Study of Compounds Class 10 ICSE — Chemistry Chapter 8
The study of compounds class 10 ICSE chapter on hydrogen chloride (Chapter 8) follows one compound from discovery to use. It traces the gas from volcanic emissions to the lab bench, then into water as hydrochloric acid. You will learn two general preparations, the full laboratory setup, physical and chemical properties, the acid’s reactions, and four identification tests. The chapter ends with 17 exercise questions and board questions from 2010 to 2014.
Download Study of Compounds Hydrogen Chloride Class 10 ICSE Chemistry Chapter 8 PDF
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What is inside Chapter 8
10 pages · 8 sections · 17 exercise questions
| Section | Title |
|---|---|
| 8.1 | OCCURRENCE |
| 8.2 | GENERAL PREPARATION OF HYDROGEN CHLORIDE GAS |
| 8.3 | LABORATORY PREPARATION OF HYDROGEN CHLORIDE GAS |
| 8.4 | PHYSICAL PROPERTIES OF HYDROGEN CHLORIDE GAS |
| 8.5 | CHEMICAL PROPERTIES OF HYDROGEN CHLORIDE GAS |
| 8.6 | HYDROCHLORIC ACID |
| 8.6.2 | Properties of hydrochloric acid |
| 8.8 | TESTS FOR HYDROGEN CHLORIDE AND HYDROCHLORIC ACID |
- EXERCISE: 17 questions
8.1 OCCURRENCE
The history of hydrogen chloride stretches back to 1648. That year, Glauber prepared the acid by heating common salt (sodium chloride) with concentrated sulphuric acid. Lavoisier later named it muriatic acid. In 1810, Davy renamed it hydrochloric acid. The names reflect growing understanding of what the gas actually was.
The chapter records two places where hydrogen chloride appears naturally. First, the gas occurs in free state in volcanic emissions. Second, hydrochloric acid is present at 0.2–0.4 percent in the gastric juice of mammals, where it helps in digestion.
Why does the history matter? The reaction Glauber used in 1648 is the same one you set up in the laboratory section that follows. The old name “muriatic acid” survives in the modern term for the acid’s salts, chlorides. For the full set of chapters, see the Class 10 ICSE Chemistry book hub.
8.2 GENERAL PREPARATION OF HYDROGEN CHLORIDE GAS
The chapter gives two general routes to hydrogen chloride gas. The first is direct combination of the elements. Moist hydrogen gas combines with chlorine in the presence of diffused sunlight:
\mathrm{H}_{2}(\mathrm{g}) + \mathrm{Cl}_{2}(\mathrm{g}) \xrightarrow[katex display="true"]\text{Sunlight}{\text{Diffused}} 2\mathrm{HCl}(\mathrm{g})[/katex]
The reaction’s speed depends on light:
| Condition | Result |
|---|---|
| Direct sunlight | Explosive reaction |
| Dark | Negligible reaction |
| Dark with activated carbon catalyst | Reaction proceeds; carbon absorbs hydrogen and increases reactivity |
A burning jet of hydrogen also burns in chlorine, forming hydrogen chloride. This route works because both elements are available and the product is a single gas.
The second route heats a metallic chloride with concentrated sulphuric acid. The chloride is converted to the metal’s sulphate salt, and hydrogen chloride escapes as gas.
Original example with potassium chloride:
\mathrm{KCl} + \mathrm{H}_2\mathrm{SO}_4 \xrightarrow{\lt 200^\circ\mathrm{C}} \mathrm{KHSO}_4 + \mathrm{HCl}\uparrow
\mathrm{KHSO}_4 + \mathrm{KCl} \xrightarrow{\gt 200^\circ\mathrm{C}} \mathrm{K}_2\mathrm{SO}_4 + \mathrm{HCl}\uparrow
Two temperature stages matter. Below 200°C the acid salt forms. Above 200°C the normal salt forms with a second mole of HCl. The same pattern appears with sodium and copper chlorides in this Class 10 ICSE chapter.
8.3 LABORATORY PREPARATION OF HYDROGEN CHLORIDE GAS
The laboratory method uses common salt and concentrated sulphuric acid. Place common salt in a flask and pour concentrated sulphuric acid through a thistle funnel. The reaction proceeds slowly in the cold. Gentle heating brings effervescence and a controlled stream of HCl gas. The gas passes through a washer bottle of concentrated sulphuric acid for drying, then collects by upward displacement of air.
The two temperature-dependent reactions are:
\mathrm{NaCl} + \mathrm{H}_2\mathrm{SO}_4 \xrightarrow{\lt 200^\circ\mathrm{C}} \mathrm{NaHSO}_4 + \mathrm{HCl}\uparrow
\mathrm{NaHSO}_4 + \mathrm{NaCl} \xrightarrow{\gt 200^\circ\mathrm{C}} \mathrm{Na}_2\mathrm{SO}_4 + \mathrm{HCl}\uparrow
The reaction is reversible, yet it goes to completion. Hydrogen chloride continuously escapes as a gas, so the reverse step never builds up. That escape drives the equilibrium forward.
Sodium chloride is preferred because it is cheap. Concentrated nitric acid is never used because it is volatile and would escape with the gas. For drying, phosphorus pentoxide and quicklime (CaO) fail because they react with hydrogen chloride:
2\mathrm{P}_2\mathrm{O}_5 + 3\mathrm{HCl} \rightarrow \mathrm{POCl}_3 + 3\mathrm{HPO}_3
\mathrm{CaO} + 2\mathrm{HCl} \rightarrow \mathrm{CaCl}_2 + \mathrm{H}_2\mathrm{O}
Collection uses downward delivery because the gas is 1.28 times heavier than air. It is never collected over water because it dissolves rapidly. To test whether a jar is full, hold a glass rod dipped in ammonium hydroxide near the mouth. Dense white fumes of ammonium chloride appear:
\mathrm{HCl} + \mathrm{NH}_4\mathrm{OH} \rightarrow \mathrm{NH}_4\mathrm{Cl} + \mathrm{H}_2\mathrm{O}
Three precautions protect the setup:
- The thistle funnel’s lower end stays dipped in the acid.
- The delivery tube dips into the drying agent (conc. H₂SO₄).
- The temperature stays near 200°C.
Overheating cracks the glass, wastes fuel, and leaves a hard sodium sulphate crust that sticks to the glass.
Three common errors come straight from this section:
| Mistake | Correction |
|---|---|
| Using concentrated nitric acid | Volatile; it escapes with the gas. Use concentrated sulphuric acid. |
| Drying with P₂O₅ or quicklime | Both react with HCl. Use concentrated sulphuric acid in a washer bottle. |
| Collecting the gas over water | Highly soluble in water. Use upward displacement of air. |
8.4 PHYSICAL PROPERTIES OF HYDROGEN CHLORIDE GAS
Hydrogen chloride gas is a colourless gas with a pungent choking smell and a sour taste. It is corrosive and irritates the nose, throat, and lungs. The chapter’s property table gives the key numbers:
| Property | Value |
|---|---|
| Vapour density | 18.25 (air is 14.4) |
| Boiling point | −83°C |
| Melting point | −113°C |
| Liquefaction | 40 atmospheres at 10°C |
| Solubility | 452 volumes in 1 volume water at room temperature |
A density comparison explains the collection method. Hydrogen chloride has a molar mass near 36.5 g/mol, while average air is about 29 g/mol. Dividing gives 36.5 ÷ 29 ≈ 1.26, so the gas is roughly 1.26 times as dense as air. That matches the vapour density figures and is why upward displacement works: the heavy gas settles and pushes air out.
Two experiments demonstrate these properties. In the candle test, pouring the gas into a jar with a burning candle extinguishes it. The heavier gas occupies the lower portion and forces the air out.
In the fountain experiment, a dry flask of the gas connects to a jet tube dipping into blue litmus solution. Pressing a dropper releases a few drops of water. The gas dissolves, pressure inside drops, and outside pressure pushes the blue solution up the tube as a red fountain. Blue to red proves the acidic nature.
8.5 CHEMICAL PROPERTIES OF HYDROGEN CHLORIDE GAS
The dry gas shows four chemical behaviours. First, it is neither combustible nor a supporter of combustion; it extinguishes a burning splint. Second, above 500°C it dissociates into its elements:
2\mathrm{HCl} \xrightarrow{\gt 500^{\circ}\mathrm{C}} \mathrm{H}_{2} + \mathrm{Cl}_{2}
Third, it reacts with heated metals that sit above hydrogen in the electrochemical series, forming the metal chloride and hydrogen. The textbook table lists sodium, calcium, magnesium, zinc, and iron. An original example with aluminium:
2\mathrm{Al} + 6\mathrm{HCl} \rightarrow 2\mathrm{AlCl}_3 + 3\mathrm{H}_2
Two conditions must hold: the metal lies above hydrogen in the series, and the mixture is heated.
Fourth, the gas combines with ammonia to give dense white fumes of ammonium chloride:
\mathrm{NH}_{3}(\mathrm{g}) + \mathrm{HCl}(\mathrm{g}) \rightarrow \mathrm{NH}_{4}\mathrm{Cl}(\mathrm{s})
Two gases become a solid. This reaction returns as the jar-full test in section 8.3 and as test 2 in section 8.8. Ammonia itself gets a full treatment in the companion chapter on ammonia.
8.6 HYDROCHLORIC ACID
Dissolving hydrogen chloride gas in water gives hydrochloric acid. The polar covalent compound ionises in water:
\mathrm{HCl} + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{H}_{3}\mathrm{O}^{+} + \mathrm{Cl}^{-}
The hydronium ion (H₃O⁺) is what makes the solution acidic. This explains a key contrast in this ICSE chapter:
| Form of HCl | Litmus | Conducts electricity | H₃O⁺ present? |
|---|---|---|---|
| Dry gas / liquefied | No change | No | No |
| Solution in water | Blue → red | Yes | Yes |
| Solution in toluene | No change | No | No |
Dry gas and liquefied gas do not turn blue litmus red, showing their non-acidic, covalent character. The toluene solution behaves the same way. Only the aqueous solution shows acidity and conductivity, because only water produces hydronium ions.
The acid is prepared by passing the gas into water until no more is absorbed. The concentrated product holds about 36 percent HCl by mass.
An inverted funnel prevents back suction. The funnel rim just touches the water surface. Water rises into the funnel, an air gap opens between rim and water, pressures equalise, and the water falls back. The cycle repeats until saturation.
Back suction is dangerous because water reaching hot concentrated sulphuric acid can cause an explosion. An empty anti-suction flask sits between the generating flask and the trough as a safety barrier.
Boiling concentrates the acid only to 22.2 percent HCl by weight at 110°C. Beyond that, HCl gas escapes with the water vapour. This constant-boiling mixture is called an azeotrope.
8.6.2 Properties of hydrochloric acid
Hydrochloric acid is a colourless, pungent, sour liquid. The concentrated acid is corrosive and causes blisters on the skin. It dissolves in water in all proportions and boils at 110°C. Its aqueous solution is strongly acidic and shows all the typical properties of acids.
Action on metals
Metals above hydrogen in the activity series form metallic chlorides and evolve hydrogen. Examples: Zn + 2HCl → ZnCl₂ + H₂↑; Mg + 2HCl → MgCl₂ + H₂↑.
Action on oxides and hydroxides
Oxides and hydroxides (bases) give salt and water only. Examples: MgO + 2HCl → MgCl₂ + H₂O; NaOH + HCl → NaCl + H₂O.
With salts of weaker acids
Carbonates, hydrogen carbonates, sulphites, and sulphides are decomposed, releasing CO₂, SO₂, or H₂S. Original examples:
\mathrm{BaCO}_3 + 2\mathrm{HCl} \rightarrow \mathrm{BaCl}_2 + \mathrm{H}_2\mathrm{O} + \mathrm{CO}_2\uparrow
\mathrm{ZnS} + 2\mathrm{HCl} \rightarrow \mathrm{ZnCl}_2 + \mathrm{H}_2\mathrm{S}\uparrow
Action on thiosulphates
Dilute HCl gives sulphur dioxide and a yellow precipitate of sulphur:
\mathrm{Na}_{2}\mathrm{S}_{2}\mathrm{O}_{3} + 2\mathrm{HCl} \rightarrow 2\mathrm{NaCl} + \mathrm{H}_{2}\mathrm{O} + \mathrm{SO}_{2} + \mathrm{S}\downarrow
This distinguishes thiosulphates from sulphites, which give no sulphur precipitate.
Reaction with nitrates
Dilute HCl does not normally react with nitrates. Lead nitrate and mercury(I) nitrate give white precipitates. Silver nitrate gives the curdy white silver chloride test precipitate, insoluble in nitric acid but soluble in ammonium hydroxide. In light, silver chloride blackens as it decomposes to silver and chlorine.
Oxidation of hydrochloric acid
Strong oxidising agents — MnO₂, PbO₂, Pb₃O₄, KMnO₄, bleaching powder — oxidise concentrated HCl to chlorine:
\mathrm{MnO}_2 + 4\mathrm{HCl} \rightarrow \mathrm{MnCl}_2 + 2\mathrm{H}_2\mathrm{O} + \mathrm{Cl}_2
Formation of aqua regia
Aqua regia mixes 3 parts concentrated HCl with 1 part concentrated nitric acid. It produces nascent chlorine, which converts gold and platinum to soluble chlorides:
\mathrm{Au} + 3[katex display="true"]\mathrm{Cl} \rightarrow \mathrm{AuCl}_3[/katex]
\mathrm{Pt} + 4[katex display="true"]\mathrm{Cl} \rightarrow \mathrm{PtCl}_4[/katex]
Decision rule for the acid:
| Acid | Reacts with |
|---|---|
| Dilute HCl | Metals (above H), oxides, hydroxides, carbonates, hydrogen carbonates, sulphites, sulphides, thiosulphates |
| Concentrated HCl | Oxidised to Cl₂ by MnO₂, PbO₂, Pb₃O₄, KMnO₄, bleaching powder; forms aqua regia |
General and industrial uses
Uses include the lab reagent, manufacture of chlorine and chlorides, dyes, drugs, paints, photographic chemicals, glucose from starch, pickling steel, purifying bone black, removing rust, cleaning metal surfaces, extracting glue from bones, and tanning. Dilute HCl is prescribed for patients with low gastric juice activity; it digests proteins and destroys microorganisms in the alimentary canal.
8.8 TESTS FOR HYDROGEN CHLORIDE AND HYDROCHLORIC ACID
Four tests identify hydrogen chloride gas and hydrochloric acid. First, the gas has a characteristic irritating smell. Second, a glass rod dipped in ammonia solution held near the vapours gives thick white fumes of ammonium chloride:
\mathrm{NH}_3 + \mathrm{HCl} \rightarrow \mathrm{NH}_4\mathrm{Cl}
Third, silver nitrate solution gives a white precipitate of silver chloride:
\mathrm{AgNO}_3 + \mathrm{HCl} \rightarrow \mathrm{AgCl}\downarrow + \mathrm{HNO}_3
The precipitate is insoluble in nitric acid but soluble in ammonium hydroxide. Fourth, heating concentrated acid with manganese dioxide liberates greenish-yellow chlorine gas:
\mathrm{MnO}_2 + 4\mathrm{HCl} \rightarrow \mathrm{MnCl}_2 + 2\mathrm{H}_2\mathrm{O} + \mathrm{Cl}_2
The chlorine turns moist starch iodide paper blue-black.
Worked identification pattern. Suppose an unknown gas gives dense white fumes with ammonia and a white precipitate with silver nitrate that dissolves in ammonium hydroxide. The gas is hydrogen chloride; the precipitate is silver chloride. The ammonia test and the silver nitrate test together rule out other gases.
What to practise from Study of Compounds Class 10 ICSE
The chapter carries one exercise block and five years of board questions. Practise them in order.
- EXERCISE — 17 questions
- Board questions 2010, 2011, 2012, 2013, 2014
Highest-value items: the labelled diagram of the laboratory preparation (question 1), the complete-and-balance set (question 9), the funnel arrangement question from 2011, the fountain experiment questions from 2010 and 2014, and the aqua regia composition question from 2010.
These board questions map back to the textbook sections. The fountain experiment questions (2010, 2014) come from section 8.4. The aqua regia composition (2010, 2012) comes from section 8.6.2. The drying agent choice (2011) comes from section 8.3. You can verify the Class 10 Chemistry syllabus on the official CISCE website. The full set of textbooks appears in the Class 10 ICSE books index.
Study of Compounds Class 10 ICSE frequently asked questions
What is in the study of compounds class 10 ICSE chapter on hydrogen chloride?
The chapter covers occurrence, general and laboratory preparation, physical and chemical properties, hydrochloric acid and its reactions, and the four identification tests. It ends with a 17-question exercise and board questions from 2010 to 2014.
Why is hydrogen chloride gas not collected over water?
Because it is highly soluble. One volume of water dissolves 452 volumes of the gas at room temperature, so the gas would dissolve before you could collect it.
Why does dry hydrogen chloride gas not turn blue litmus red?
Acidity needs hydronium ions (H₃O⁺). These form only when the gas dissolves in water, so dry gas shows no acid character.
What does the fountain experiment demonstrate?
It demonstrates high solubility and the acidic nature of the gas. The dissolving gas lowers the pressure inside the flask, and the blue litmus solution rises and turns red.
Why can aqua regia dissolve gold and platinum?
Aqua regia is 3 parts concentrated HCl with 1 part concentrated nitric acid. It produces nascent chlorine, which converts gold to AuCl₃ and platinum to PtCl₄, both soluble chlorides.
Reference: ICSE Class 10 Chemistry textbook material, chapter 8: Study of Compounds Hydrogen Chloride — CISCE-aligned.
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