ChemistryICSE Class 10 · Chapter 8

Hydrogen Chloride ICSE Class 10: Preparation, Properties and Tests

Updated 14 October 2026 · Fact-checked

Hydrogen chloride is prepared in the laboratory by heating sodium chloride with concentrated sulphuric acid below 200 °C, dried with concentrated sulphuric acid and collected by upward displacement of air. It is very soluble in water (fountain experiment), forms dense white fumes with ammonia, and its solution, hydrochloric acid, shows typical acid properties.

What Hydrogen Chloride is about

Hydrogen chloride (HCl) is a colourless, pungent, covalent gas that is heavier than air and extremely soluble in water. Its solution in water is hydrochloric acid, a strong acid.

The CISCE 2027 syllabus asks for: the laboratory preparation from sodium chloride (reactants, products, conditions, equation, apparatus diagram, procedure, observation, precautions, collection, identification); a simple experiment to show it is heavier than air; the fountain experiment for solubility; preparing hydrochloric acid with the special arrangement that prevents back suction; its reaction with ammonia; the acidic properties of its solution with metals, oxides, hydroxides, carbonates, hydrogen carbonates, sulphides and sulphites; aqua regia (composition and uses); precipitation with silver nitrate and lead nitrate; and tests for HCl gas and hydrochloric acid.

As the syllabus says for all of Chemistry, study each process in terms of reactants, products, conditions, observations, balanced equations and diagrams.

Weightage: CISCE publishes no chapter or unit weightage for ICSE Chemistry. Hydrogen Chloride is in the 8. Study of Compounds A. Hydrogen Chloride 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

Laboratory preparation

Reactants: sodium chloride (rock salt) and concentrated sulphuric acid.
Condition: gentle heating, below 200 °C.
Equation: $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$
Why not higher temperature? Above about 500 °C the reaction goes further, $2 N a C l + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H C l$ , but this needs more fuel, the glass apparatus may crack, and sodium sulphate forms a hard crust that sticks to the flask.
Why concentrated sulphuric acid? It is non-volatile, so it displaces the more volatile HCl from its salt. (Dilute acid would leave the very soluble HCl dissolved in the water.)
Drying: pass the gas through concentrated sulphuric acid. Quicklime (CaO) and phosphorus pentoxide cannot be used, because they react with HCl.
Collection: by upward displacement of air (gas jar upright), because HCl is heavier than air (vapour density 18.25 compared with about 14.4 for air). It is not collected over water because it is very soluble.
Gas jar full? A glass rod dipped in ammonia solution held at the mouth gives dense white fumes.

Worked example

Why is hydrogen chloride not dried over quicklime?

Try it first, then show the solutionSolution

Quicklime (CaO) is a basic oxide.
It reacts with HCl: $C a O + 2 H C l \to C a C l_{2} + H_{2} O$ .
So the gas would be absorbed, not just dried.

Answer: Quicklime reacts with HCl; concentrated H₂SO₄ is used instead.

Density and solubility: the fountain experiment

Heavier than air: an open gas jar of HCl held mouth upwards still gives white fumes with an ammonia rod after a while, while a jar held mouth downwards soon does not, because the gas has fallen out.

Fountain experiment (high solubility):

1. A dry round-bottom flask is filled with dry HCl gas and fitted with a cork carrying a long jet tube (and a dropper of water).
2. The flask is clamped upside down with the jet tube dipping into a trough of water coloured with blue litmus.
3. A few drops of water are squeezed in. HCl dissolves in them, the pressure inside falls, and the outside atmospheric pressure pushes water up the tube.
4. Observation: water rushes in as a fountain and the blue litmus turns red.
5. Inference: HCl is highly soluble in water and the solution is acidic.

Worked example

In the fountain experiment, why does the water rise into the flask, and why does it turn red?

Try it first, then show the solutionSolution

HCl dissolves in the first few drops of water, creating a partial vacuum.
Atmospheric pressure on the trough water forces it up the jet.
The dissolved HCl forms hydrochloric acid, which turns blue litmus red.

Answer: Partial vacuum from high solubility; the solution is acidic.

Making hydrochloric acid without back suction

HCl gas is so soluble that, if the delivery tube dips straight into water, water would be sucked back into the hot generating flask and could crack it.

Special arrangement: the delivery tube is joined to an inverted funnel whose rim just touches the surface of the water in a beaker.

How it prevents back suction: when gas dissolves and water rises into the funnel, the water level in the beaker drops below the funnel's rim. Air enters, the pressure is restored, and the water falls back. The wide funnel also gives a large surface for the gas to dissolve.

Worked example

State two advantages of using an inverted funnel to dissolve HCl gas in water.

Try it first, then show the solutionSolution

It prevents back suction of water into the generating flask.
It provides a large surface area, so the gas dissolves quickly.

Answer: Prevents back suction; increases surface area for absorption.

Reaction with ammonia and acidic properties

With ammonia: dense white fumes of ammonium chloride: $N H_{3} + H C l \to N H_{4} C l$ .

Hydrochloric acid is a strong monobasic acid: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ . It reacts as a typical acid:

Active metals → chloride + hydrogen: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ ; $F e + 2 H C l \to F e C l_{2} + H_{2}$ .
Oxides and hydroxides → chloride + water: $C u O + 2 H C l \to C u C l_{2} + H_{2} O$ ; $N a O H + H C l \to N a C l + H_{2} O$ .
Carbonates and hydrogen carbonates → CO₂: $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ ; $N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ .
Sulphites and hydrogen sulphites → SO₂: $N a_{2} S O_{3} + 2 H C l \to 2 N a C l + H_{2} O + S O_{2}$ .
Sulphides → H₂S: $F e S + 2 H C l \to F e C l_{2} + H_{2} S$ .

Note that iron gives iron(II) chloride with hydrochloric acid, not iron(III) chloride.

Worked example

Write the equation for the action of dilute hydrochloric acid on sodium hydrogen sulphite, and name a test for the gas.

Try it first, then show the solutionSolution

$N a H S O_{3} + H C l \to N a C l + H_{2} O + S O_{2}$
SO₂ turns acidified potassium dichromate solution (or paper) from orange to green.

Answer: Equation above; dichromate test.

Precipitation reactions and aqua regia

With silver nitrate: a white, curdy precipitate of silver chloride, insoluble in dilute nitric acid but soluble in ammonium hydroxide:
$A g N O_{3} + H C l \to A g C l + H N O_{3}$

With lead nitrate: a white precipitate of lead chloride, which dissolves on heating and reappears on cooling:
$P b (N O_{3})_{2} + 2 H C l \to P b C l_{2} + 2 H N O_{3}$

Aqua regia ('royal water'): a mixture of 3 parts concentrated hydrochloric acid and 1 part concentrated nitric acid by volume. Nitric acid oxidises HCl to give nascent chlorine:
$H N O_{3} + 3 H C l \to N O C l + 2 H_{2} O + 2 C l$
The nascent chlorine converts the noble metals gold and platinum into soluble chlorides. Uses: dissolving gold and platinum, cleaning and etching metals, and in testing and refining noble metals.

Worked example

Gold does not dissolve in concentrated HCl or concentrated HNO₃ separately, but dissolves in aqua regia. Explain.

Try it first, then show the solutionSolution

Neither acid alone attacks gold.
In aqua regia, nitric acid oxidises hydrochloric acid, releasing nascent chlorine.
Nascent chlorine is very reactive and converts gold to its soluble chloride.

Answer: Nascent chlorine formed in aqua regia attacks gold.

Tests for HCl gas and hydrochloric acid

HCl gas:

Colourless, pungent gas that fumes in moist air.
Turns moist blue litmus red (dry litmus is unaffected: dry HCl is not acidic).
A rod dipped in ammonia solution gives dense white fumes of NH₄Cl.
Passed into silver nitrate solution, it gives a white precipitate.

Hydrochloric acid (chloride ion):

Add silver nitrate solution: white curdy precipitate of AgCl, insoluble in dilute nitric acid.
Warm with manganese dioxide: greenish-yellow chlorine gas, which bleaches moist litmus: $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$ .

Worked example

How would you distinguish between dilute hydrochloric acid and dilute nitric acid using silver nitrate?

Try it first, then show the solutionSolution

Add silver nitrate solution to each.
Hydrochloric acid: white precipitate of AgCl.
Nitric acid: no precipitate (silver nitrate does not react).

Answer: White precipitate only with hydrochloric acid.

Chemistry in real life: Hydrogen Chloride

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.

Acid properties of hydrochloric acid

Suppose a plumber uses dilute hydrochloric acid to clean hard white scale (calcium carbonate) from bathroom tiles and notices fizzing.

Explain the fizzing with an equation and name a test for the gas.

Try it first, then show the solutionSolution

The scale is calcium carbonate.
$C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$
The fizzing is carbon dioxide; it turns lime water milky.

Answer: CO₂ from the carbonate; lime water test.

Memory hook: Acid on carbonate always fizzes CO₂.

Aqua regia

Suppose a goldsmith wants to check whether a ring is pure gold or gold-coated brass, and an assayer mentions aqua regia.

What is aqua regia, and why does it dissolve gold when neither acid alone does?

Try it first, then show the solutionSolution

Aqua regia is 3 volumes of concentrated HCl mixed with 1 volume of concentrated HNO₃.
Nitric acid oxidises hydrochloric acid to give nascent chlorine: $H N O_{3} + 3 H C l \to N O C l + 2 H_{2} O + 2 C l$ .
Nascent chlorine converts gold into a soluble chloride.

Answer: A 3 : 1 mixture of conc. HCl and HNO₃; its nascent chlorine attacks gold.

Memory hook: Royal water for royal metals.

Preventing back suction

Suppose a student bubbles HCl gas straight into a beaker of water through a narrow tube and the teacher warns that the flask might crack.

Explain the danger and the safe arrangement.

Try it first, then show the solutionSolution

HCl dissolves so fast that the pressure in the tube falls and water is sucked back into the hot flask.
Cold water meeting hot glass can crack it.
Use an inverted funnel with its rim just touching the water: if water rises, the level outside drops below the rim, air enters and the water falls back.

Answer: Back suction; prevent it with an inverted funnel.

Memory hook: A very soluble gas sucks water back; the funnel lets air in.

Topics in this chapter

Hydrogen Chloride formula sheet

Laboratory preparation (below 200 °C): $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$; Concentrated acid, gentle heat.
Above 500 °C (not used): $2 N a C l + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H C l$; Needs more fuel; crust of Na₂SO₄.
With ammonia: $N H_{3} + H C l \to N H_{4} C l$; Dense white fumes.
With silver nitrate: $A g N O_{3} + H C l \to A g C l + H N O_{3}$; White curdy precipitate.
With lead nitrate: $P b (N O_{3})_{2} + 2 H C l \to P b C l_{2} + 2 H N O_{3}$; White precipitate, soluble in hot water.
Aqua regia: $H N O_{3} + 3 H C l \to N O C l + 2 H_{2} O + 2 C l$; 3 : 1 by volume; nascent chlorine.
Test with MnO₂: $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$; Greenish-yellow gas.

Hydrogen Chloride: diagrams to practise

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

Laboratory preparation of hydrogen chloride

Draw a labelled diagram of the laboratory preparation and collection of HCl gas.

Draw

A round-bottom flask on a tripod and wire gauze over a burner, containing sodium chloride and concentrated sulphuric acid.
A thistle funnel through the cork, its end dipping below the mixture.
A delivery tube leading to a wash bottle containing concentrated sulphuric acid (gas enters through the long tube, leaves through the short tube).
A second tube leading to the bottom of an upright gas jar (upward displacement of air).

Label

sodium chloride + conc. H₂SO₄
thistle funnel
heat (below 200 °C)
conc. H₂SO₄ (drying agent)
HCl gas
gas jar (upright)

Watch out: In the wash bottle, the incoming tube dips into the acid; the outgoing tube stays above it.

Fountain experiment

Draw and label the fountain experiment to show the solubility of HCl.

Draw

An inverted round-bottom flask clamped to a stand, filled with dry HCl.
A cork with a long jet tube passing down into a trough of water containing blue litmus.
A dropper filled with water through the same cork.
The fountain of red liquid spraying inside the flask.

Label

dry HCl gas
jet tube
dropper with water
blue litmus solution
red fountain

Watch out: Write 'partial vacuum' inside the flask.

Absorption of HCl in water (inverted funnel)

Show the arrangement used to prepare hydrochloric acid without back suction.

Draw

The delivery tube from the HCl generator attached to the stem of an inverted funnel.
The funnel's rim just touching the surface of water in a beaker.

Label

HCl gas
inverted funnel
rim just touching water
water / hydrochloric acid

Watch out: Do not dip the funnel deep into the water: that would defeat its purpose.

How the board asks Hydrogen Chloride

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

Question type	How it is asked	Method
Laboratory preparation	Section B, 3-4 marks: equation, why concentrated H₂SO₄, drying agent, method of collection, why not collected over water, diagram.	Learn the preparation as a checklist: reactants, condition, equation, drying, collection, test.
Fountain experiment	Describe or explain: why the fountain forms, colour of litmus, which property is shown.	Solubility → partial vacuum → atmospheric pressure → fountain; colour shows acidity.
Back suction	Diagram or reason: how the inverted funnel prevents back suction.	Funnel rim touches water; level falls below the rim; air enters; water falls back.
Reactions of the acid	Equations with metals, oxides, carbonates, sulphites, sulphides, silver nitrate and lead nitrate.	Use the general patterns; iron gives FeCl₂.
Tests and identification	Give a test for HCl gas; distinguish HCl from HNO₃; aqua regia composition.	Ammonia fumes; AgNO₃ precipitate insoluble in HNO₃; 3 : 1 HCl : HNO₃.

Tricks to remember Hydrogen Chloride

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

Salt + Sulphuric, Slowly (below 200)
S-S-S: Salt, concentrated Sulphuric acid, Slow gentle heat. The product is the bisulphate (NaHSO₄).
Use it when: Lab preparation equation and conditions.
Heavy and thirsty
HCl is heavy (upward displacement of air) and thirsty (never over water).
Use it when: Method of collection.
Aqua regia: three kings and one queen
3 HCl : 1 HNO₃ dissolves the royal metals (gold and platinum).
Use it when: Composition and uses.
Drying: like dries like
An acidic gas is dried by an acid (conc. H₂SO₄), never by a base (CaO).
Use it when: Choosing a drying agent.

Hydrogen Chloride: board-level solved questions

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

Example 1: Collection · 1 mark · MCQBoard level

Hydrogen chloride gas is collected by:
(a) downward displacement of water
(b) upward displacement of air
(c) downward displacement of air
(d) upward displacement of water

Try it first, then show the solutionSolution

Heavier than air: upward displacement of air (jar upright).
Highly soluble: never over water.

Answer: (b)

Example 2: Drying agent · 1 mark · Assertion-reasonBoard level

Assertion (A): Concentrated sulphuric acid is used to dry hydrogen chloride.
Reason (R): Concentrated sulphuric acid does not react with hydrogen chloride.
(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.

Try it first, then show the solutionSolution

A drying agent must absorb water without reacting with the gas; conc. H₂SO₄ does not react with HCl.

Answer: (a)

Example 3: Lab preparation · 3 marks · Short answerBoard level

For the laboratory preparation of hydrogen chloride: (a) give the equation; (b) why is the temperature kept below 200 °C? (c) which property of concentrated sulphuric acid is used?

Try it first, then show the solutionSolution

(a) $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$
(b) A higher temperature needs more fuel, may crack the flask, and forms a hard crust of Na₂SO₄.
(c) It is non-volatile, so it displaces the volatile HCl.

Answer: As above.

Example 4: Fountain experiment · 3 marks · Short answerHarder board level

In the fountain experiment with HCl: (a) what is the colour of the fountain if the trough contains blue litmus solution? (b) which two properties of HCl are shown? (c) why must the flask be dry?

Try it first, then show the solutionSolution

(a) Red.
(b) High solubility in water and acidic nature of its solution.
(c) Moisture in the flask would dissolve the gas beforehand, so no partial vacuum would form when the drops are added.

Answer: As above.

Example 5: Precipitation reactions · 4 marks · Long answerHarder board level

Write equations and state observations when hydrochloric acid is added to (a) silver nitrate solution, (b) lead nitrate solution. How can you distinguish the two precipitates?

Try it first, then show the solutionSolution

(a) $A g N O_{3} + H C l \to A g C l + H N O_{3}$ : white curdy precipitate.
(b) $P b (N O_{3})_{2} + 2 H C l \to P b C l_{2} + 2 H N O_{3}$ : white precipitate.
AgCl dissolves in ammonium hydroxide; PbCl₂ does not, but dissolves in hot water.
So heating (or adding NH₄OH) tells them apart.

Answer: As above.

Example 6: Unknown gas · 3 marks · Short answerHarder board level

A colourless gas X fumes in moist air, turns moist blue litmus red, and gives dense white fumes with a rod dipped in ammonia. When passed into silver nitrate solution, it gives a white precipitate. (a) Identify X. (b) Write the equation with ammonia. (c) Name a drying agent for X.

Try it first, then show the solutionSolution

All tests point to hydrogen chloride.
$N H_{3} + H C l \to N H_{4} C l$
Concentrated sulphuric acid.

Answer: (a) HCl (b) as above (c) conc. H₂SO₄

Example 7: Iron and the acid · 3 marks · Short answerHOTS / competency

Iron filings are added to dilute hydrochloric acid. (a) Write the equation. (b) A student writes FeCl₃ as the product. Explain the error. (c) How could iron(III) chloride be made?

Try it first, then show the solutionSolution

(a) $F e + 2 H C l \to F e C l_{2} + H_{2}$
(b) Dilute HCl is not an oxidising agent; it gives iron(II) chloride and the hydrogen formed keeps iron in the +2 state.
(c) By direct combination: $2 F e + 3 C l_{2} \to 2 F e C l_{3}$ .

Answer: As above.

Example 8: Dry versus moist · 2 marks · Short answerHarder board level

Dry HCl gas does not change the colour of dry blue litmus paper, but changes moist blue litmus to red. Explain.

Try it first, then show the solutionSolution

Dry HCl is covalent and has no H₃O⁺ ions.
With moisture it ionises: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ , and H₃O⁺ turns litmus red.

Answer: Acidity needs water to form hydronium ions.

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 + 1 + 1 marks.

Setting up the preparation

Suppose a student sets up: a round-bottom flask with sodium chloride, a thistle funnel for adding acid, a delivery tube leading through a wash bottle, and a gas jar for collection.

(i) · 1 mark
What liquid should be in the wash bottle, and why?
Answer
Answer: Concentrated sulphuric acid, to dry the gas without reacting with it.
(ii) · 1 mark
How should the gas jar be placed, and why?
Answer
Answer: Upright (upward displacement of air), because HCl is heavier than air.
(iii) · 1 mark
How will the student know the jar is full?
Answer
Answer: A rod dipped in ammonia solution held at the mouth gives dense white fumes.
(iv) · 1 mark
The thistle funnel must dip below the reaction mixture. Why?
Answer
Answer: So that the gas does not escape through the funnel.
(v) · 1 mark
Write the equation for the reaction in the flask.
Answer
Answer: $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$

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.

Give two reasons why hydrogen chloride is collected by upward displacement of air and not over water. (2 marks) (2 marks)

Step	Marks
It is heavier than air (vapour density 18.25 > about 14.4)	1
It is highly soluble in water	1

Describe the fountain experiment for HCl: what is done, what is seen and what it shows. (3 marks) (3 marks)

Step	Marks
Flask of dry HCl inverted with a jet tube in blue litmus solution; a few drops of water introduced	1
Water rushes up as a fountain and turns red	1
HCl is highly soluble (partial vacuum formed) and its solution is acidic	1

Write balanced equations for the reaction of hydrochloric acid with (a) zinc, (b) copper(II) oxide, (c) sodium sulphite, (d) iron(II) sulphide. (4 marks) (4 marks)

Step	Marks
$Z n + 2 H C l \to Z n C l_{2} + H_{2}$	1
$C u O + 2 H C l \to C u C l_{2} + H_{2} O$	1
$N a_{2} S O_{3} + 2 H C l \to 2 N a C l + H_{2} O + S O_{2}$	1
$F e S + 2 H C l \to F e C l_{2} + H_{2} S$	1

Where students lose marks in Hydrogen Chloride

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

Mistake: Using the high-temperature equation
CISCE examiners' comments, 2024: for the laboratory preparation, candidates wrote the equation for the higher temperature or wrong products.
Wrong working
2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl (in the laboratory)
Right working
$N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$ (below 200 °C)
Do this: Laboratory = below 200 °C = sodium bisulphate.
Usually costs: 1 mark.
Mistake: Wrong method of collection
CISCE examiners' comments, 2024: candidates wrote 'upward displacement of gas' or 'downward delivery of air'.
Wrong working
Collected by downward displacement of air.
Right working
Collected by upward displacement of air (heavier than air).
Do this: Heavy gas pushes air UP out of an upright jar.
Usually costs: 1 mark.
Mistake: Wrong property of sulphuric acid
CISCE examiners' comments, 2024: candidates wrote 'dehydrating agent' or 'dibasic acid' instead of 'non-volatile' for the property used in the preparation.
Wrong working
Concentrated sulphuric acid is used because it is a dehydrating agent.
Right working
It is non-volatile, so it displaces the more volatile HCl from NaCl.
Do this: Preparation of a volatile acid from its salt = non-volatile nature.
Usually costs: 1 mark.
Mistake: Calling the fountain colour wrongly
A slip worth checking in your own answer (not taken from a CISCE report): the colour depends on which litmus is in the trough; with blue litmus the fountain is red, with red litmus there is no change.
Wrong working
The fountain turns blue.
Right working
Blue litmus in the trough turns red in the flask (acidic solution).
Do this: HCl fountain = red; NH₃ fountain = blue.
Usually costs: 1 mark.
Mistake: Iron(III) from dilute HCl
A slip worth checking in your own answer (not taken from a CISCE report): writing iron(III) chloride as the product of iron with dilute hydrochloric acid.
Wrong working
2Fe + 6HCl → 2FeCl₃ + 3H₂
Right working
$F e + 2 H C l \to F e C l_{2} + H_{2}$
Do this: Dilute acid + iron = iron(II); chlorine + iron = iron(III).
Usually costs: 1 mark.

Practice questions on Hydrogen Chloride

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
The drying agent used for hydrogen chloride gas is:
(a) quicklime
(b) phosphorus pentoxide
(c) concentrated sulphuric acid
(d) calcium chloride solution
Hint
Acid dries acid.
Answer
(c)
Q2 · 1 mark · MCQBoard level
Aqua regia contains concentrated HCl and concentrated HNO₃ in the volume ratio:
(a) 1 : 3
(b) 3 : 1
(c) 1 : 1
(d) 2 : 1
Hint
Three of HCl.
Answer
(b)
Q3 · 1 mark · MCQHarder board level
Statement I: HCl gas can be collected over water.
Statement II: HCl gas is heavier than air.
(a) Both true
(b) Both false
(c) I true, II false
(d) I false, II true
Hint
Solubility.
Answer
(d)
Q4 · 2 marks · Short answerBoard level
Give one observation and the equation when (a) a rod dipped in ammonia is brought near HCl gas, (b) dilute HCl is added to sodium carbonate.
Hint
Fumes; fizz.
Answer
(a) Dense white fumes: $N H_{3} + H C l \to N H_{4} C l$ . (b) Brisk effervescence: $N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$ .
Q5 · 3 marks · Short answerHarder board level
Explain with a diagram how back suction is prevented when hydrogen chloride is dissolved in water.
Hint
Inverted funnel.
Answer
Delivery tube attached to an inverted funnel whose rim just touches the water. When water rises in the funnel, the level in the beaker falls below the rim, air enters and the water falls back. (Diagram as in the checklist.)
Q6 · 2 marks · Very short answerBoard level
Name: (a) the gas formed when hydrochloric acid reacts with iron(II) sulphide; (b) the precipitate formed with silver nitrate; (c) the property of HCl shown by the fountain experiment; (d) the metals dissolved by aqua regia.
Hint
Half a mark each.
Answer
(a) Hydrogen sulphide (b) Silver chloride (c) High solubility (and acidity) (d) Gold and platinum
Q7 · 2 marks · Short answerHOTS / competency
Why is hydrochloric acid prepared from sodium chloride using concentrated sulphuric acid and not concentrated nitric acid?
Hint
Volatility.
Answer
Nitric acid is itself volatile (and an oxidising agent that would oxidise HCl); only the non-volatile sulphuric acid can displace the volatile HCl cleanly.
Q8 · 2 marks · Short answerHarder board level
Write the equation for the action of hydrochloric acid on manganese dioxide and state how this is used as a test for the acid.
Hint
Greenish-yellow gas.
Answer
$M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$ ; on warming, a greenish-yellow gas (chlorine) that bleaches moist litmus shows the acid contains chloride (HCl).
Q9 · 3 marks · Short answerHarder board level
A colourless solution gives a white precipitate with silver nitrate that is insoluble in nitric acid but soluble in ammonium hydroxide. Another sample gives effervescence with sodium hydrogen carbonate. Identify the solution and write both equations.
Hint
A chloride that is also an acid.
Answer
Hydrochloric acid. $A g N O_{3} + H C l \to A g C l + H N O_{3}$ ; $N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ .
Q10 · 2 marks · Short answerHOTS / competency
In a fountain experiment, the flask was not completely dry and no fountain formed. A second attempt used red litmus in the trough. Explain both results.
Hint
Gas dissolved early; red stays red.
Answer
Moisture already dissolved the HCl, so no partial vacuum formed when water was added. With red litmus, the acidic solution would not change colour, so the acidity would not be seen.
Q11 · 3 marks · Short answerHarder board level
State three differences between HCl gas and hydrochloric acid.
Hint
Particles, litmus, conductivity.
Answer
HCl gas is covalent and molecular; the acid contains H₃O⁺ and Cl⁻ ions. Dry gas has no effect on dry litmus; the acid turns blue litmus red. The gas does not conduct electricity; the acid conducts.

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.Reactants for lab preparation of HCl?Show answer
Sodium chloride and concentrated sulphuric acid.
2.Temperature for the lab preparation?Show answer
Below 200 °C.
3.Drying agent for HCl?Show answer
Concentrated sulphuric acid.
4.Composition of aqua regia?Show answer
3 parts conc. HCl : 1 part conc. HNO₃ by volume.
5.Colour of AgCl precipitate?Show answer
White (curdy).
6.Product of HCl with ammonia?Show answer
Ammonium chloride (dense white fumes).
7.Why is HCl not collected over water?Show answer
It is highly soluble in water.
8.What does the fountain experiment show?Show answer
High solubility of HCl and the acidic nature of its solution.

Best order to study it

Learn the preparation checklist and its diagram.
Learn the density test and the fountain experiment.
Learn the inverted funnel arrangement for hydrochloric acid.
Write equations for the acid with each type of compound and the precipitation reactions.
Finish with aqua regia and the tests.

Quick revision checklist

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

Can you write the lab equation, conditions, drying agent and collection method?
Can you explain why concentrated (not dilute) H₂SO₄ is used and why the temperature is kept low?
Can you describe the fountain experiment and the inverted funnel arrangement?
Can you write equations for HCl with Zn, Fe, CuO, NaOH, CaCO₃, NaHCO₃, Na₂SO₃, FeS, AgNO₃ and Pb(NO₃)₂?
Can you state the composition and uses of aqua regia?

Hydrogen Chloride: frequently asked questions

Is aqua regia new in the ICSE 2027 Chemistry syllabus?

Yes. The 2027 syllabus adds aqua regia (composition and uses) and tests for HCl gas and hydrochloric acid to this chapter.

Do I need the manufacture of HCl on a large scale?

No. The syllabus asks only for the laboratory preparation from sodium chloride.

Why does HCl fume in moist air?

It dissolves in water vapour in the air to form tiny droplets of hydrochloric acid, which look like fumes.

Why is iron(III) chloride not formed when iron reacts with hydrochloric acid?

Dilute hydrochloric acid is not an oxidising agent, so iron forms iron(II) chloride; iron(III) chloride is made by heating iron in chlorine.

Based on: CISCE ICSE 2027 Chemistry (Science Paper 2) syllabus; CISCE ICSE 2027 Chemistry (Science Paper 2) specimen question paper and marking scheme; CISCE Analysis of Pupil Performance, ICSE Chemistry (Science Paper 2), 2025; CISCE Analysis of Pupil Performance, ICSE Chemistry (Science Paper 2), 2024