Hydrogen Chloride Dissolves So Fast It Pulls Water Up Into a Fountain
Prepare hydrogen chloride in the laboratory with every condition and precaution, show it is heavier than air and extremely soluble through the fountain experiment, prevent back suction when making hydrochloric acid, write its acidic reactions, and learn aqua regia and the tests for chloride ions.
What makes hydrogen chloride such a dramatic gas to study?
Invert a dry flask full of hydrogen chloride over a trough of water with a jet tube dipping in, and within seconds water shoots up the tube and sprays into the flask like a fountain, turning red as it goes if blue litmus has been added. No pump is involved. The gas dissolves in the water so quickly that it leaves an almost empty space behind, and the air outside pushes water in to fill it.
That single demonstration shows the three properties that shape everything about hydrogen chloride:
- It is extremely soluble in water, which decides how it must be collected and how hydrochloric acid is made safely
- Its solution is acidic, because it ionises completely in water
- It is heavier than air, which decides the direction of collection
Hydrogen chloride is also familiar in another form. Hydrochloric acid — hydrogen chloride dissolved in water — is present in your stomach, where it helps digestion, and it is sold in dilute form as a cleaner for tiles and metal surfaces.
This part follows the ICSE pattern for studying a compound:
- Laboratory preparation, with reactants, conditions, apparatus, precautions, collection and identification
- Physical properties — density and solubility — with the experiments that show them, and the arrangement that prevents back suction
- Chemical properties as an acid
- Aqua regia, precipitation reactions and tests for the gas, the acid and the chloride ion
Much of the chemistry is already familiar. The acidic reactions come from the acids chapter, the precipitation tests from analytical chemistry, and the ionisation from electrolysis. What is new is the practical detail — the conditions, the precautions and the reasons behind each choice.
One distinction to keep clear throughout. Hydrogen chloride is the covalent gas, , made of molecules. Hydrochloric acid is its solution in water, containing hydronium and chloride ions. Dry hydrogen chloride does not behave as an acid, which is why the difference matters.
This page covers the ICSE Class 10 Chemistry study of hydrogen chloride: laboratory preparation, density and solubility with the fountain experiment, acidic properties, aqua regia and tests.
That single demonstration shows the three properties that shape everything about hydrogen chloride:
- It is extremely soluble in water, which decides how it must be collected and how hydrochloric acid is made safely
- Its solution is acidic, because it ionises completely in water
- It is heavier than air, which decides the direction of collection
Hydrogen chloride is also familiar in another form. Hydrochloric acid — hydrogen chloride dissolved in water — is present in your stomach, where it helps digestion, and it is sold in dilute form as a cleaner for tiles and metal surfaces.
This part follows the ICSE pattern for studying a compound:
- Laboratory preparation, with reactants, conditions, apparatus, precautions, collection and identification
- Physical properties — density and solubility — with the experiments that show them, and the arrangement that prevents back suction
- Chemical properties as an acid
- Aqua regia, precipitation reactions and tests for the gas, the acid and the chloride ion
Much of the chemistry is already familiar. The acidic reactions come from the acids chapter, the precipitation tests from analytical chemistry, and the ionisation from electrolysis. What is new is the practical detail — the conditions, the precautions and the reasons behind each choice.
One distinction to keep clear throughout. Hydrogen chloride is the covalent gas, , made of molecules. Hydrochloric acid is its solution in water, containing hydronium and chloride ions. Dry hydrogen chloride does not behave as an acid, which is why the difference matters.
This page covers the ICSE Class 10 Chemistry study of hydrogen chloride: laboratory preparation, density and solubility with the fountain experiment, acidic properties, aqua regia and tests.
How is hydrogen chloride prepared in the laboratory, collected and identified?
Sodium chloride is heated gently with concentrated sulphuric acid below about 200 °C; the gas is dried through concentrated sulphuric acid and collected by upward displacement of air, and identified by its fumes with ammonia and its effect on blue litmus.
Reactants: sodium chloride (rock salt) and concentrated sulphuric acid.
Condition: gentle heating, keeping the temperature **below about **:
**Above about **, the sodium hydrogen sulphate reacts with more salt:
Why the lower temperature is preferred:
- Sodium sulphate forms a hard crust that sticks to the flask and is difficult to remove
- Higher heating wastes fuel and can crack the glass
- The lower-temperature reaction gives the gas conveniently without these problems
Apparatus:
- A round-bottom flask containing the salt, fitted with a thistle funnel for adding the acid and a delivery tube
- A wash bottle of concentrated sulphuric acid through which the gas passes to be dried
- Gas jars for collection
Drying agent: concentrated sulphuric acid. Quicklime cannot be used, because it is basic and would react with the acidic gas.
Collection: by upward displacement of air — the delivery tube goes to the bottom of an upright gas jar, because hydrogen chloride is heavier than air. It is not collected over water, because it is extremely soluble.
Precautions:
- The lower end of the thistle funnel must dip below the acid, or the gas escapes through the funnel
- All joints must be airtight
- The temperature should be kept low, for the reasons above
- The gas is irritating and corrosive, so the preparation is done in a well-ventilated place or fume cupboard
Identification of the gas:
- Colourless, with a pungent, choking smell, and fumes in moist air
- Turns moist blue litmus paper red
- Gives dense white fumes with a glass rod dipped in ammonia solution:
- Gives a white precipitate with silver nitrate solution on a glass rod
Worked example — mass of salt needed. What mass of sodium chloride gives of hydrogen chloride by the reaction below ? Na , Cl , H .
At STP that gas would occupy .
An everyday note. Industrial hydrochloric acid is mostly made by other routes on a large scale, but this laboratory method uses cheap, easily stored materials — common salt and sulphuric acid — which is why schools use it.
The boundary case — why concentrated sulphuric acid works as both reactant and drier. It reacts with salt because it is less volatile and drives out the more volatile hydrogen chloride; it does not react with the hydrogen chloride gas itself. So the same acid can make the gas in the flask and dry it in the wash bottle.
Reactants: sodium chloride (rock salt) and concentrated sulphuric acid.
Condition: gentle heating, keeping the temperature **below about **:
**Above about **, the sodium hydrogen sulphate reacts with more salt:
Why the lower temperature is preferred:
- Sodium sulphate forms a hard crust that sticks to the flask and is difficult to remove
- Higher heating wastes fuel and can crack the glass
- The lower-temperature reaction gives the gas conveniently without these problems
Apparatus:
- A round-bottom flask containing the salt, fitted with a thistle funnel for adding the acid and a delivery tube
- A wash bottle of concentrated sulphuric acid through which the gas passes to be dried
- Gas jars for collection
Drying agent: concentrated sulphuric acid. Quicklime cannot be used, because it is basic and would react with the acidic gas.
Collection: by upward displacement of air — the delivery tube goes to the bottom of an upright gas jar, because hydrogen chloride is heavier than air. It is not collected over water, because it is extremely soluble.
Precautions:
- The lower end of the thistle funnel must dip below the acid, or the gas escapes through the funnel
- All joints must be airtight
- The temperature should be kept low, for the reasons above
- The gas is irritating and corrosive, so the preparation is done in a well-ventilated place or fume cupboard
Identification of the gas:
- Colourless, with a pungent, choking smell, and fumes in moist air
- Turns moist blue litmus paper red
- Gives dense white fumes with a glass rod dipped in ammonia solution:
- Gives a white precipitate with silver nitrate solution on a glass rod
Worked example — mass of salt needed. What mass of sodium chloride gives of hydrogen chloride by the reaction below ? Na , Cl , H .
At STP that gas would occupy .
An everyday note. Industrial hydrochloric acid is mostly made by other routes on a large scale, but this laboratory method uses cheap, easily stored materials — common salt and sulphuric acid — which is why schools use it.
The boundary case — why concentrated sulphuric acid works as both reactant and drier. It reacts with salt because it is less volatile and drives out the more volatile hydrogen chloride; it does not react with the hydrogen chloride gas itself. So the same acid can make the gas in the flask and dry it in the wash bottle.
How do you show hydrogen chloride is heavier than air and very soluble, and how is back suction prevented?
Pouring the gas downward into a jar of moist blue litmus shows it is heavier than air; the fountain experiment shows its extreme solubility and acidity; and an inverted funnel just touching the water prevents back suction when making hydrochloric acid.
1. Hydrogen chloride is heavier than air.
Its vapour density is , **greater than that of air, about .
Experiment: hold a gas jar of hydrogen chloride above an open empty gas jar containing moist blue litmus paper, and tilt it as if pouring water. After a short time the litmus in the lower jar turns red. The gas has flowed downward into the lower jar, as only a gas heavier than air would.
2. The fountain experiment — solubility.
Apparatus: a dry round-bottom flask filled with dry hydrogen chloride, closed with a cork carrying a long jet tube, inverted and clamped so that the tube dips into a trough of water containing blue litmus solution.
Procedure: a few drops of water are introduced into the flask, for example from a dropper fitted through the cork, to start the process.
Observation: water rushes up the tube and enters the flask as a fountain, and the blue litmus turns red.
Explanation:
- The first drops of water dissolve a large amount of hydrogen chloride
- The pressure inside the flask falls sharply
- Atmospheric pressure on the water in the trough pushes water up the tube, and more gas dissolves as it enters
Inference: hydrogen chloride is extremely soluble in water, and its solution is acidic.
3. Preventing back suction when making hydrochloric acid.
The problem: if hydrogen chloride were led directly into water through a tube dipping below the surface, the gas would dissolve so rapidly that water would be sucked back up the delivery tube into the hot flask — which could crack or burst it.
The special arrangement: the delivery tube ends in an inverted funnel whose rim just touches the surface of the water.
- If water starts to rise into the funnel, the level in the beaker drops below the rim
- Air then enters the funnel, the pressure is equalised and the water falls back
- The wide funnel also gives a large surface area, so the gas dissolves efficiently
Worked comparison — the two fountains. Ammonia also gives a fountain experiment, but it turns red litmus blue, because ammonia solution is alkaline. Hydrogen chloride turns blue litmus red. Both gases are extremely soluble; they differ in the nature of the solution they form.
An everyday example of pressure pushing liquid. Sucking the air out of a drinking straw lets the atmosphere push the drink up into your mouth. In the fountain experiment, the dissolving gas does the sucking, and the atmosphere does the pushing.
The boundary case — why the flask and gas must be dry. If the flask were wet, the gas would dissolve before the experiment began, the pressure would already be low, and no dramatic fountain would appear. Dryness is what stores up the effect.**
1. Hydrogen chloride is heavier than air.
Its vapour density is , **greater than that of air, about .
Experiment: hold a gas jar of hydrogen chloride above an open empty gas jar containing moist blue litmus paper, and tilt it as if pouring water. After a short time the litmus in the lower jar turns red. The gas has flowed downward into the lower jar, as only a gas heavier than air would.
2. The fountain experiment — solubility.
Apparatus: a dry round-bottom flask filled with dry hydrogen chloride, closed with a cork carrying a long jet tube, inverted and clamped so that the tube dips into a trough of water containing blue litmus solution.
Procedure: a few drops of water are introduced into the flask, for example from a dropper fitted through the cork, to start the process.
Observation: water rushes up the tube and enters the flask as a fountain, and the blue litmus turns red.
Explanation:
- The first drops of water dissolve a large amount of hydrogen chloride
- The pressure inside the flask falls sharply
- Atmospheric pressure on the water in the trough pushes water up the tube, and more gas dissolves as it enters
Inference: hydrogen chloride is extremely soluble in water, and its solution is acidic.
3. Preventing back suction when making hydrochloric acid.
The problem: if hydrogen chloride were led directly into water through a tube dipping below the surface, the gas would dissolve so rapidly that water would be sucked back up the delivery tube into the hot flask — which could crack or burst it.
The special arrangement: the delivery tube ends in an inverted funnel whose rim just touches the surface of the water.
- If water starts to rise into the funnel, the level in the beaker drops below the rim
- Air then enters the funnel, the pressure is equalised and the water falls back
- The wide funnel also gives a large surface area, so the gas dissolves efficiently
Worked comparison — the two fountains. Ammonia also gives a fountain experiment, but it turns red litmus blue, because ammonia solution is alkaline. Hydrogen chloride turns blue litmus red. Both gases are extremely soluble; they differ in the nature of the solution they form.
An everyday example of pressure pushing liquid. Sucking the air out of a drinking straw lets the atmosphere push the drink up into your mouth. In the fountain experiment, the dissolving gas does the sucking, and the atmosphere does the pushing.
The boundary case — why the flask and gas must be dry. If the flask were wet, the gas would dissolve before the experiment began, the pressure would already be low, and no dramatic fountain would appear. Dryness is what stores up the effect.**
What are the acidic reactions of hydrochloric acid?
Hydrochloric acid ionises completely in water and shows all the typical reactions of a strong acid — with active metals, metal oxides and hydroxides, carbonates, bicarbonates, sulphites, bisulphites and sulphides, and with ammonia.
Ionisation in water:
1. With active metals — salt and hydrogen.
2. With metal oxides — salt and water.
3. With hydroxides — salt and water.
4. With carbonates and bicarbonates — salt, water and carbon dioxide.
5. With sulphites and bisulphites — salt, water and sulphur dioxide, on warming.
6. With sulphides — salt and hydrogen sulphide.
7. With ammonia — dense white fumes of ammonium chloride.
Worked example — volume of gas from a carbonate. What volume of carbon dioxide at STP is released when of calcium carbonate reacts completely with dilute hydrochloric acid, and what mass of hydrogen chloride is used?
- Carbon dioxide: mol
- Hydrogen chloride used: mol
Worked check — balancing the sulphite equation. Sodium ; sulphur ; oxygen on the left and on the right; hydrogen ; chlorine . Balanced.
An everyday example. Dilute hydrochloric acid is used to clean rust and scale from metal surfaces before painting or galvanising, and to remove hard-water deposits — the carbonate reaction above dissolving calcium carbonate scale.
The boundary case — metals that do not react. Copper, silver and gold lie below hydrogen in the activity series and give no hydrogen with dilute hydrochloric acid. Iron gives iron(II) chloride, not iron(III) chloride, a detail that is marked in equations. And lead reacts only slowly, because insoluble lead chloride coats the metal.
Ionisation in water:
1. With active metals — salt and hydrogen.
2. With metal oxides — salt and water.
3. With hydroxides — salt and water.
4. With carbonates and bicarbonates — salt, water and carbon dioxide.
5. With sulphites and bisulphites — salt, water and sulphur dioxide, on warming.
6. With sulphides — salt and hydrogen sulphide.
7. With ammonia — dense white fumes of ammonium chloride.
Worked example — volume of gas from a carbonate. What volume of carbon dioxide at STP is released when of calcium carbonate reacts completely with dilute hydrochloric acid, and what mass of hydrogen chloride is used?
- Carbon dioxide: mol
- Hydrogen chloride used: mol
Worked check — balancing the sulphite equation. Sodium ; sulphur ; oxygen on the left and on the right; hydrogen ; chlorine . Balanced.
An everyday example. Dilute hydrochloric acid is used to clean rust and scale from metal surfaces before painting or galvanising, and to remove hard-water deposits — the carbonate reaction above dissolving calcium carbonate scale.
The boundary case — metals that do not react. Copper, silver and gold lie below hydrogen in the activity series and give no hydrogen with dilute hydrochloric acid. Iron gives iron(II) chloride, not iron(III) chloride, a detail that is marked in equations. And lead reacts only slowly, because insoluble lead chloride coats the metal.
What is aqua regia, and how do you test for hydrogen chloride, hydrochloric acid and chloride ions?
Aqua regia is three volumes of concentrated hydrochloric acid mixed with one volume of concentrated nitric acid, which dissolves gold and platinum; chloride ions are identified by the white precipitate they give with silver nitrate, insoluble in nitric acid but soluble in ammonium hydroxide.
1. Aqua regia.
- Composition: 3 parts concentrated hydrochloric acid to 1 part concentrated nitric acid, by volume
- Name: royal water, because it dissolves the royal or noble metals
How it works. Nitric acid oxidises hydrochloric acid, producing nascent chlorine, a very reactive form:
The nascent chlorine attacks gold and platinum:
Uses:
- Dissolving gold and platinum, which neither acid dissolves alone
- Refining and recovering noble metals
- Cleaning metal deposits from laboratory apparatus
2. Precipitation with silver nitrate.
- A curdy white precipitate of silver chloride
- Insoluble in dilute nitric acid
- Soluble in excess ammonium hydroxide
3. Precipitation with lead nitrate.
- A white precipitate of lead chloride
- Soluble in hot water, reappearing as crystals on cooling
4. Tests.
For hydrogen chloride gas:
- Turns moist blue litmus red
- Dense white fumes with a rod dipped in ammonia solution
For hydrochloric acid:
- Turns blue litmus red
- Warmed with manganese dioxide, gives greenish-yellow chlorine gas:
For the chloride ion in any solution:
- Add dilute nitric acid, then silver nitrate solution
- A white precipitate, insoluble in nitric acid and soluble in ammonium hydroxide, confirms chloride
Why nitric acid is added first. Carbonates and sulphites also give white precipitates with silver nitrate. Nitric acid destroys them first, so any white precipitate that forms afterwards must be silver chloride.
Worked check — balancing the aqua regia equation.
- Hydrogen: on the left; on the right
- Nitrogen:
- Oxygen: on the left; on the right
- Chlorine: on the left; on the right
Balanced.
Worked example — mass of precipitate. What mass of silver chloride forms when excess silver nitrate is added to a solution containing of hydrogen chloride? Ag .
An everyday example. Jewellers and gold refiners use aqua regia to dissolve gold for purification. The same mixture that dissolves a gold ring leaves ordinary glass untouched, which is why it can be handled in glass vessels.
The boundary case. Neither concentrated hydrochloric acid nor concentrated nitric acid dissolves gold on its own. Only the mixture works, because only together do they produce the reactive chlorine and the chloride ions that hold gold in solution.
1. Aqua regia.
- Composition: 3 parts concentrated hydrochloric acid to 1 part concentrated nitric acid, by volume
- Name: royal water, because it dissolves the royal or noble metals
How it works. Nitric acid oxidises hydrochloric acid, producing nascent chlorine, a very reactive form:
The nascent chlorine attacks gold and platinum:
Uses:
- Dissolving gold and platinum, which neither acid dissolves alone
- Refining and recovering noble metals
- Cleaning metal deposits from laboratory apparatus
2. Precipitation with silver nitrate.
- A curdy white precipitate of silver chloride
- Insoluble in dilute nitric acid
- Soluble in excess ammonium hydroxide
3. Precipitation with lead nitrate.
- A white precipitate of lead chloride
- Soluble in hot water, reappearing as crystals on cooling
4. Tests.
For hydrogen chloride gas:
- Turns moist blue litmus red
- Dense white fumes with a rod dipped in ammonia solution
For hydrochloric acid:
- Turns blue litmus red
- Warmed with manganese dioxide, gives greenish-yellow chlorine gas:
For the chloride ion in any solution:
- Add dilute nitric acid, then silver nitrate solution
- A white precipitate, insoluble in nitric acid and soluble in ammonium hydroxide, confirms chloride
Why nitric acid is added first. Carbonates and sulphites also give white precipitates with silver nitrate. Nitric acid destroys them first, so any white precipitate that forms afterwards must be silver chloride.
Worked check — balancing the aqua regia equation.
- Hydrogen: on the left; on the right
- Nitrogen:
- Oxygen: on the left; on the right
- Chlorine: on the left; on the right
Balanced.
Worked example — mass of precipitate. What mass of silver chloride forms when excess silver nitrate is added to a solution containing of hydrogen chloride? Ag .
An everyday example. Jewellers and gold refiners use aqua regia to dissolve gold for purification. The same mixture that dissolves a gold ring leaves ordinary glass untouched, which is why it can be handled in glass vessels.
The boundary case. Neither concentrated hydrochloric acid nor concentrated nitric acid dissolves gold on its own. Only the mixture works, because only together do they produce the reactive chlorine and the chloride ions that hold gold in solution.
Exam tip
What does a full-mark hydrogen chloride answer include?
For the preparation, give reactants, the temperature condition, the equation, the drying agent, the collection method with its reason and one precaution; for tests, give the reagent, the observation and the conclusion.
- Write the equation for the reaction below about 200 °C:
- Give the reasons for the low temperature: sodium sulphate crust, fuel, glass cracking
- Name concentrated sulphuric acid as the drying agent, and say why quicklime is unsuitable
- Collect by upward displacement of air, with the reason — heavier than air and very soluble
- Mention the thistle funnel dipping into the acid as a precaution
- Describe the fountain experiment in four parts: apparatus, procedure, observation, inference
- Explain back suction and the inverted funnel with the rim just touching the water
- Write iron(II) chloride, not iron(III) chloride, for iron with the acid
- Give the ratio of aqua regia by volume as , hydrochloric to nitric
- For chloride ions, add dilute nitric acid before silver nitrate, and state both solubility results
The misconception to name. Hydrogen chloride gas is not an acid until it meets water. Dry hydrogen chloride does not turn dry blue litmus red, because it contains molecules, not hydronium ions — which is why tests always specify moist litmus.
A second trap. Writing aqua regia as three parts nitric acid to one part hydrochloric acid. The hydrochloric acid is the larger share, and reversing the ratio is marked wrong.
- Write the equation for the reaction below about 200 °C:
- Give the reasons for the low temperature: sodium sulphate crust, fuel, glass cracking
- Name concentrated sulphuric acid as the drying agent, and say why quicklime is unsuitable
- Collect by upward displacement of air, with the reason — heavier than air and very soluble
- Mention the thistle funnel dipping into the acid as a precaution
- Describe the fountain experiment in four parts: apparatus, procedure, observation, inference
- Explain back suction and the inverted funnel with the rim just touching the water
- Write iron(II) chloride, not iron(III) chloride, for iron with the acid
- Give the ratio of aqua regia by volume as , hydrochloric to nitric
- For chloride ions, add dilute nitric acid before silver nitrate, and state both solubility results
The misconception to name. Hydrogen chloride gas is not an acid until it meets water. Dry hydrogen chloride does not turn dry blue litmus red, because it contains molecules, not hydronium ions — which is why tests always specify moist litmus.
A second trap. Writing aqua regia as three parts nitric acid to one part hydrochloric acid. The hydrochloric acid is the larger share, and reversing the ratio is marked wrong.
Did you know
Why does a bottle of concentrated hydrochloric acid give off white fumes when opened?
Unscrew the cap of a bottle of concentrated hydrochloric acid on a humid day and wisps of white mist drift out of the neck, even though the acid itself is colourless and the gas above it is colourless too. The fumes appear only where the escaping gas meets the air.
Concentrated hydrochloric acid holds a great deal of dissolved hydrogen chloride, and some of that gas constantly escapes from the surface into the space above the liquid. When the bottle is opened, the gas meets the water vapour in the air.
Because hydrogen chloride is so extremely soluble, it grabs water vapour at once, forming countless tiny droplets of hydrochloric acid. Those droplets scatter light, just like the droplets in a cloud or in the mist from a kettle, and so the invisible gas becomes a visible white fume.
The fumes are thicker on a humid day and thinner in dry weather — a direct sign that water vapour from the air is taking part.
The white fumes with ammonia are something different. When a rod dipped in ammonia solution is held near the same bottle, far denser white fumes form. These are not droplets of acid but tiny solid crystals of ammonium chloride:
Two kinds of white fume from one gas — liquid droplets with moisture, solid crystals with ammonia — and the second is the one used as a test, because moist air alone could never produce such a dense cloud.
The same property that makes the fumes also makes the gas dangerous to breathe. Hydrogen chloride dissolves in the moisture of the eyes, nose and lungs just as eagerly as it does in air, forming acid on those surfaces. That is why concentrated hydrochloric acid is opened in a fume cupboard or a well-ventilated space, and why the bottle is closed again as soon as possible.
Concentrated hydrochloric acid holds a great deal of dissolved hydrogen chloride, and some of that gas constantly escapes from the surface into the space above the liquid. When the bottle is opened, the gas meets the water vapour in the air.
Because hydrogen chloride is so extremely soluble, it grabs water vapour at once, forming countless tiny droplets of hydrochloric acid. Those droplets scatter light, just like the droplets in a cloud or in the mist from a kettle, and so the invisible gas becomes a visible white fume.
The fumes are thicker on a humid day and thinner in dry weather — a direct sign that water vapour from the air is taking part.
The white fumes with ammonia are something different. When a rod dipped in ammonia solution is held near the same bottle, far denser white fumes form. These are not droplets of acid but tiny solid crystals of ammonium chloride:
Two kinds of white fume from one gas — liquid droplets with moisture, solid crystals with ammonia — and the second is the one used as a test, because moist air alone could never produce such a dense cloud.
The same property that makes the fumes also makes the gas dangerous to breathe. Hydrogen chloride dissolves in the moisture of the eyes, nose and lungs just as eagerly as it does in air, forming acid on those surfaces. That is why concentrated hydrochloric acid is opened in a fume cupboard or a well-ventilated space, and why the bottle is closed again as soon as possible.
Exam relevance
How is hydrogen chloride chemistry examined in JEE and NEET?
This is foundation work for Class 12 The p-Block Elements, where hydrogen chloride and hydrochloric acid are studied among the compounds of group 17, and for Class 11 Equilibrium — both examined in JEE Main and NEET Chemistry — and for the salt analysis section of JEE Main practical chemistry.
Where the preparation and properties lead. Class 12 The p-Block Elements covers the laboratory preparation of hydrogen chloride from sodium chloride and sulphuric acid, its high solubility and its acidic reactions. The equations on this page are used there with little change, and questions on the preparation conditions and on why the gas fumes in moist air are standard.
Where aqua regia leads. The same chapter describes how aqua regia dissolves gold and platinum, often with an equation showing the formation of chloro-complexes of the noble metals. The composition and the reason neither acid works alone are typical objective questions in both exams.
Where the strong acid behaviour leads. Class 11 Equilibrium treats hydrochloric acid as the standard strong acid in pH calculations. The complete ionisation shown on this page is what allows the hydronium ion concentration to be taken as equal to the acid concentration in those numericals.
Where the chloride test leads. JEE Main practical chemistry includes the chemical principles of detecting anions, and chloride is one of them. The silver nitrate test, with its solubility in ammonia, is the basis of that detection; the dissolution in ammonia is explained in Class 12 Coordination Compounds by the formation of a silver-ammonia complex.
Where the manganese dioxide reaction leads. The preparation of chlorine from hydrochloric acid and manganese dioxide is a redox reaction, and identifying the oxidising and reducing agents in it — manganese dioxide oxidises chloride ions to chlorine — is a Class 11 Redox Reactions question.
Question types to expect. At this level: full preparation descriptions, experiments, equations and tests. In competitive papers: preparation conditions, aqua regia reactions, redox identification, pH of strong acids and anion detection principles.
The single trap that costs marks. Writing iron(III) chloride as the product of iron with hydrochloric acid. Hydrochloric acid gives iron(II) chloride, because it is not an oxidising acid — a distinction that resurfaces with nitric acid, which behaves very differently.
A second trap. Forgetting why silver chloride dissolves in ammonia. It forms a soluble complex, not an ordinary salt, and assertion-reason questions test that explanation.
Board versus competitive emphasis. The ICSE paper marks the complete practical description and every equation; a competitive paper marks a specific condition, reaction product or explanation. The transferable habit is linking each practical choice to a property of the gas — solubility, density or acidity — because every procedural detail follows from one of those.
Where the preparation and properties lead. Class 12 The p-Block Elements covers the laboratory preparation of hydrogen chloride from sodium chloride and sulphuric acid, its high solubility and its acidic reactions. The equations on this page are used there with little change, and questions on the preparation conditions and on why the gas fumes in moist air are standard.
Where aqua regia leads. The same chapter describes how aqua regia dissolves gold and platinum, often with an equation showing the formation of chloro-complexes of the noble metals. The composition and the reason neither acid works alone are typical objective questions in both exams.
Where the strong acid behaviour leads. Class 11 Equilibrium treats hydrochloric acid as the standard strong acid in pH calculations. The complete ionisation shown on this page is what allows the hydronium ion concentration to be taken as equal to the acid concentration in those numericals.
Where the chloride test leads. JEE Main practical chemistry includes the chemical principles of detecting anions, and chloride is one of them. The silver nitrate test, with its solubility in ammonia, is the basis of that detection; the dissolution in ammonia is explained in Class 12 Coordination Compounds by the formation of a silver-ammonia complex.
Where the manganese dioxide reaction leads. The preparation of chlorine from hydrochloric acid and manganese dioxide is a redox reaction, and identifying the oxidising and reducing agents in it — manganese dioxide oxidises chloride ions to chlorine — is a Class 11 Redox Reactions question.
Question types to expect. At this level: full preparation descriptions, experiments, equations and tests. In competitive papers: preparation conditions, aqua regia reactions, redox identification, pH of strong acids and anion detection principles.
The single trap that costs marks. Writing iron(III) chloride as the product of iron with hydrochloric acid. Hydrochloric acid gives iron(II) chloride, because it is not an oxidising acid — a distinction that resurfaces with nitric acid, which behaves very differently.
A second trap. Forgetting why silver chloride dissolves in ammonia. It forms a soluble complex, not an ordinary salt, and assertion-reason questions test that explanation.
Board versus competitive emphasis. The ICSE paper marks the complete practical description and every equation; a competitive paper marks a specific condition, reaction product or explanation. The transferable habit is linking each practical choice to a property of the gas — solubility, density or acidity — because every procedural detail follows from one of those.
Key takeaways
What must you be able to do from this part?
One preparation, three experiments, seven kinds of acidic reaction, aqua regia and a set of tests.
- Preparation: below about
- Low temperature avoids a sodium sulphate crust, saves fuel and protects the glass
- Dried by concentrated sulphuric acid, not quicklime
- Collected by upward displacement of air — heavier than air, very soluble
- Precaution: thistle funnel dipping into the acid; airtight joints
- Identified by moist blue litmus turning red and white fumes with ammonia
- ** of NaCl** gives of HCl
- Heavier than air: vapour density ; gas poured downward reddens moist blue litmus
- Fountain experiment: dry flask of dry HCl, jet into blue litmus water — fountain forms, litmus turns red; very soluble and acidic
- Back suction is prevented by an inverted funnel with its rim just touching the water
- Acidic reactions: metals give salt and hydrogen; oxides and hydroxides give salt and water; carbonates give CO2; sulphites give SO2; sulphides give H2S; ammonia gives NH4Cl
- **Iron gives ; copper gives no reaction
- Aqua regia: 3 parts concentrated HCl to 1 part concentrated HNO3; nascent chlorine dissolves gold and platinum
- Silver nitrate gives curdy white AgCl, insoluble in nitric acid, soluble in ammonium hydroxide
- Lead nitrate gives white PbCl2, soluble in hot water
- Hydrochloric acid with manganese dioxide gives greenish-yellow chlorine
- Chloride ion test**: dilute nitric acid, then silver nitrate — white precipitate soluble in ammonia
The sharpest self-test is a blank page and one gas. Write out the preparation of hydrogen chloride as if for a practical exam — reactants, equation, condition, apparatus, drying, collection, precaution and identification — then check each heading against this page and note the one you forgot.
- Preparation: below about
- Low temperature avoids a sodium sulphate crust, saves fuel and protects the glass
- Dried by concentrated sulphuric acid, not quicklime
- Collected by upward displacement of air — heavier than air, very soluble
- Precaution: thistle funnel dipping into the acid; airtight joints
- Identified by moist blue litmus turning red and white fumes with ammonia
- ** of NaCl** gives of HCl
- Heavier than air: vapour density ; gas poured downward reddens moist blue litmus
- Fountain experiment: dry flask of dry HCl, jet into blue litmus water — fountain forms, litmus turns red; very soluble and acidic
- Back suction is prevented by an inverted funnel with its rim just touching the water
- Acidic reactions: metals give salt and hydrogen; oxides and hydroxides give salt and water; carbonates give CO2; sulphites give SO2; sulphides give H2S; ammonia gives NH4Cl
- **Iron gives ; copper gives no reaction
- Aqua regia: 3 parts concentrated HCl to 1 part concentrated HNO3; nascent chlorine dissolves gold and platinum
- Silver nitrate gives curdy white AgCl, insoluble in nitric acid, soluble in ammonium hydroxide
- Lead nitrate gives white PbCl2, soluble in hot water
- Hydrochloric acid with manganese dioxide gives greenish-yellow chlorine
- Chloride ion test**: dilute nitric acid, then silver nitrate — white precipitate soluble in ammonia
The sharpest self-test is a blank page and one gas. Write out the preparation of hydrogen chloride as if for a practical exam — reactants, equation, condition, apparatus, drying, collection, precaution and identification — then check each heading against this page and note the one you forgot.