the solutionSolution

Sodium is less dense than water and the reaction is so exothermic that it melts (low melting point) into a ball.
Calcium is denser than water and reacts less vigorously, so it sinks and does not melt.

Answer: Density and the heat of reaction.

Hydrogen from dilute acids and from alkalis

Dilute acids (HCl or H₂SO₄) with metals above hydrogen:

$M g + 2 H C l \to M g C l_{2} + H_{2}$ and $M g + H_{2} S O_{4} \to M g S O_{4} + H_{2}$
$2 A l + 6 H C l \to 2 A l C l_{3} + 3 H_{2}$ and $2 A l + 3 H_{2} S O_{4} \to A l_{2} (S O_{4})_{3} + 3 H_{2}$
$Z n + 2 H C l \to Z n C l_{2} + H_{2}$ and $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$
$F e + 2 H C l \to F e C l_{2} + H_{2}$ and $F e + H_{2} S O_{4} \to F e S O_{4} + H_{2}$

Why other metals are not used:

Na, K, Ca: react violently/explosively with acids; unsafe.
Lead: forms an insoluble coating of PbCl₂ or PbSO₄ that stops the reaction.
Cu, Hg, Ag, Au: below hydrogen; no reaction.
Dilute nitric acid: it is an oxidising agent; it oxidises the hydrogen formed to water and itself gives oxides of nitrogen (NO), so little or no hydrogen is collected.

Alkalis (NaOH or KOH, on warming) with Al, Zn and Pb:

$Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$ (sodium zincate)
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$ (sodium aluminate)
$P b + 2 N a O H \to N a_{2} P b O_{2} + H_{2}$ (sodium plumbite)
With KOH: $Z n + 2 K O H \to K_{2} Z n O_{2} + H_{2}$

Unique nature: Al, Zn and Pb react with both acids and alkalis (they are amphoteric metals); most metals react only with acids.

Worked example

Write the equation for aluminium with warm sodium hydroxide solution and name the salt formed.

Try it first, then show the solutionSolution

Aluminium forms the aluminate ion AlO₂⁻ and hydrogen.
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$

Answer: Sodium aluminate (NaAlO₂).

Laboratory preparation of hydrogen

Reactants: granulated zinc and dilute hydrochloric acid (or dilute sulphuric acid).
Equation: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$
Apparatus: a Woulfe bottle (or flat-bottomed flask) with a thistle funnel whose end dips below the acid (so gas cannot escape through it), and a delivery tube.
Collection: by downward displacement of water (over water in a trough), because hydrogen is almost insoluble in water. It is lighter than air, but is not collected by displacement of air because a hydrogen-air mixture is explosive and it would mix with air.

Why zinc? It reacts at a moderate, steady rate (Na, K, Ca too violent; Fe slow and gives impure gas; Mg costly). Granulated zinc has a large surface and impurities (like copper) that speed up the reaction; pure zinc reacts very slowly.

Impurities and removal: the gas may contain H₂S, SO₂, CO₂, oxides of nitrogen, PH₃, AsH₃ and water vapour (from impurities in zinc and acid). Pass through:

lead nitrate solution (removes H₂S),
silver nitrate solution (removes PH₃ and AsH₃),
potassium hydroxide solution (removes SO₂, CO₂ and oxides of nitrogen),
then dry with fused calcium chloride or phosphorus pentoxide (or quicklime), and collect over mercury if dry gas is needed.

Precautions: make the apparatus airtight; keep flames away; let the first few bubbles escape (they carry air); test a small sample: if it burns quietly with a pop, the air is gone.

Worked example

Why must all the air be expelled from the apparatus before hydrogen is collected and tested?

Try it first, then show the solutionSolution

Hydrogen mixed with air explodes when lit.
A test tube of the gas is lit away from the apparatus: a loud pop means air is still present; a quiet burn means pure hydrogen.

Answer: To avoid an explosion of the hydrogen-air mixture.

Industrial manufacture: Bosch process

Step 1: water gas. Steam is passed over white-hot coke at about 1000 °C:
$C + H_{2} O \to C O + H_{2}$ (endothermic). The mixture of CO and H₂ is water gas.

Step 2: Bosch reaction. Water gas is mixed with excess steam and passed over a catalyst of iron(III) oxide (Fe₂O₃) with chromium(III) oxide (Cr₂O₃) as promoter at about 450 °C:
$C O + H_{2} + H_{2} O \to C O_{2} + 2 H_{2}$
The carbon monoxide is converted into carbon dioxide, and the steam gives more hydrogen.

Step 3: separation.

CO₂: dissolved in water under pressure (about 30 atm), or absorbed in caustic potash solution: $2 K O H + C O_{2} \to K_{2} C O_{3} + H_{2} O$
CO (unreacted traces): absorbed in ammoniacal copper(I) chloride solution.

The remaining gas is hydrogen.

Worked example

In the Bosch process, name the catalyst and promoter and give the temperature.

Try it first, then show the solutionSolution

Catalyst: iron(III) oxide; promoter: chromium(III) oxide.
Temperature: about 450 °C.

Answer: Fe₂O₃, Cr₂O₃, about 450 °C.

Oxidation, reduction and redox

	Oxidation	Reduction
Oxygen	addition of oxygen	removal of oxygen
Hydrogen	removal of hydrogen	addition of hydrogen
Electrons	loss of electrons	gain of electrons

A redox reaction is one in which oxidation and reduction happen together.

Oxidising agent: supplies oxygen, removes hydrogen, or accepts electrons; it is itself reduced. Examples: O₂, Cl₂, conc. HNO₃, CuO.
Reducing agent: removes oxygen, supplies hydrogen, or donates electrons; it is itself oxidised. Examples: H₂, C, CO, metals.

Examples:

$C u O + H_{2} \to C u + H_{2} O$ : CuO is reduced (loses O), H₂ is oxidised (gains O). H₂ = reducing agent; CuO = oxidising agent.
$H_{2} S + C l_{2} \to 2 H C l + S$ : H₂S loses hydrogen (oxidised); Cl₂ gains hydrogen (reduced).
$Z n + C u S O_{4} \to Z n S O_{4} + C u$ : Zn → Zn²⁺ + 2e⁻ (oxidation); Cu²⁺ + 2e⁻ → Cu (reduction).
$2 M g + O_{2} \to 2 M g O$ : Mg loses 2 electrons each (oxidised); O gains them (reduced).

Worked example

In $P b O + C \to P b + C O$ , identify the substance oxidised, the substance reduced, the oxidising agent and the reducing agent.

Try it first, then show the solutionSolution

C gains oxygen: oxidised; so C is the reducing agent.
PbO loses oxygen: reduced; so PbO is the oxidising agent.

Answer: Oxidised C, reduced PbO; oxidising agent PbO, reducing agent C.

Chemistry in real life: Study of the First Element: Hydrogen

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.

Mass of hydrogen from zinc

Suppose a school lab has 13 g of granulated zinc and plenty of dilute hydrochloric acid for a hydrogen demonstration.

What mass of hydrogen can be made? (Zn = 65, H = 1, Cl = 35.5)

Try it first, then show the solutionSolution

$Z n + 2 H C l \to Z n C l_{2} + H_{2}$ : 65 g of zinc gives 2 g of hydrogen.
13 g gives 13 × 2/65 = 0.4 g.

Answer: 0.4 g of hydrogen.

Memory hook: Use the equation's mass ratio.

Aluminium cookware and caustic soda

Suppose someone cleans an aluminium pressure cooker with a strong caustic soda drain cleaner and notices bubbles and pitting.

Explain with an equation.

Try it first, then show the solutionSolution

Aluminium is amphoteric and reacts with sodium hydroxide.
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$ : the bubbles are hydrogen; the metal is eaten away.

Answer: Aluminium reacts with alkali to give hydrogen and sodium aluminate.

Memory hook: ZAP metals hate strong alkalis too.

Hydrogen safety

Suppose a student wants to light the end of the delivery tube as soon as hydrogen starts bubbling from the zinc and acid.

Explain the danger and the correct procedure.

Try it first, then show the solutionSolution

The flask still contains air, so the first gas is a hydrogen-air mixture, which explodes when lit.
Let the gas bubble for a while, collect a test tube full and light it away from the apparatus.
Only when it burns quietly is the gas pure enough.

Answer: Risk of explosion; test samples first.

Memory hook: Loud pop means air; quiet pop means go.

Topics in this chapter

Study of the First Element: Hydrogen formula sheet

Laboratory preparation: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$; Granulated zinc; collect over water.
Iron and steam: $3 F e + 4 H_{2} O ⇌ F e_{3} O_{4} + 4 H_{2}$; Reversible; remove H₂.
Water gas: $C + H_{2} O \to C O + H_{2}$; About 1000 °C.
Bosch reaction: $C O + H_{2} + H_{2} O \to C O_{2} + 2 H_{2}$; Fe₂O₃ catalyst, Cr₂O₃ promoter, about 450 °C.
Zinc with alkali: $Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$; Sodium zincate.
Aluminium with alkali: $2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$; Sodium aluminate.

Study of the First Element: Hydrogen: diagrams to practise

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

Laboratory preparation of hydrogen

Draw a labelled diagram for the preparation and collection of hydrogen from zinc and dilute acid.

Draw

A Woulfe bottle (or flat-bottomed flask) with granulated zinc covered by dilute HCl.
A thistle funnel whose lower end dips below the acid.
A delivery tube leading to a beehive shelf in a trough of water.
An inverted gas jar full of water over the shelf, collecting hydrogen.

Label

thistle funnel
dilute HCl
granulated zinc
delivery tube
trough of water
beehive shelf
hydrogen gas

Watch out: Show bubbles rising into the jar; the funnel end must be under the liquid.

Action of steam on a metal

Show how steam is passed over heated iron (or magnesium, zinc) and the hydrogen collected.

Draw

A flask of boiling water producing steam.
A hard-glass tube with iron filings, heated strongly by a burner.
Delivery tube to an inverted jar over water collecting hydrogen.

Label

water (boiling)
steam
iron filings
strong heat
hydrogen collected over water

Watch out: Label the reversible equation underneath.

How the school exam asks Study of the First Element: Hydrogen

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

Question type	How it is asked	Method
Metals with water, steam and acids	Equations and observations for Na, K, Ca, Mg, Al, Zn, Fe; why Cu or Pb is not used.	Use the activity series; give one observation per reaction.
Laboratory preparation	Diagram; equation; why zinc; collection method; impurities and how they are removed; precautions.	Learn the checklist; know which solution removes which impurity.
Bosch process	Equations, catalyst, promoter, temperature, removal of CO₂ and CO.	Two steps, then separation.
Redox	Identify oxidised, reduced, oxidising agent, reducing agent; define in terms of electrons.	Track oxygen, hydrogen or electrons for each reactant.
Amphoteric metals	Equations for Zn, Al, Pb with NaOH or KOH; name the salts.	-ate (zincate, aluminate) and plumbite.

Tricks to remember Study of the First Element: Hydrogen

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

OIL RIG
Oxidation Is Loss, Reduction Is Gain (of electrons).
Use it when: Electronic concept of redox.
Agents do the opposite
The oxidising agent gets reduced; the reducing agent gets oxidised.
Use it when: Identifying agents.
Cold-hot-steam ladder
K Na Ca: cold water. Mg: hot water. Al Zn Fe: steam. Below Fe: nothing.
Use it when: Hydrogen from water.
'Pb Shows Ag the Kitchen Door' for impurity removal
Pb(NO₃)₂ for H₂S; AgNO₃ for PH₃/AsH₃; KOH for acid gases; then Dry.
Use it when: Purifying hydrogen.
Zinc, Aluminium, Lead: 'ZAP both ways'
These three react with both acids and alkalis.
Use it when: Amphoteric metals.

Study of the First Element: Hydrogen: exam-level solved questions

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

Example 1: Metal for the lab preparation · 1 mark · MCQExam level

Which metal is used in the laboratory preparation of hydrogen?
(a) sodium
(b) copper
(c) granulated zinc
(d) lead

Try it first, then show the solutionSolution

Zinc reacts at a moderate rate.

Answer: (c)

Example 2: Redox agent · 1 mark · MCQHarder exam level

In the reaction $C u O + H_{2} \to C u + H_{2} O$ , the oxidising agent is:
(a) CuO
(b) H₂
(c) Cu
(d) H₂O

Try it first, then show the solutionSolution

CuO gives oxygen to hydrogen and is itself reduced, so it is the oxidising agent.

Answer: (a)

Example 3: Metals and water · 3 marks · Short answerExam level

Write balanced equations and one observation for the reactions of (a) calcium with cold water, (b) magnesium with steam, (c) aluminium with steam.

Try it first, then show the solutionSolution

(a) $C a + 2 H_{2} O \to C a (O H)_{2} + H_{2}$ : calcium sinks, effervescence, solution turns milky.
(b) $M g + H_{2} O \to M g O + H_{2}$ : burns with a dazzling white light; white residue.
(c) $2 A l + 3 H_{2} O \to A l_{2} O_{3} + 3 H_{2}$ : white residue of aluminium oxide.

Answer: As above.

Example 4: Reasons for metals not used · 4 marks · Long answerHarder exam level

Give reasons why the following are not used to prepare hydrogen from dilute acids: (a) potassium, (b) lead, (c) copper. (d) Why is dilute nitric acid not used?

Try it first, then show the solutionSolution

(a) Reaction is violent and dangerous.
(b) An insoluble coating of PbCl₂ or PbSO₄ forms and stops the reaction.
(c) Copper is below hydrogen in the activity series.
(d) It is an oxidising agent; it oxidises hydrogen to water.

Answer: As above.

Example 5: Reversible reaction · 3 marks · Short answerHarder exam level

Steam is passed over red-hot iron. (a) Write the equation. (b) Why is the reaction reversible? (c) How is it made to go to completion?

Try it first, then show the solutionSolution

(a) $3 F e + 4 H_{2} O ⇌ F e_{3} O_{4} + 4 H_{2}$
(b) Hydrogen formed can reduce Fe₃O₄ back to iron and steam.
(c) A continuous stream of steam sweeps the hydrogen away as it forms.

Answer: As above.

Example 6: Bosch process · 4 marks · Long answerExam level

Write the equations for the formation of water gas and for the Bosch reaction. Name the catalyst and promoter, and explain how CO₂ and CO are removed.

Try it first, then show the solutionSolution

$C + H_{2} O \to C O + H_{2}$ (about 1000 °C)
$C O + H_{2} + H_{2} O \to C O_{2} + 2 H_{2}$ (about 450 °C)
Catalyst Fe₂O₃, promoter Cr₂O₃.
CO₂: dissolved in water under pressure (or KOH); CO: ammoniacal copper(I) chloride.

Answer: As above.

Example 7: Electronic redox · 3 marks · Short answerHOTS / competency

For $F e + C u S O_{4} \to F e S O_{4} + C u$ : (a) write the two half changes with electrons, (b) identify the oxidising and reducing agents, (c) explain why the blue colour fades.

Try it first, then show the solutionSolution

(a) Fe → Fe²⁺ + 2e⁻ (oxidation); Cu²⁺ + 2e⁻ → Cu (reduction).
(b) Oxidising agent: Cu²⁺ (CuSO₄); reducing agent: Fe.
(c) Blue Cu²⁺ ions are removed as copper; pale green Fe²⁺ ions form.

Answer: As above.

Example 8: Unknown metal · 4 marks · Long answerHOTS / competency

A metal M reacts with dilute HCl and with warm NaOH solution, giving the same gas both times. Its oxide is yellow when hot and white when cold. (a) Identify M and the gas. (b) Write both equations. (c) What is special about M?

Try it first, then show the solutionSolution

(a) Zinc; hydrogen.
(b) $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ and $Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$
(c) It is amphoteric: it reacts with both acids and alkalis.

Answer: M = zinc; amphoteric.

Case-study practice (Section B style)

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

Purifying laboratory hydrogen

Suppose impure zinc and commercial acid are used, so the hydrogen contains hydrogen sulphide, sulphur dioxide, phosphine and moisture. A student sets up wash bottles A, B and C, then a drying tube D.

(i) · 1 mark
What should A contain to remove hydrogen sulphide?
Answer
Answer: Lead nitrate solution.
(ii) · 1 mark
What should B contain to remove phosphine?
Answer
Answer: Silver nitrate solution.
(iii) · 2 marks
What should C contain to remove sulphur dioxide, and why does it not absorb hydrogen?
Answer
Answer: Potassium hydroxide solution; SO₂ is acidic and reacts with the alkali, but hydrogen is neutral.
(iv) · 1 mark
Name a drying agent for D and how the dry gas should be collected.
Answer
Answer: Fused calcium chloride (or P₂O₅); collect over mercury.

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.

Define oxidation and reduction in terms of electrons. (2 marks) (2 marks)

Step	Marks
Oxidation: loss of electrons	1
Reduction: gain of electrons	1

Write equations for the action of warm NaOH on (a) zinc, (b) lead, (c) aluminium. (3 marks) (3 marks)

Step	Marks
$Z n + 2 N a O H \to N a_{2} Z n O_{2} + H_{2}$	1
$P b + 2 N a O H \to N a_{2} P b O_{2} + H_{2}$	1
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$	1

Laboratory preparation of hydrogen: equation, reason for using granulated zinc, method of collection with reason, one precaution. (4 marks) (4 marks)

Step	Marks
$Z n + 2 H C l \to Z n C l_{2} + H_{2}$	1
Granulated zinc: large surface and impurities make the reaction steady	1
Downward displacement of water: hydrogen is almost insoluble	1
Thistle funnel end under the acid / expel air before collecting / no flame near	1

Where students lose marks in Study of the First Element: Hydrogen

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

Mistake: Oxidising agent and reducing agent swapped
CISCE examiners' comments on the ICSE Class 10 paper, 2025: some candidates struggled with the concepts of oxidising agent and reducing agent and chose the wrong option.
Wrong working
In CuO + H₂ → Cu + H₂O, CuO is the reducing agent because it is reduced.
Right working
CuO is reduced, so it is the oxidising agent; H₂ is the reducing agent.
Do this: The agent causes the change in the other substance and itself undergoes the opposite.
Usually costs: 1 mark.
Mistake: Wrong product with hot water or steam
Students write a hydroxide for steam reactions.
Wrong working
Zn + 2H₂O → Zn(OH)₂ + H₂ (steam)
Right working
$Z n + H_{2} O \to Z n O + H_{2}$ : steam gives the oxide.
Do this: Cold or hot water → hydroxide; steam → oxide.
Usually costs: 1 mark.
Mistake: Collecting hydrogen by displacement of air
Students reason only from density.
Wrong working
Hydrogen is lighter than air, so it is collected by downward displacement of air.
Right working
It is collected over water: it is almost insoluble, and a hydrogen-air mixture is explosive.
Do this: Give both reasons for collecting over water.
Usually costs: 1 mark.
Mistake: Nitric acid reason incomplete
Students say 'it is too strong' instead of naming its oxidising nature.
Wrong working
Dilute nitric acid is not used because it is dangerous.
Right working
It is an oxidising agent and oxidises the hydrogen to water.
Do this: Use the word 'oxidising'.
Usually costs: 1 mark.
Mistake: Bosch catalyst confused
Iron catalysts in Haber's process (Class 10) and Bosch get mixed up.
Wrong working
Bosch catalyst: finely divided iron with molybdenum.
Right working
Iron(III) oxide with chromium(III) oxide as promoter.
Do this: Bosch = Fe₂O₃ + Cr₂O₃.
Usually costs: 1 mark.

Practice questions on Study of the First Element: Hydrogen

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 · MCQExam level
Which metal does NOT liberate hydrogen from dilute hydrochloric acid?
(a) Mg
(b) Fe
(c) Zn
(d) Cu
Hint
Below H.
Answer
(d)
Q2 · 1 mark · MCQExam level
Water gas is a mixture of:
(a) CO₂ and H₂
(b) CO and H₂
(c) CO and N₂
(d) H₂ and steam
Hint
Coke + steam.
Answer
(b)
Q3 · 2 marks · Short answerExam level
Write the equations for the reactions of (a) iron with dilute sulphuric acid, (b) aluminium with dilute hydrochloric acid.
Hint
Iron(II).
Answer
(a) $F e + H_{2} S O_{4} \to F e S O_{4} + H_{2}$ (b) $2 A l + 6 H C l \to 2 A l C l_{3} + 3 H_{2}$
Q4 · 2 marks · Short answerHarder exam level
In $H_{2} S + C l_{2} \to 2 H C l + S$ , name (a) the substance oxidised, (b) the oxidising agent. Explain in terms of hydrogen.
Hint
Who loses H?
Answer
(a) H₂S is oxidised: it loses hydrogen. (b) Cl₂ is the oxidising agent: it gains hydrogen (is reduced).
Q5 · 3 marks · Short answerExam level
Explain why (a) hydrogen is not collected by downward displacement of air, (b) the end of the thistle funnel dips into the acid, (c) the first few test tubes of gas are rejected.
Hint
Explosive mixture.
Answer
(a) It mixes with air to form an explosive mixture (and it is almost insoluble, so water works well). (b) So that hydrogen cannot escape through the funnel. (c) They contain air mixed with hydrogen.
Q6 · 3 marks · Short answerHarder exam level
Arrange K, Mg, Fe and Cu in order of increasing reactivity with water and describe the condition each needs to give hydrogen.
Hint
Cold, hot, steam, none.
Answer
Cu < Fe < Mg < K. K: cold water (violent). Mg: hot water (slow) or steam. Fe: red-hot with steam (reversible). Cu: no hydrogen even with steam.
Q7 · 3 marks · Short answerHarder exam level
Express in electron terms: (a) $2 N a + C l_{2} \to 2 N a C l$ , (b) $M g + 2 H C l \to M g C l_{2} + H_{2}$ . Name the reducing agent in each.
Hint
Metals lose electrons.
Answer
(a) Na → Na⁺ + e⁻ (oxidation); Cl₂ + 2e⁻ → 2Cl⁻ (reduction). Reducing agent Na. (b) Mg → Mg²⁺ + 2e⁻; 2H⁺ + 2e⁻ → H₂. Reducing agent Mg.
Q8 · 2 marks · Short answerHarder exam level
Why does the Bosch process use excess steam, and why is the CO₂ removed under pressure with water?
Hint
More hydrogen; solubility.
Answer
Excess steam converts as much CO as possible into CO₂ and gives extra hydrogen. CO₂ is far more soluble in water under pressure than hydrogen, so it dissolves while hydrogen passes on.
Q9 · 2 marks · Short answerHarder exam level
Calculate the mass of aluminium needed to make 0.6 g of hydrogen by reaction with sodium hydroxide solution. (Al = 27, H = 1)
Hint
2Al gives 3H₂.
Answer
$2 A l + 2 N a O H + 2 H_{2} O \to 2 N a A l O_{2} + 3 H_{2}$ : 54 g Al gives 6 g H₂. Mass = 0.6 × 54/6 = 5.4 g.
Q10 · 3 marks · Short answerHOTS / competency
A student finds that a piece of very pure zinc reacts very slowly with dilute sulphuric acid, but the reaction speeds up greatly when a few drops of copper sulphate solution are added. Explain, with an equation for what happens first.
Hint
Displacement first.
Answer
Zinc displaces copper: $Z n + C u S O_{4} \to Z n S O_{4} + C u$ . The copper deposited on the zinc acts like the impurity in commercial zinc: zinc-copper contact speeds up the release of hydrogen.
Q11 · 3 marks · Short answerHOTS / competency
Three metals P, Q and R: P gives hydrogen with cold water; Q gives it only with steam and also with NaOH solution; R gives it with neither water, steam nor dilute acid. (a) Suggest P, Q and R. (b) Write the equation for Q with steam. (c) Why can R not displace hydrogen?
Hint
Activity series; amphoteric.
Answer
(a) P: sodium (or K, Ca); Q: zinc (or aluminium); R: copper. (b) $Z n + H_{2} O \to Z n O + H_{2}$ (c) Copper is below hydrogen in the activity series.

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.Colour of the flame when hydrogen burns?Show answer
Pale blue.
2.Metal that reacts with hot water but not cold?Show answer
Magnesium.
3.Name of the CO + H₂ mixture?Show answer
Water gas.
4.Catalyst in the Bosch process?Show answer
Iron(III) oxide (Cr₂O₃ promoter).
5.Solution that removes H₂S from hydrogen?Show answer
Lead nitrate solution.
6.Salt formed when zinc reacts with NaOH?Show answer
Sodium zincate, Na₂ZnO₂.
7.Oxidation in terms of electrons?Show answer
Loss of electrons.
8.Why is lead not used with dilute HCl?Show answer
Insoluble PbCl₂ coats it and stops the reaction.

Best order to study it

Learn hydrogen's dual position.
Learn the reactions with cold water, hot water and steam in activity-series order.
Learn reactions with dilute acids, the reasons for unsuitable metals, and alkali reactions.
Learn the lab preparation checklist and impurity removal.
Learn the Bosch process: two equations, conditions, separation.
Practise redox identification in oxygen, hydrogen and electron terms.

Quick revision checklist

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

Can you write the equations for K, Na, Ca with cold water and Mg with hot water?
Can you explain why the iron-steam reaction is reversible?
Can you give a reason for not using each of K, Pb, Cu and dilute HNO₃?
Can you write the Zn, Al and Pb reactions with NaOH?
Can you name the reagent that removes each impurity from hydrogen?
Can you identify oxidising and reducing agents in any given reaction?

Study of the First Element: Hydrogen: frequently asked questions

Does magnesium react with hot water to give MgO or Mg(OH)₂?

With hot (boiling) water it gives magnesium hydroxide: Mg + 2H_2O \rightarrow Mg(OH)_2 + H_2. With steam it gives magnesium oxide: Mg + H_2O \rightarrow MgO + H_2. Some books write MgO for hot water too; give the hydroxide for water and the oxide for steam.

Why is impure zinc better than pure zinc?

Impurities such as copper in contact with zinc speed up the reaction with the acid (they form tiny cells). Very pure zinc reacts very slowly with dilute acid.

Does the syllabus need the uses of hydrogen?

The 2028 Class IX syllabus does not list uses of hydrogen. It does ask for position, preparation (lab, from water, acids, alkalis), the Bosch process and redox.

Is every reaction that gives hydrogen a redox reaction?

Yes, in this chapter: the metal (or carbon) is oxidised by losing electrons, and hydrogen in water, acid or alkali is reduced to H₂ by gaining electrons.

Based on: CISCE ICSE 2028 Science (52) Chemistry (Science Paper 2) syllabus, Classes IX and X; CISCE Analysis of Pupil Performance, ICSE (Class 10) Chemistry (Science Paper 2), 2025