ChemistryICSE Class 10 · Hydrogen Chloride

Laboratory Preparation of Hydrogen Chloride Gas (ICSE Class 10)

Q: How is hydrogen chloride prepared in the laboratory?

By heating sodium chloride with concentrated sulphuric acid below 200 °C, drying the gas with concentrated H₂SO₄ and collecting it by upward displacement of air.

Q: Why is HCl not collected over water?

It is extremely soluble in water and would dissolve.

Q: Why is concentrated sulphuric acid used to dry HCl but not ammonia?

It does not react with acidic HCl, but it reacts with basic ammonia to form ammonium sulphate.

Q: Why is the temperature kept below 200 °C?

To save fuel, avoid cracking the glass and avoid a hard crust of Na₂SO₄; the acid salt NaHSO₄ forms instead.

Updated 15 October 2026 · Fact-checked

Hydrogen chloride is prepared by heating sodium chloride with concentrated sulphuric acid below 200 °C: NaCl + H₂SO₄ → NaHSO₄ + HCl. The gas is dried by passing it through concentrated sulphuric acid and collected by upward displacement of air, because it is heavier than air and very soluble in water.

Understand Laboratory Preparation of Hydrogen Chloride

Reactants and equation. Sodium chloride (rock salt) and concentrated sulphuric acid are heated gently in a round-bottomed flask fitted with a thistle funnel and a delivery tube:

$N a C l + H_{2} S O_{4} < 200^{\circ} C N a H S O_{4} + H C l$

Why concentrated sulphuric acid? It is non-volatile, so it displaces the volatile hydrogen chloride from its salt. (With dilute acid, the very soluble HCl would simply stay dissolved in the water.)

Why below 200 °C? At a much higher temperature (above about 500 °C) the reaction goes further, $2 N a C l + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H C l$ . This needs more fuel, may crack the glass flask, and leaves sodium sulphate as a hard crust that is difficult to remove. So the low-temperature reaction, giving the acid salt NaHSO₄, is used.

Drying. The gas is bubbled through concentrated sulphuric acid in a wash bottle. Quicklime (CaO) and phosphorus pentoxide cannot be used: quicklime is basic and reacts with HCl; P₂O₅ also reacts with it.

Collection. By upward displacement of air: the delivery tube goes to the bottom of an upright gas jar, and the heavier HCl pushes the air up and out. HCl is heavier than air (vapour density 18.25, against about 14.4 for air). It is not collected over water because it is extremely soluble.

Gas jar full? A glass rod dipped in ammonia solution held at the mouth gives dense white fumes of ammonium chloride: $N H_{3} + H C l \to N H_{4} C l$ . Moist blue litmus held at the mouth turns red.

Properties to quote. HCl is a colourless gas with a pungent, choking smell; it fumes in moist air (it dissolves in water vapour, forming tiny droplets of acid). Dry HCl gas does not change dry litmus; moist HCl turns blue litmus red because it forms H₃O⁺ ions only in water.

Laboratory Preparation of Hydrogen Chloride: formulas and key facts

Preparation (below 200 °C): $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$
Above 500 °C (not used): $2 N a C l + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H C l$
Test with ammonia: $N H_{3} + H C l \to N H_{4} C l$
Preparation at a glance: Reactants sodium chloride and concentrated sulphuric acid
Condition gentle heat, below 200 °C
Drying agent concentrated sulphuric acid
Collection upward displacement of air
Jar full dense white fumes with an ammonia rod
Why not these drying agents: Quicklime basic, reacts with HCl
Phosphorus pentoxide reacts with HCl
Concentrated sulphuric acid does not react with HCl, so it is used
Ammonia and hydrogen chloride compared: Ammonia lighter than air, dried by quicklime, downward displacement of air
Hydrogen chloride heavier than air, dried by conc. H₂SO₄, upward displacement of air
Both too soluble to collect over water

Reactants	sodium chloride and concentrated sulphuric acid
Condition	gentle heat, below 200 °C
Drying agent	concentrated sulphuric acid
Collection	upward displacement of air
Jar full	dense white fumes with an ammonia rod

Quicklime	basic, reacts with HCl
Phosphorus pentoxide	reacts with HCl
Concentrated sulphuric acid	does not react with HCl, so it is used

Ammonia	lighter than air, dried by quicklime, downward displacement of air
Hydrogen chloride	heavier than air, dried by conc. H₂SO₄, upward displacement of air
Both	too soluble to collect over water

How to solve Laboratory Preparation of Hydrogen Chloride questions

Answer every preparation question with the same checklist and give a reason for each choice.

1Reactants: NaCl + concentrated H₂SO₄; equation giving NaHSO₄.
2Condition: below 200 °C, with the reasons (fuel, cracking, Na₂SO₄ crust).
3Property of H₂SO₄ used: non-volatile.
4Drying: concentrated H₂SO₄; say why CaO and P₂O₅ are unsuitable.
5Collection: upward displacement of air; reasons: heavier than air, very soluble.
6Test: white fumes with ammonia; moist blue litmus turns red.

Laboratory Preparation of Hydrogen Chloride 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.

Scaling the preparation

Suppose a teacher needs about 4.5 L of dry HCl gas at S.T.P. for a class fountain experiment and prepares it from rock salt. (Na = 23, Cl = 35.5, H = 1, S = 32, O = 16)

What masses of sodium chloride and sulphuric acid give 4.48 L of HCl, and what mass of solid is left in the flask?

Try it first, then show the solutionSolution

Moles of HCl $= 4.48/22.4 = 0.2$ ; NaCl : H₂SO₄ : NaHSO₄ : HCl = 1 : 1 : 1 : 1.
NaCl $= 0.2 \times 58.5 = 11.7$ g; H₂SO₄ $= 0.2 \times 98 = 19.6$ g.
NaHSO₄ left $= 0.2 \times 120 = 24$ g (check: 11.7 + 19.6 = 24 + 7.3 g HCl).

Answer: 11.7 g NaCl and 19.6 g H₂SO₄; 24 g of sodium hydrogen sulphate remains.

Memory hook: One salt, one acid, one HCl: below 200 °C everything is 1 : 1.

Tricks to remember Laboratory Preparation of Hydrogen Chloride

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

Heavy gas, upright jar
HCl is heavy: it sits in an upright jar like water in a cup (upward displacement of air).
Use it when: Collection method.
Acid dries acid
An acidic gas is dried by an acidic drier (conc. H₂SO₄); a basic gas (NH₃) by a basic drier (CaO).
Use it when: Choosing drying agents.
Two hundred, half salt
Below 200 °C you get the half-replaced salt NaHSO₄.
Use it when: Writing the correct equation.

Laboratory Preparation of Hydrogen Chloride: solved examples

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

Example 1: Drying agent · 1 mark · MCQBoard level

Hydrogen chloride gas is dried by passing it through:
(a) quicklime
(b) concentrated sulphuric acid
(c) phosphorus pentoxide
(d) solid sodium hydroxide

Try it first, then show the solutionSolution

Basic driers (CaO, NaOH) react with the acidic gas; P₂O₅ also reacts.
Concentrated sulphuric acid does not react with HCl.

Answer: (b).

Example 2: Why concentrated acid, and what is left behind · 2 marks · Short answerBoard level

(a) Why must concentrated, not dilute, sulphuric acid be heated with rock salt to obtain hydrogen chloride gas? (b) Write the equation for the reaction below 200 °C and name the solid left in the flask.

Try it first, then show the solutionSolution

(a) Concentrated sulphuric acid is non-volatile, so on heating it displaces the volatile HCl from the salt. With the dilute acid the very soluble HCl would stay dissolved in the water and little gas would escape.
(b) $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$ ; the residue is sodium hydrogen sulphate (NaHSO₄).

Answer: (a) It is non-volatile and contains little water, so HCl gas is driven off. (b) As above; sodium hydrogen sulphate remains.

Example 3: Reasons in the set-up · 3 marks · Short answerHarder board level

Give reasons: (a) HCl is collected by upward displacement of air; (b) it is not collected over water; (c) the reaction is carried out below 200 °C.

Try it first, then show the solutionSolution

(a) It is heavier than air (VD 18.25 against about 14.4).
(b) It is extremely soluble in water.
(c) Higher temperatures need more fuel, may crack the glass and leave a hard Na₂SO₄ crust.

Answer: Heavier than air; very soluble; avoid fuel waste, cracking and crust.

Example 4: Two equations, two temperatures · 3 marks · Short answerHarder board level

Write the equations for the reaction of sodium chloride with concentrated sulphuric acid (a) below 200 °C and (b) above 500 °C. Which is used in the laboratory and why?

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) $2 N a C l + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H C l$ .
(a) is used: it needs less heat, does not endanger the glass flask, and NaHSO₄ is easy to remove.

Answer: Below 200 °C reaction is used.

Example 5: Spot the faults · 3 marks · Short answerHOTS / competency

A student prepares HCl by heating sodium chloride with dilute sulphuric acid, passes the gas through a tower of quicklime, and collects it in an inverted gas jar. Identify three faults and correct them.

Try it first, then show the solutionSolution

Dilute acid: HCl would stay dissolved in the water. Use concentrated sulphuric acid.
Quicklime reacts with HCl. Dry with concentrated sulphuric acid.
Inverted jar suits gases lighter than air. HCl is heavier: use an upright jar (upward displacement of air).

Answer: Use concentrated H₂SO₄ as reactant and drier, and collect in an upright jar.

Example 6: Masses and volumes · 4 marks · Long answerHOTS / competency

11.7 g of sodium chloride is heated with excess concentrated sulphuric acid below 200 °C. Calculate (a) the volume of HCl at S.T.P., (b) the mass of HCl, (c) the volume of ammonia at S.T.P. that would just react with all of it, and the mass of ammonium chloride formed. (Na = 23, Cl = 35.5, N = 14, H = 1)

Try it first, then show the solutionSolution

Moles of NaCl $= 11.7/58.5 = 0.2$ = moles of HCl.
(a) $0.2 \times 22.4 = 4.48$ L. (b) $0.2 \times 36.5 = 7.3$ g.
(c) $N H_{3} + H C l \to N H_{4} C l$ : 0.2 mol NH₃ $= 4.48$ L.
NH₄Cl $= 0.2 \times 53.5 = 10.7$ g.

Answer: (a) 4.48 L (b) 7.3 g (c) 4.48 L of NH₃; 10.7 g NH₄Cl.

Where students lose marks in Laboratory Preparation of Hydrogen Chloride

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

Mistake: Using the high-temperature equation
In 2024 (Q8(iv)) examiners noted the higher-temperature equation or wrong products given for the laboratory preparation.
Wrong working
2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl
Right working
NaCl + H₂SO₄ → NaHSO₄ + HCl (below 200 °C)
Do this: The laboratory reaction gives NaHSO₄.
Usually costs: 1 mark.
Mistake: Wrong collection method
The same report found 'upward displacement of gas' or 'downward delivery of air' written.
Do this: Write 'upward displacement of air' (the gas is delivered downwards into an upright jar).
Usually costs: 1 mark.
Mistake: Wrong property of sulphuric acid
Candidates wrote 'dehydrating agent' or 'dibasic acid' instead of 'non-volatile' (2024 Q8(iv)).
Do this: Displacing a volatile acid from its salt uses the non-volatile nature.
Usually costs: 1 mark.
Mistake: Confusing HCl collection with ammonia
In 2024 (Q1(xii)) candidates mixed up the collection of ammonia with that of HCl.
Do this: HCl: heavy, upright jar. NH₃: light, inverted jar.
Usually costs: 1 mark.

Exam tips

Practise a labelled diagram: flask with NaCl and thistle funnel, wash bottle of conc. H₂SO₄, upright gas jar.
Learn 'below 200 °C' with its three reasons.
Learn the comparison table of HCl and NH₃ preparations; questions often pair them.
In definitions or reasons, use the words 'heavier than air' and 'highly soluble in water'.

Practice questions on Laboratory Preparation of 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
Hydrogen chloride gas is collected by:
(a) downward displacement of air
(b) upward displacement of air
(c) displacement of water
(d) displacement of mercury only
Hint
Is it heavier or lighter than air?
Answer
(b).
Q2 · 1 mark · Very short answerBoard level
Name the salt left in the flask when HCl is prepared below 200 °C.
Hint
Acid salt.
Answer
Sodium hydrogen sulphate (sodium bisulphate), NaHSO₄.
Q3 · 2 marks · Short answerBoard level
How would you know that a gas jar is full of hydrogen chloride? Write the equation.
Hint
Ammonia rod.
Answer
Dense white fumes form when a rod dipped in ammonia solution is held at the mouth. $N H_{3} + H C l \to N H_{4} C l$ .
Q4 · 1 mark · Very short answerHarder board level
Why does hydrogen chloride gas fume in moist air?
Hint
Solubility.
Answer
It dissolves in the water vapour of the air, forming tiny droplets of hydrochloric acid that appear as fumes.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Quicklime cannot be used to dry hydrogen chloride.
Reason (R): Quicklime is a basic oxide and reacts with hydrogen chloride.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
CaO + 2HCl → CaCl₂ + H₂O.
Answer
(a).
Q6 · 2 marks · Short answerHarder board level
Calculate the vapour density of hydrogen chloride and compare its density with that of air (VD of air about 14.4). (H = 1, Cl = 35.5)
Hint
VD = M/2.
Answer
M = 36.5, so VD $= 18.25$ . This is greater than about 14.4 for air, so HCl is heavier than air.
Q7 · 2 marks · Short answerHarder board level
Why is dry HCl gas without action on dry blue litmus, while moist HCl turns it red?
Hint
Ions form only in water.
Answer
Dry HCl is a covalent molecule and contains no H₃O⁺ ions. In water it ionises, $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ , and the hydronium ions turn blue litmus red.
Q8 · 2 marks · Short answerHOTS / competency
Suppose a student uses an inverted gas jar and finds that an ammonia rod held at its mouth shows only faint fumes even after a long time. Explain what went wrong and correct it.
Hint
Density.
Answer
HCl is heavier than air, so in an inverted jar it falls out of the open mouth instead of filling the jar. Use an upright jar with the delivery tube reaching to the bottom (upward displacement of air).

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.Drying agent for HCl?Show answer
Concentrated sulphuric acid.
2.Vapour density of HCl?Show answer
18.25.
3.Property of H₂SO₄ used in the preparation?Show answer
Non-volatile nature.
4.Product with NaCl above 500 °C?Show answer
Na₂SO₄ and HCl.
5.Colour of HCl gas?Show answer
Colourless (fumes in moist air).

Laboratory Preparation of Hydrogen Chloride: frequently asked questions

How is hydrogen chloride prepared in the laboratory?

By heating sodium chloride with concentrated sulphuric acid below 200 °C, drying the gas with concentrated H₂SO₄ and collecting it by upward displacement of air.

Why is HCl not collected over water?

It is extremely soluble in water and would dissolve.

Why is concentrated sulphuric acid used to dry HCl but not ammonia?

It does not react with acidic HCl, but it reacts with basic ammonia to form ammonium sulphate.

Why is the temperature kept below 200 °C?

To save fuel, avoid cracking the glass and avoid a hard crust of Na₂SO₄; the acid salt NaHSO₄ forms instead.