ChemistryICSE Class 10 · Hydrogen Chloride

Properties and Tests of Hydrochloric Acid and HCl Gas (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

Hydrochloric acid is a strong monobasic acid. It reacts with active metals to give hydrogen, with oxides and hydroxides to give chlorides and water, and with carbonates, sulphites and sulphides to give CO₂, SO₂ and H₂S. It gives a white precipitate of AgCl with silver nitrate and of PbCl₂ with lead nitrate.

Notice that each type of reactant gives its own product: H₂, CO₂, SO₂ or H₂S gas, white fumes, or a white chloride precipitate.

Illustration drawn for QuestionBuddy.

Understand Properties and Tests of Hydrochloric Acid

Strong, monobasic acid. In water, hydrogen chloride ionises completely: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$ . One molecule gives one hydronium ion, so it is monobasic and forms only one series of salts, the chlorides.

Acidic properties.

Active metals give the chloride and hydrogen: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ ; $F e + 2 H C l \to F e C l_{2} + H_{2}$ . Iron forms iron(II) chloride, FeCl₂, not iron(III) chloride: hydrochloric acid is not an oxidising acid, so iron goes only to the +2 state.
Oxides and hydroxides give chloride and 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 give carbon dioxide with brisk effervescence: $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ ; $N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ .
Sulphites and hydrogen sulphites give sulphur dioxide: $N a_{2} S O_{3} + 2 H C l \to 2 N a C l + H_{2} O + S O_{2}$ .
Sulphides give hydrogen sulphide (rotten-egg smell): $F e S + 2 H C l \to F e C l_{2} + H_{2} S$ .

Reaction with ammonia. HCl gas and ammonia form dense white fumes of ammonium chloride: $N H_{3} + H C l \to N H_{4} C l$ .

Precipitation reactions (chloride ion).

Silver nitrate: a white, curdy precipitate of silver chloride, insoluble in dilute nitric acid but soluble in excess ammonium hydroxide: $A g N O_{3} + H C l \to A g C l + H N O_{3}$ .
Lead nitrate: a white precipitate of lead chloride, which dissolves on heating and reappears on cooling, and does not dissolve in ammonium hydroxide: $P b (N O_{3})_{2} + 2 H C l \to P b C l_{2} + 2 H N O_{3}$ .

Aqua regia. A mixture of 3 parts concentrated hydrochloric acid and 1 part concentrated nitric acid by volume. Nitric acid oxidises HCl, releasing nascent chlorine, which converts the noble metals gold and platinum into soluble chlorides; neither acid alone dissolves them. It is used to dissolve gold and platinum, to clean and etch metals, and in testing and refining noble metals.

Tests.

HCl gas: fumes in moist air; turns moist blue litmus red; dense white fumes with a rod dipped in ammonia; white precipitate when passed into silver nitrate solution.
Hydrochloric acid: white curdy precipitate with silver nitrate, insoluble in dilute HNO₃; warmed with manganese dioxide it gives greenish-yellow chlorine: $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$ .

Properties and Tests of Hydrochloric Acid: formulas and key facts

Ionisation (monobasic): $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$
With silver nitrate: $A g N O_{3} + H C l \to A g C l + H N O_{3}$
With lead nitrate: $P b (N O_{3})_{2} + 2 H C l \to P b C l_{2} + 2 H N O_{3}$
With manganese dioxide (test): $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$
With iron and with a sulphide: $F e + 2 H C l \to F e C l_{2} + H_{2}; F e S + 2 H C l \to F e C l_{2} + H_{2} S$
White chloride precipitates compared: Silver chloride: white, curdy; insoluble in dilute nitric acid; dissolves in excess ammonium hydroxide | Lead chloride: white; dissolves on heating, reappears on cooling; insoluble in ammonium hydroxide
Aqua regia: Composition 3 volumes conc. HCl to 1 volume conc. HNO₃
Active agent nascent chlorine
Dissolves gold and platinum
Uses dissolving and refining noble metals, cleaning and etching metals

Composition	3 volumes conc. HCl to 1 volume conc. HNO₃
Active agent	nascent chlorine
Dissolves	gold and platinum
Uses	dissolving and refining noble metals, cleaning and etching metals

How to solve Properties and Tests of Hydrochloric Acid questions

Hydrochloric acid questions are equation-and-observation questions. Use the acid patterns and learn the two white precipitates.

1Classify the other reactant: metal, oxide/hydroxide, carbonate, sulphite, sulphide, or a salt that gives a precipitate (AgNO₃, Pb(NO₃)₂).
2Write the product pattern and balance: chlorides need two HCl for a divalent metal (ZnCl₂, CaCl₂, FeCl₂).
3Name and test the gas: CO₂ (lime water milky), SO₂ (dichromate green), H₂S (rotten eggs), H₂ (pop).
4For precipitates, give colour, then the behaviour with dilute HNO₃, heat or excess NH₄OH.
5For aqua regia, give the 3 : 1 ratio and the reason it dissolves gold.

Properties and Tests of Hydrochloric Acid 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.

Acid cleaning of limescale

Suppose a cleaner uses dilute hydrochloric acid to remove 5 g of limescale (calcium carbonate) from tiles in a small bathroom. (Ca = 40, C = 12, O = 16, H = 1, Cl = 35.5)

Write the equation, calculate the volume of gas released at S.T.P. and the mass of HCl used, and explain why the bathroom should be ventilated.

Try it first, then show the solutionSolution

$C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ .
Moles of CaCO₃ $= 5/100 = 0.05$ ; CO₂ $= 0.05 \times 22.4 = 1.12$ L; HCl $= 0.1$ mol $= 3.65$ g.
Fumes of HCl from the acid are choking and corrosive, and CO₂ builds up in a closed space, so the room needs fresh air.

Answer: 1.12 L of CO₂; 3.65 g of HCl used; ventilate because of acid fumes and CO₂.

Memory hook: Acid plus carbonate fizzes: salt, water and carbon dioxide.

Tricks to remember Properties and Tests of Hydrochloric Acid

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

Silver stays, lead leaves with heat
AgCl stays put in hot water but runs away into ammonia; PbCl₂ melts away (dissolves) in hot water but ignores ammonia.
Use it when: Telling the two white chloride precipitates apart.
Aqua regia, 3 and 1
'Three Hydrochloric, one Nitric: the royal water rules gold.'
Use it when: Composition and use of aqua regia.
Iron stays two
Fe + HCl gives FeCl₂, never FeCl₃.
Use it when: Writing the iron equation.

Properties and Tests of Hydrochloric Acid: solved examples

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

Example 1: Aqua regia ratio · 1 mark · MCQBoard level

Aqua regia is a mixture of concentrated hydrochloric acid and concentrated nitric acid in the volume ratio:
(a) 1 : 1
(b) 1 : 3
(c) 3 : 1
(d) 2 : 1

Try it first, then show the solutionSolution

Three parts HCl to one part HNO₃.

Answer: (c) 3 : 1.

Example 2: Two salts, one acid · 2 marks · Short answerBoard level

Write the balanced equation and one observation when dilute hydrochloric acid is added to (a) sodium sulphite, (b) iron(II) sulphide.

Try it first, then show the solutionSolution

(a) $N a_{2} S O_{3} + 2 H C l \to 2 N a C l + H_{2} O + S O_{2}$ : a gas with the smell of burning sulphur.
(b) $F e S + 2 H C l \to F e C l_{2} + H_{2} S$ : a gas with a rotten-egg smell.

Answer: SO₂ (burning sulphur smell) and H₂S (rotten eggs).

Example 3: Two white precipitates · 3 marks · Short answerHarder board level

Two unlabelled bottles hold silver nitrate solution and lead nitrate solution. A few drops of dilute hydrochloric acid give a white precipitate with each. (a) Describe a test using heat that tells the two precipitates apart. (b) Describe a second test using another reagent. (c) Write the equation for each precipitation.

Try it first, then show the solutionSolution

(a) Heat each test tube: lead chloride dissolves in hot water and reappears as crystals on cooling; silver chloride does not dissolve.
(b) Add excess ammonium hydroxide: silver chloride dissolves; lead chloride does not.
(c) $A g N O_{3} + H C l \to A g C l + H N O_{3}$ ; $P b (N O_{3})_{2} + 2 H C l \to P b C l_{2} + 2 H N O_{3}$ .

Answer: Heat: PbCl₂ dissolves (reappears on cooling), AgCl does not. Excess NH₄OH: AgCl dissolves, PbCl₂ does not.

Example 4: Why aqua regia dissolves gold · 3 marks · Short answerHarder board level

Gold does not dissolve in concentrated hydrochloric acid or in concentrated nitric acid alone. (a) What is aqua regia? (b) Explain how it dissolves gold. (c) Give two uses.

Try it first, then show the solutionSolution

(a) 3 volumes of concentrated HCl mixed with 1 volume of concentrated HNO₃.
(b) Nitric acid oxidises hydrochloric acid, releasing nascent chlorine, which converts gold into a soluble chloride.
(c) Dissolving gold and platinum; cleaning and etching metals (also testing purity of gold).

Answer: As above.

Example 5: Masses with silver nitrate · 3 marks · Short answerHOTS / competency

A solution containing 0.73 g of hydrogen chloride is treated with excess silver nitrate solution. (a) Calculate the mass of precipitate. (b) How would you show the precipitate is a chloride and not a carbonate? (H = 1, Cl = 35.5, Ag = 108)

Try it first, then show the solutionSolution

Moles of HCl $= 0.73/36.5 = 0.02$ = moles of AgCl.
(a) AgCl (M = 143.5): $0.02 \times 143.5 = 2.87$ g.
(b) Add dilute nitric acid: AgCl does not dissolve (silver carbonate would dissolve with effervescence).

Answer: (a) 2.87 g (b) insoluble in dilute HNO₃.

Example 6: Chlorine from the acid · 4 marks · Long answerHOTS / competency

8.7 g of manganese dioxide is warmed with excess concentrated hydrochloric acid. (a) Write the equation. (b) Calculate the volume of chlorine at S.T.P. and the mass of HCl used. (c) Suggest a test to show the gas is chlorine. (Mn = 55, O = 16, H = 1, Cl = 35.5)

Try it first, then show the solutionSolution

(a) $M n O_{2} + 4 H C l \to M n C l_{2} + 2 H_{2} O + C l_{2}$ .
Moles of MnO₂ $= 8.7/87 = 0.1$ .
(b) Cl₂ $= 0.1 \times 22.4 = 2.24$ L; HCl $= 0.4 \times 36.5 = 14.6$ g.
(c) Greenish-yellow gas that bleaches moist blue litmus (turning it red first); it turns moist starch-iodide paper blue-black.

Answer: (b) 2.24 L Cl₂; 14.6 g HCl. (c) Bleaches moist litmus; starch-iodide turns blue-black.

Where students lose marks in Properties and Tests of Hydrochloric Acid

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

Mistake: Silver chloride said to be insoluble in excess NH₄OH
In 2025 (Q5(iv)) examiners noted candidates claiming that the silver chloride precipitate is insoluble in excess ammonium hydroxide.
Do this: AgCl dissolves in excess NH₄OH; PbCl₂ does not.
Usually costs: 1 mark.
Mistake: Observation confused with product
The same report found candidates naming the product instead of describing what is seen, or giving only the colour.
Wrong working
Silver chloride is formed.
Right working
A white precipitate forms, which dissolves in excess ammonium hydroxide.
Do this: An observation describes colour, state and what happens with excess reagent.
Usually costs: 1 mark.
Mistake: Cl instead of Cl₂
In 2024 (Q5(iv)) examiners noted Cl or 'chloride ion' written for the gas from MnO₂ and HCl.
Do this: The gas is chlorine, Cl₂.
Usually costs: 1 mark.
Mistake: FeCl₃ from iron and HCl
Students write the higher chloride.
Do this: Fe + 2HCl → FeCl₂ + H₂.
Usually costs: 1 mark.

Exam tips

Learn the white precipitates of AgCl and PbCl₂ with their three distinguishing behaviours (dilute HNO₃, heat, excess NH₄OH).
For gases, name and test: CO₂, SO₂, H₂S, H₂, Cl₂.
Aqua regia: give the ratio by volume and the word 'nascent chlorine'.
Write 'dilute' for the acidic reactions and 'concentrated' for MnO₂ and aqua regia.

Practice questions on Properties and Tests of Hydrochloric Acid

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 precipitate formed when hydrochloric acid is added to silver nitrate solution is:
(a) yellow and soluble in ammonia
(b) white and soluble in ammonium hydroxide
(c) white and soluble in dilute nitric acid
(d) black
Hint
AgCl.
Answer
(b).
Q2 · 1 mark · Very short answerBoard level
Why is hydrochloric acid called monobasic?
Hint
Number of H₃O⁺ ions.
Answer
One molecule gives one hydronium ion in water (one replaceable hydrogen).
Q3 · 2 marks · Short answerBoard level
Write balanced equations for dilute hydrochloric acid with (a) zinc, (b) copper(II) oxide.
Hint
Chloride + H₂; chloride + water.
Answer
(a) $Z n + 2 H C l \to Z n C l_{2} + H_{2}$ (b) $C u O + 2 H C l \to C u C l_{2} + H_{2} O$ .
Q4 · 2 marks · Short answerHarder board level
A student writes $2 F e + 6 H C l \to 2 F e C l_{3} + 3 H_{2}$ . Correct the equation and give the reason.
Hint
Oxidation state of iron.
Answer
$F e + 2 H C l \to F e C l_{2} + H_{2}$ . Dilute hydrochloric acid is not an oxidising acid, so iron is converted only to iron(II) chloride.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Lead chloride precipitate dissolves when the test tube is heated.
Reason (R): Lead chloride is more soluble in hot water than in cold water.
(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
It reappears on cooling.
Answer
(a).
Q6 · 2 marks · Short answerHarder board level
Give the action of dilute hydrochloric acid on sodium hydrogen carbonate with equation and test for the gas.
Hint
Effervescence.
Answer
$N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ . Brisk effervescence; the gas turns lime water milky.
Q7 · 3 marks · Short answerHarder board level
2.8 g of iron filings react with excess dilute hydrochloric acid. Calculate the volume of hydrogen at S.T.P. and the mass of iron(II) chloride formed. (Fe = 56, Cl = 35.5)
Hint
1 : 1 : 1.
Answer
Moles of Fe $= 0.05$ . H₂ $= 1.12$ L. FeCl₂ (M = 127) $= 0.05 \times 127 = 6.35$ g.
Q8 · 2 marks · Short answerHOTS / competency
Suppose a goldsmith dips a ring into concentrated hydrochloric acid and then into concentrated nitric acid separately, and nothing happens; but in a 3 : 1 mixture of the two, the ring's surface dissolves. Explain, and say what this suggests about the ring.
Hint
Nascent chlorine; brass would have reacted with nitric acid alone.
Answer
The mixture is aqua regia: nitric acid oxidises HCl to nascent chlorine, which converts gold into a soluble chloride. Since neither acid alone attacked it, the surface is gold (a base metal such as brass would have reacted with nitric acid).

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.Gas from HCl and a sulphide?Show answer
Hydrogen sulphide.
2.Composition of aqua regia?Show answer
3 volumes conc. HCl : 1 volume conc. HNO₃.
3.Precipitate with lead nitrate?Show answer
White lead chloride, soluble in hot water.
4.Product of MnO₂ and conc. HCl that is a gas?Show answer
Chlorine.
5.Basicity of HCl?Show answer
1 (monobasic).

Properties and Tests of Hydrochloric Acid: frequently asked questions

What is the test for hydrochloric acid?

Add silver nitrate solution: a white curdy precipitate of AgCl, insoluble in dilute nitric acid but soluble in ammonium hydroxide.

What is aqua regia used for?

Dissolving noble metals such as gold and platinum, testing and refining them, and cleaning or etching metals.

How do you tell AgCl from PbCl₂?

PbCl₂ dissolves in hot water and reappears on cooling; AgCl does not dissolve on heating but dissolves in excess ammonium hydroxide.

Does hydrochloric acid react with copper?

No. Copper is below hydrogen in the activity series and hydrochloric acid is not an oxidising acid.