LIWA

Study of Compounds: Hydrogen Chloride

ICSE Class 10 · Chemistry · Chapter 8

What this chapter covers

Key definitions

Hydrogen chloride is a colourless, pungent-smelling gas with the formula HCl, in which one hydrogen atom is joined to one chlorine atom by a single polar covalent bond.

Hydrochloric acid is the solution of hydrogen chloride gas in water.

Upward displacement of air is the method of collecting a gas that is heavier than air, in which the gas is led to the bottom of an upright jar and pushes the lighter air out through the top.

The fountain experiment is a demonstration in which water is drawn up into a flask of a very soluble gas and sprays out as a fountain, because the gas dissolving in a little water lowers the pressure inside the flask.

Back suction is the rising of water from the absorbing vessel back up the delivery tube into the generating flask, which happens when the gas dissolves faster than it is being produced.

Aqua regia is a freshly prepared mixture of three parts of concentrated hydrochloric acid and one part of concentrated nitric acid by volume.

Nascent chlorine is chlorine in the atomic state at the moment it is produced in a reaction, and it is far more reactive than ordinary chlorine gas.

A precipitate is an insoluble solid that separates out from a solution as a result of a chemical reaction.

Key points

Questions answered

Define hydrogen chloride and hydrochloric acid, and state the difference between them.
Hydrogen chloride is a colourless, pungent gas, HCl, in which hydrogen and chlorine are joined by a single polar covalent bond. Hydrochloric acid is the solution of hydrogen chloride in water. The dry gas shows no acidic properties; the solution is a strong acid because the molecules ionise in water to give H3O+ and Cl- ions.
Write the equation for the laboratory preparation of hydrogen chloride and state the condition.
NaCl + H2SO4 → NaHSO4 + HCl, by gentle heating below 200 °C.
Give two reasons why the temperature in the laboratory preparation is kept below 200 °C.
A higher temperature needs more fuel and may crack the glass flask, and the sodium sulphate then formed sticks to the flask as a hard crust that is difficult to remove.
Why is hydrogen chloride dried with concentrated sulphuric acid and not with quicklime?
Quicklime is basic and reacts with the acidic gas, CaO + 2HCl → CaCl2 + H2O. Concentrated sulphuric acid absorbs water without reacting with hydrogen chloride.
Give two reasons why hydrogen chloride is collected by upward displacement of air.
It is heavier than air (vapour density 18.25 against about 14.4 for air), and it is highly soluble in water, so it cannot be collected over water.
In the fountain experiment, why does water rise into the flask, and why does it turn red?
The first drops of water dissolve so much of the gas that the pressure inside falls far below atmospheric pressure; the outside air pressure then pushes the water up into the flask. It turns red because the solution of hydrogen chloride is acidic and changes blue litmus to red.
Explain how the inverted funnel arrangement prevents back suction when hydrochloric acid is prepared.
If water rises into the funnel, the level in the beaker falls below the rim of the funnel. Air enters, the pressure inside becomes atmospheric, and the water falls back, so it cannot reach the delivery tube. The funnel also gives a large surface for absorbing the gas.
What is observed when a glass rod dipped in ammonia solution is held near hydrogen chloride? Write the equation.
Dense white fumes of ammonium chloride form. NH3 + HCl → NH4Cl.

Past board exam questions

ICSE 2026 board paper · 1 mark

Rita added dilute hydrochloric acid to four metals. She recorded her observations in the table given below. While noting her observations she made some errors. | | Metals | Observations | |---|---|---| | 1 | copper | a gas was given off | | 2 | iron | a gas was given off | | 3 | magnesium | no gas was given off | | 4 | zinc | a gas was given off | Which observation(s) made by Rita is/are correct?

  1. 1 only
  2. 3 only
  3. Both 1 and 4
  4. Both 2 and 4
Show answer
Both 2 and 4

ICSE 2026 sample paper · 2 marks

Write balanced chemical equations for the following: (a) Laboratory preparation of hydrochloric acid from a less volatile acid. (b) Bromine gas is passed over ethene in the presence of carbon tetrachloride.

Show answer
(a) NaCl + conc. H₂SO₄ → NaHSO₄ + HCl (below 200 °C) (b) C₂H₄ + Br₂ → C₂H₄Br₂ (CH₂Br–CH₂Br, 1,2-dibromoethane)

ICSE 2025 compartment paper · 1 mark

A white precipitate is formed when dilute hydrochloric acid reacts with ‘X’. The white precipitate is soluble in excess of NH₄OH and insoluble in dilute HNO₃. Identify ‘X’.

  1. AgNO₃
  2. NH₄Cl
  3. AgCl
  4. CaCl₂
Show answer
AgNO₃

ICSE 2025 sample paper · 1 mark

Assertion (A): Dry hydrogen chloride gas is collected by the upward displacement of air. Reason (R): Hydrogen chloride gas is lighter than air.

  1. Both A and R are true and R is the correct explanation of A
  2. Both A and R are true but R is not the correct explanation of A
  3. A is true but R is false
  4. A is false but R is true
Show answer
A is true but R is false

ICSE 2024 sample paper · 5 marks

The setup described below is that of the fountain experiment with hydrogen chloride gas in the flask. (Setup: an inverted round-bottom flask filled with HCl gas is clamped to a retort stand. A jet tube passes through the cork into the flask, where it ends in a narrow jet from which the fountain emerges; its lower end dips into litmus solution in a trough below. A dropper also passes through the cork into the flask.) The fountain starts when a few drops of water from the dropper are introduced into the flask. Instead of the drops of water, Pooja started the fountain by introducing a few drops of Sodium hydroxide into the flask. (a) Explain why the litmus solution gets sucked up when Sodium hydroxide is used. (b) What will be the colour of the fountain when Sodium hydroxide is used? Justify your answer. (c) If instead of HCl gas, ammonia gas is filled in the flask and water is introduced from the dropper, will there be a different observation? Justify your answer.

Show answer
(a) Sodium hydroxide reacts with (neutralises) some of the hydrogen chloride gas: NaOH + HCl → NaCl + H₂O. This removes gas from the flask and creates a partial vacuum (low pressure) inside it, so the atmospheric pressure on the litmus solution in the trough pushes it up the jet tube into the flask. (b) Red. Only a few drops of NaOH are added, so they neutralise only a little of the HCl. The blue litmus solution that rushes in dissolves the remaining hydrogen chloride gas, forming hydrochloric acid, which turns blue litmus red. (c) Yes. Ammonia is also highly soluble in water, so a fountain is still formed, but ammonia dissolves to form ammonium hydroxide, which is basic: the fountain is …

ICSE 2024 board paper · 3 marks

Hydrogen chloride gas is prepared in the laboratory by the action of concentrated sulphuric acid on sodium chloride. (a) Give balanced chemical equation for the above reaction. (b) State the method of collection of the gas formed above. (c) What is the property of sulphuric acid that makes it a suitable reagent for the reaction?

Show answer
(a) NaCl + H₂SO₄ (conc.) —(below 200 °C)→ NaHSO₄ + HCl (b) By upward displacement of air (HCl is heavier than air and highly soluble in water). (c) Concentrated sulphuric acid is a non-volatile (less volatile) acid, so it displaces the more volatile HCl from its salt.

ICSE 2023 sample paper · 1 mark

Drying agent used to dry Hydrogen chloride gas:

  1. Concentrated Sulphuric acid
  2. Calcium oxide
  3. Sulphurous acid
  4. Calcium hydroxide
Show answer
Concentrated Sulphuric acid

ICSE 2023 board paper · 1 mark

In the laboratory preparation, HCl gas is dried by passing through:

  1. dilute nitric acid
  2. concentrated sulphuric acid
  3. dilute sulphuric acid
  4. acidified water
Show answer
concentrated sulphuric acid