Study of Compounds: Hydrogen Chloride | ICSE Class 10 Chemistry Notes

This note covers hydrogen chloride and hydrochloric acid, laboratory preparation, drying and collection, density, solubility, the fountain experiment, prevention of back suction, reaction with ammonia, acidic reactions, precipitation tests and aqua regia.

What is hydrogen chloride, and how does it differ from hydrochloric acid?

Hydrogen chloride, HCl, is a compound of hydrogen and chlorine. A compound contains elements chemically combined in a fixed proportion. An element contains one type of atom; an atom is its smallest unit retaining chemical identity. Each HCl molecule contains one hydrogen atom and one chlorine atom. A molecule is a group of chemically bonded atoms.

HCl is a polar covalent compound: its atoms share an electron pair unequally. Electrons are negatively charged particles. Chlorine attracts the shared pair more strongly than hydrogen. This unequal sharing must not be confused with the formation of freely moving ions in the dry gas.

Definition: Hydrochloric acid is the aqueous solution of hydrogen chloride. Aqueous means dissolved in water; a solution is a uniform mixture formed by dissolving a substance in a solvent. An acid produces hydronium ions in water. Dry hydrogen chloride gas and hydrochloric acid therefore describe different physical forms of the same hydrogen chloride compound.

What changes when the gas dissolves?

Ionisation is the formation of ions from molecules. An ion is an atom or group of atoms carrying an electrical charge. Hydrogen chloride ionises in water, producing hydronium ions and chloride ions. A hydronium ion is a water molecule combined with a hydrogen ion.

In the following equation, H₂O means water, H₃O⁺ means a hydronium ion and Cl⁻ means a chloride ion, a chlorine atom carrying one negative charge. The superscript + indicates one positive charge, and − indicates one negative charge. The arrow → means “produces”; a plus sign between substances means “reacts with” or “and”.

HCl + H₂O → H₃O⁺ + Cl⁻

The symbols H, Cl and O represent hydrogen, chlorine and oxygen. A lowered number gives the number of atoms of that element in a formula; where no lowered number appears, one atom is indicated.

Litmus is an acid-base indicator, a substance whose colour helps distinguish acidic and basic conditions. Blue litmus becomes red in acid. Moisture on a litmus strip allows the gas to dissolve and show its acidic behaviour.

How is hydrogen chloride prepared in the laboratory?

The starting materials, or reactants, are solid sodium chloride and concentrated sulphuric acid. Sodium chloride has the formula NaCl, where Na represents sodium. Sulphuric acid has the formula H₂SO₄, where S represents sulphur. Concentrated means that the solution contains a relatively large proportion of acid.

On gentle warming, the substances formed, or products, are hydrogen chloride gas and sodium hydrogen sulphate, NaHSO₄. Keep the laboratory preparation below 200 °C; °C means degrees Celsius, the temperature unit used here.

NaCl + H₂SO₄ → NaHSO₄ + HCl

Concentrated sulphuric acid is non-volatile under these preparation conditions, meaning that it does not readily escape as vapour. Hydrogen chloride is volatile and leaves the reaction mixture as a gas. The acid thus releases hydrogen chloride from the chloride salt. A salt is an ionic compound containing positive and negative ions; a chloride salt contains chloride ions.

What does the apparatus show?

The drawing shows a dropper containing concentrated sulphuric acid above a test tube containing sodium chloride. A second view shows a corked test tube connected to a bent delivery tube, with moist litmus paper held near its outlet. A separate guard tube containing calcium chloride is also drawn.

See Fig. 2.4 in your NCERT textbook

A delivery tube carries the gas away from the reaction vessel. A cork closes the vessel around this tube. A guard tube is an additional tube containing a substance that removes moisture from the passing gas.

1. Place dry sodium chloride in a clean, dry reaction vessel fitted for the passage of gas into a delivery tube.
2. Add concentrated sulphuric acid to the salt and warm gently when needed to maintain the evolution of hydrogen chloride.
3. Pass the gas through a suitable drying arrangement when dry hydrogen chloride is required; then lead it into a dry collection vessel.
4. Identify the evolved gas using moist blue litmus and the ammonia test, which produces dense white fumes.

The observation is the evolution of a colourless gas. Moist blue litmus turns red. “Colourless” describes the gas itself; it does not mean that the gas cannot produce visible droplets or solid particles when it meets another substance.

Note: This is a supervised laboratory preparation. Keep the apparatus dry when collecting dry gas, and use gentle heating. Water in the apparatus would absorb hydrogen chloride before it reaches the collection vessel.

Why are drying, density and collection closely connected?

Drying a gas means removing water vapour from it. A drying agent must remove moisture without consuming the gas being collected. Concentrated sulphuric acid is suitable for drying hydrogen chloride. A guard tube containing calcium chloride can also remove moisture from the gas.

Calcium chloride is CaCl₂, where Ca represents calcium. Do not confuse the purpose of a drying tube with the purpose of a collecting vessel: one removes moisture, while the other receives the gas.

How is the gas collected?

Density is mass per unit volume. Hydrogen chloride is denser than air under comparable conditions. It is collected in a dry, upright gas jar by upward displacement of air: gas entering near the bottom fills the jar and pushes air upwards.

This method is also described as downward delivery of the gas. The two phrases refer to different movements. Hydrogen chloride is delivered downwards into the jar, while the displaced air leaves upwards. State the movement of both substances to avoid ambiguity.

Hydrogen chloride is highly soluble in water. Solubility describes how much of a substance can dissolve in a solvent under specified conditions; a solvent is the liquid that dissolves it. Collection over water would absorb the gas instead of preserving a sample.

How can its greater density be demonstrated?

In a supervised demonstration, a gas jar of hydrogen chloride can be tilted above an air-filled jar so that the gas is delivered downwards. Moist blue litmus placed in the receiving jar turns red as hydrogen chloride reaches it.

The downward transfer illustrates that hydrogen chloride is heavier than air. The litmus observation detects the acidic gas after it dissolves in moisture. It is the transfer arrangement together with this observation that demonstrates density; a red litmus strip alone does not measure density.

How does the fountain experiment demonstrate solubility?

The fountain experiment shows the high solubility of hydrogen chloride in water. It also shows the acidic nature of the resulting solution when blue litmus solution is used. The movement of liquid depends on a pressure difference created by the absorption of gas.

Pressure is force per unit area. Atmospheric pressure is the pressure exerted by the surrounding air. When much of the gas in a closed flask dissolves, the pressure inside becomes lower than atmospheric pressure outside.

What arrangement is required?

Draw an inverted flask filled with dry hydrogen chloride and fitted with an airtight stopper. Show a water-filled dropper and a glass tube passing through the stopper. The tube has a jet inside the flask and its lower end dips into blue litmus solution in a trough. Label the flask, dropper, jet, tube and trough.

A jet is a narrow outlet through which liquid emerges as a stream. The apparatus must be airtight so that outside air does not enter freely and destroy the pressure difference needed to raise the water.

1. Fill the dry flask with hydrogen chloride and arrange the delivery tube with its lower end immersed in blue litmus solution.
2. Introduce a little water from the dropper into the flask. Hydrogen chloride dissolves rapidly in this water.
3. As gas is removed from the space inside the flask, its internal pressure falls below the pressure acting on the liquid in the trough.
4. Atmospheric pressure pushes the litmus solution up the tube. It enters through the jet as a fountain and becomes red in the acidic solution.

The observation is a fountain of liquid entering the flask and a change from blue to red. The inference, meaning the conclusion drawn from an observation, is that hydrogen chloride dissolves readily and forms an acidic solution.

Separate the two conclusions carefully. The rapid entry of liquid demonstrates absorption of the gas and the pressure change. The colour change demonstrates acidity. A fountain without an indicator would still demonstrate high solubility, but would not itself provide the litmus evidence for acidity.

How is hydrochloric acid prepared without back suction?

Hydrochloric acid is prepared by absorbing hydrogen chloride gas in water. Its high solubility makes absorption easy, but creates an apparatus problem: if a narrow delivery tube is dipped deeply into water, rapid absorption can lower the gas pressure and draw water backwards.

Back suction is the unwanted movement of liquid from the receiving vessel into the gas delivery system. In a heated preparation apparatus, water returning to the hot vessel is dangerous. The absorbing arrangement must therefore allow gas to dissolve while interrupting backward flow.

Why is an inverted funnel used?

Draw a delivery tube joined to the stem of an inverted funnel. Place the funnel's wide mouth just below the water surface in a beaker. Label the hydrogen chloride inlet, funnel stem, broad mouth and water level. Keep the rim close to the surface, rather than deeply submerged.

The inverted funnel has two functions. Its broad mouth gives a large area of contact between gas and water, helping absorption. Its shallow position allows the water seal at the rim to break if water is drawn into the funnel.

1. Hydrogen chloride reaches the funnel and dissolves in the water touching its broad mouth.
2. Rapid absorption may reduce pressure inside the funnel and draw water upwards into its wider part.
3. As water enters the funnel, the water level outside can fall below the shallowly immersed rim, allowing air to enter.
4. The entering air restores pressure, and the water in the funnel falls back into the beaker instead of continuing into the reaction vessel.

The important detail is the position of the rim. A wide funnel deeply submerged in water does not provide the intended shallow water seal. A labelled diagram should show the mouth just below the water surface, because this position explains how the protection works.

Compare this with the fountain experiment. Both depend on rapid dissolution and a fall in gas pressure. In the fountain, the pressure difference deliberately raises liquid into a flask. In acid preparation, the apparatus is arranged to prevent liquid from reaching the gas generator.

How does hydrogen chloride react with ammonia?

Ammonia is a compound of nitrogen and hydrogen with the formula NH₃; N represents nitrogen. When ammonia gas meets hydrogen chloride gas, the product is ammonium chloride, NH₄Cl. This solid appears as dense white fumes made of fine particles suspended in the air.

NH₃ + HCl → NH₄Cl

The formula NH₄Cl contains the ammonium ion, NH₄⁺, which is a positively charged group containing one nitrogen atom and four hydrogen atoms. It is different from the uncharged ammonia molecule, NH₃. Ammonium chloride contains ammonium ions and chloride ions.

How is the reaction used as a test?

A glass rod dipped in ammonia solution is brought near the hydrogen chloride outlet. Ammonia escaping from the solution meets the gas and produces dense white fumes. Record both the observation and the substance responsible: ammonium chloride.

A second test uses moist blue litmus, which becomes red. This establishes acidic behaviour after hydrogen chloride dissolves in the moisture. The ammonia test and moist-litmus test provide different observations and should not be described as the same chemical change.

Hydrogen chloride itself is colourless and has a pungent odour, meaning a sharp, irritating smell. The white fumes in the ammonia test belong to the reaction product. Colour, odour and reaction with a test reagent are separate descriptions; none should replace the others.

How does hydrochloric acid react with metals, oxides and hydroxides?

The acidic properties of hydrochloric acid arise from its hydronium ions. A chloride salt is a compound containing chloride ions with a positive ion. Hydrochloric acid forms chloride salts in its reactions with suitable metals, metal oxides and metal hydroxides.

What happens with a reactive metal?

Zinc, represented by Zn, reacts with dilute hydrochloric acid to form zinc chloride, ZnCl₂, and hydrogen gas, H₂. Dilute means containing a relatively small amount of dissolved acid compared with water. Bubbles of hydrogen are produced as the metal reacts.

Zn + 2HCl → ZnCl₂ + H₂

A number before a formula is a coefficient: it multiplies the entire formula. Thus 2HCl represents two HCl units. A balanced equation has equal numbers of each kind of atom on both sides; coefficients are changed during balancing, not the formulae of substances.

Hydrogen gives a characteristic pop sound when a small sample is tested with a burning splint in a supervised experiment. Do not assume every metal reacts with dilute hydrochloric acid. Copper does not react with it under the ordinary conditions of this test.

What happens with a metal oxide?

A metal oxide is a compound of a metal and oxygen. Copper(II) oxide, CuO, reacts with hydrochloric acid to give copper(II) chloride, CuCl₂, and water. Cu represents copper; (II) indicates that copper has oxidation state +2 in these compounds.

An oxidation state is the charge assigned to an atom by the rules used to account for electrons in compounds. The black copper(II) oxide dissolves, producing a blue-green solution containing copper(II) chloride.

CuO + 2HCl → CuCl₂ + H₂O

This reaction produces water rather than hydrogen gas. The difference between copper metal and copper(II) oxide matters: the oxide reacts with dilute hydrochloric acid even though copper metal does not.

What happens with a metal hydroxide?

A hydroxide contains the hydroxide ion, OH⁻. Sodium hydroxide, NaOH, is an alkali, meaning a base soluble in water. A base neutralises an acid. Sodium hydroxide reacts with hydrochloric acid to form sodium chloride and water.

NaOH + HCl → NaCl + H₂O

Neutralisation is the reaction of an acid with a base to form salt and water. With the indicator phenolphthalein, the pink alkaline solution becomes colourless as sufficient hydrochloric acid is added to neutralise the alkali.

What happens when hydrochloric acid reacts with carbonates, sulphides and sulphites?

These reactions produce gases, but the gases are different. Identify the starting ion before predicting the products. Effervescence means the formation and escape of gas bubbles through a liquid; it is an observation, not the name of a particular gas.

What do carbonates and hydrogencarbonates produce?

A carbonate contains the carbonate ion, CO₃²⁻, where C represents carbon and ²⁻ means two negative charges. Sodium carbonate, Na₂CO₃, reacts with hydrochloric acid to give sodium chloride, water and carbon dioxide, CO₂.

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

A hydrogencarbonate contains the hydrogencarbonate ion, HCO₃⁻. Sodium hydrogencarbonate, NaHCO₃, gives the same types of products but requires a different coefficient for hydrochloric acid. The alternative name “hydrogen carbonate” refers to the same ion.

NaHCO₃ + HCl → NaCl + H₂O + CO₂

Carbon dioxide is colourless and odourless. It turns lime water, a solution of calcium hydroxide, Ca(OH)₂, milky through the formation of solid calcium carbonate, CaCO₃. Parentheses group atoms in a formula; the lowered 2 in Ca(OH)₂ applies to both O and H.

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

The solid forming in a solution is called a precipitate. With excess carbon dioxide, the milkiness disappears because soluble calcium hydrogencarbonate, Ca(HCO₃)₂, forms. Excess means more of a reactant than is needed for the preceding reaction.

CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂

How do sulphides and sulphites differ?

A sulphide contains sulphide ions, S²⁻. Sodium sulphide, Na₂S, reacts with dilute hydrochloric acid to release hydrogen sulphide, H₂S. Hydrogen sulphide is a colourless gas with the characteristic smell of rotten eggs.

Na₂S + 2HCl → 2NaCl + H₂S

Hydrogen sulphide turns paper moistened with lead acetate solution black because lead sulphide forms. Lead acetate is a soluble lead salt used in this gas test; lead sulphide is the black solid produced. This observation distinguishes it from carbon dioxide.

A sulphite contains sulphite ions, SO₃²⁻. Sodium sulphite, Na₂SO₃, reacts with dilute hydrochloric acid to form sodium chloride, water and sulphur dioxide, SO₂. Sulphur dioxide is colourless, with a pungent, suffocating smell like burning sulphur.

Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂

Sulphur dioxide turns acidified potassium dichromate paper green. Potassium dichromate is the testing reagent; acidified means that acid has been added to it. Sulphur dioxide can also turn lime water milky, so that observation alone does not distinguish it from carbon dioxide.

How do precipitation reactions help identify hydrochloric acid?

Precipitation occurs when dissolved substances react to form a solid that separates from solution. The chloride ions in hydrochloric acid produce characteristic white precipitates with silver nitrate solution and lead nitrate solution.

What happens with silver nitrate?

Silver nitrate, AgNO₃, contains silver, represented by Ag, and nitrate ions, NO₃⁻. A nitrate ion contains one nitrogen atom and three oxygen atoms with an overall negative charge. With hydrochloric acid it forms silver chloride, AgCl, as a curdy white precipitate. Nitric acid, HNO₃, is the other product in the molecular equation.

AgNO₃ + HCl → AgCl + HNO₃

Curdy describes the appearance of the white solid. The silver chloride precipitate dissolves in ammonia solution. Its formation and subsequent dissolution are useful observations in identifying chloride ions; neither observation measures the strength or concentration of an acid.

What happens with lead nitrate?

Lead nitrate, Pb(NO₃)₂, contains lead, represented by Pb. Reaction with hydrochloric acid produces lead chloride, PbCl₂, as a white precipitate. Lead chloride dissolves in hot water, a useful property for distinguishing this precipitate from silver chloride in the tests described here.

Pb(NO₃)₂ + 2HCl → PbCl₂ + 2HNO₃

The coefficient 2 before HCl in the lead nitrate equation supplies two chloride units for each lead chloride formula unit. In the silver nitrate equation, one chloride unit is required for each silver chloride formula unit.

Note: A chloride test is not uniquely a test for hydrochloric acid. Sodium chloride solution also contains chloride ions. Use evidence of acidity, such as blue litmus turning red, alongside the chloride test when identifying hydrochloric acid.

What is aqua regia, and why does its composition matter?

Aqua regia, meaning “royal water”, is a freshly prepared mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3:1 by volume. A volume ratio compares the volumes of the liquids taken, not their masses.

State the acid names in the same order as the ratio: hydrochloric acid : nitric acid = 3 : 1. Reversing the names while retaining 3:1 gives the wrong composition. Both acids are concentrated; a mixture of unspecified dilute acids does not state the required preparation.

What is its characteristic use?

Aqua regia can dissolve gold, even though neither of these acids can do so alone. It is one of the few reagents able to dissolve gold and platinum. A reagent is a substance or mixture used to bring about a chemical reaction or test.

It is a highly corrosive, fuming liquid. Corrosive means capable of chemically attacking materials and damaging living tissue. Its use in dissolving gold and platinum belongs to the mixture, and must not be attributed to hydrochloric acid alone.

Keep the different forms in this chapter distinct. Dry hydrogen chloride is a gas; hydrochloric acid is its solution in water; aqua regia is a mixture of two concentrated acids. Their names specify different compositions, so their properties cannot simply be interchanged.

Glossary

• Hydrogen chloride — A compound of hydrogen and chlorine that forms hydrochloric acid when dissolved in water.
• Hydrochloric acid — The aqueous solution of hydrogen chloride, containing hydronium ions and chloride ions.
• Ionisation — Formation of electrically charged ions from molecules, as hydrogen chloride reacts with water.
• Hydronium ion — A positively charged ion formed when a hydrogen ion combines with a water molecule.
• Indicator — A substance whose colour change helps distinguish acidic and basic conditions in a sample.
• Drying agent — A substance used to remove moisture from a gas without consuming the gas being collected.
• Density — Mass per unit volume, used to compare a gas with air under comparable conditions.
• Back suction — Unwanted backward movement of liquid from a receiving vessel into the gas delivery system.
• Atmospheric pressure — Pressure exerted by surrounding air, which can drive liquid into a lower-pressure flask.
• Neutralisation — A reaction between an acid and a base that produces salt and water.
• Effervescence — Formation and escape of gas bubbles through a liquid during a reaction.
• Precipitate — A solid that forms and separates from a solution during a chemical reaction.
• Aqua regia — Freshly prepared concentrated hydrochloric acid and concentrated nitric acid in a three-to-one ratio by volume.

Common errors and misconceptions

• Misconception: Dry hydrogen chloride turns dry blue litmus red. Correct: Moisture is needed for the gas to form an acidic solution on the indicator.
• Misconception: Hydrogen chloride is collected over water because it dissolves readily. Correct: High solubility prevents this collection method; use a dry jar with upward displacement of air.
• Misconception: The fountain occurs because the gas expands. Correct: Dissolution reduces gas pressure inside the flask, and atmospheric pressure drives the liquid upwards.
• Misconception: The absorbing funnel should be deeply submerged. Correct: Its rim must be close to the water surface so that air can enter if the water seal breaks.
• Misconception: White fumes with ammonia mean that hydrogen chloride is white. Correct: Hydrogen chloride is colourless; solid ammonium chloride particles produce the white fumes.
• Misconception: Copper and copper(II) oxide both fail to react with dilute hydrochloric acid. Correct: Copper does not react, but its oxide produces copper(II) chloride and water.
• Misconception: Every gas that turns lime water milky is carbon dioxide. Correct: Sulphur dioxide can also do so; use the other observations and the starting substance.
• Misconception: A white precipitate with silver nitrate proves a solution is hydrochloric acid. Correct: It detects chloride ions; separate evidence of acidity is also needed.

Exam-style questions with model answers

Q1. Dry hydrogen chloride is tested first with dry blue litmus and then with moist blue litmus. State and explain each observation. [2 marks]

1. Dry blue litmus does not change colour because water is absent and the dry gas does not form hydronium ions on the paper.
2. Moist blue litmus turns red because hydrogen chloride dissolves in its water and produces hydronium ions responsible for acidic behaviour.

Q2. Hydrogen chloride is prepared from solid sodium chloride and concentrated sulphuric acid. Give the heating condition, balanced equation, reason for using concentrated sulphuric acid and one identification test with its observation. [4 marks]

1. Warm the mixture gently, keeping the laboratory preparation below 200 °C, rather than heating the reaction vessel strongly.
2. The balanced equation is NaCl + H₂SO₄ → NaHSO₄ + HCl; sodium hydrogen sulphate remains while hydrogen chloride gas escapes.
3. Concentrated sulphuric acid is non-volatile under these conditions and releases volatile hydrogen chloride from sodium chloride.
4. Bring ammonia solution on a glass rod near the gas outlet. Dense white fumes of ammonium chloride form.

Q3. Hydrogen chloride is denser than air and highly soluble in water. Explain why its collection uses an upright dry jar with gas delivered near the bottom, rather than collection over water. Give three points. [3 marks]

1. The upright jar allows the denser hydrogen chloride to collect below the air, displacing that air upwards as more gas enters near the bottom.
2. The collecting jar must be dry because moisture would absorb some of the hydrogen chloride entering it.
3. Collection over water is unsuitable because the gas would dissolve readily in the water instead of accumulating as a gas sample.

Q4. An airtight inverted flask contains dry hydrogen chloride. A tube with a jet inside the flask dips into blue litmus solution in an open trough. A little water is injected into the flask. Explain the resulting fountain and colour change in five stages. [5 marks]

1. The injected water dissolves hydrogen chloride rapidly. This begins removing gas molecules from the space inside the flask because hydrogen chloride is highly soluble in water.
2. The removal of gas lowers the pressure inside the airtight flask relative to the atmospheric pressure outside. Air cannot freely enter to remove this difference.
3. Atmospheric pressure acting on the exposed liquid in the trough pushes blue litmus solution up the tube towards the region of lower pressure.
4. The solution enters through the narrow jet and forms a fountain inside the flask. Further dissolution of hydrogen chloride helps maintain the inflow.
5. The litmus turns red because dissolved hydrogen chloride forms an acidic solution containing hydronium ions. This colour change demonstrates acidity alongside the evidence of high solubility.

Q5. Hydrogen chloride enters an inverted funnel whose broad rim is just below the water surface in a beaker. Explain the broad mouth's function and how this shallow arrangement prevents back suction when gas absorption lowers the pressure. [4 marks]

1. The broad mouth provides a large contact area between hydrogen chloride and water, allowing the gas to be absorbed efficiently.
2. If absorption lowers the pressure inside the funnel, water begins to rise into its wide portion from the surrounding beaker.
3. The outside water level can fall below the shallowly immersed rim. The water seal then breaks and air enters the funnel.
4. Entering air restores the pressure, allowing water to fall back into the beaker instead of being drawn into the gas-generating vessel.

Q6. In separate experiments, dilute hydrochloric acid is added to zinc and to copper(II) oxide. For each experiment, give a balanced equation and one observation. [4 marks]

1. With zinc, the balanced equation is Zn + 2HCl → ZnCl₂ + H₂. Zinc chloride and hydrogen are the products.
2. Bubbles of hydrogen appear as the zinc reacts with the dilute hydrochloric acid; this is gas evolution.
3. With copper(II) oxide, the balanced equation is CuO + 2HCl → CuCl₂ + H₂O. The products are copper(II) chloride and water.
4. The black copper(II) oxide dissolves and a blue-green solution develops because copper(II) chloride forms in the solution.

Q7. Dilute hydrochloric acid is added separately to sodium carbonate, sodium hydrogencarbonate, sodium sulphide and sodium sulphite. Give the balanced equation and gas identity for each reaction. Then explain why lime water alone cannot distinguish the carbonate gas from the sulphite gas. [5 marks]

1. Sodium carbonate produces carbon dioxide: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂. The reaction also forms sodium chloride and water.
2. Sodium hydrogencarbonate also produces carbon dioxide: NaHCO₃ + HCl → NaCl + H₂O + CO₂. Its equation uses one HCl unit for each NaHCO₃ unit.
3. Sodium sulphide produces hydrogen sulphide: Na₂S + 2HCl → 2NaCl + H₂S. This gas turns moist lead acetate paper black.
4. Sodium sulphite produces sulphur dioxide: Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂. This gas turns acidified potassium dichromate paper green.
5. Both carbon dioxide and sulphur dioxide can turn lime water milky. That shared observation does not distinguish them, so use an additional test such as acidified potassium dichromate paper.

Q8. State the composition of freshly prepared aqua regia, specifying the acid concentrations and volume ratio. State its characteristic use. [2 marks]

1. Aqua regia contains concentrated hydrochloric acid and concentrated nitric acid in the ratio 3:1 by volume, with hydrochloric acid named first.
2. It is used to dissolve gold and platinum, metals for which hydrochloric acid alone does not have this dissolving action.

Q9. Separate samples of hydrochloric acid are treated with silver nitrate solution and lead nitrate solution. Give each balanced equation and precipitate observation. Explain why the silver nitrate result alone cannot identify hydrochloric acid rather than sodium chloride solution. [3 marks]

1. AgNO₃ + HCl → AgCl + HNO₃. Silver chloride forms as a curdy white precipitate that dissolves in ammonia solution.
2. Pb(NO₃)₂ + 2HCl → PbCl₂ + 2HNO₃. Lead chloride forms as a white precipitate that dissolves in hot water.
3. Both hydrochloric acid and sodium chloride solution contain chloride ions. The silver nitrate result therefore detects chloride; additional evidence of acidity is needed to identify the acid.

Key takeaways

• Hydrogen chloride is a gas; hydrochloric acid is its aqueous solution, whose hydronium ions produce acidic behaviour.
• Sodium chloride and concentrated sulphuric acid produce hydrogen chloride on gentle warming, with sodium hydrogen sulphate remaining.
• Collect hydrogen chloride in a dry upright jar by upward displacement of air because it is denser than air.
• The fountain experiment links rapid gas absorption to lower internal pressure, liquid inflow and the acidic colour change.
• An inverted funnel with its rim just below water provides broad contact and helps prevent back suction.
• Ammonia reacts with hydrogen chloride to form solid ammonium chloride, observed as dense white fumes.
• Different salts release different gases with hydrochloric acid: carbonates give carbon dioxide, sulphides hydrogen sulphide and sulphites sulphur dioxide.
• Silver nitrate and lead nitrate give white chloride precipitates; these tests must be combined with evidence of acidity.
• Aqua regia contains concentrated hydrochloric and nitric acids in a three-to-one volume ratio and can dissolve gold and platinum.

Test yourself

Why must blue litmus be moist when testing dry hydrogen chloride?

Water allows hydrogen chloride to dissolve and form hydronium ions, which turn blue litmus red.

What remains when sodium chloride reacts with concentrated sulphuric acid under gentle heating?

Sodium hydrogen sulphate remains in the reaction vessel while hydrogen chloride escapes as a gas.

Which substance moves upwards during collection by upward displacement of air?

Air moves upwards out of the upright jar as the denser hydrogen chloride enters near the bottom.

What supplies the pressure that raises liquid in the fountain experiment?

Atmospheric pressure on the liquid in the trough pushes it towards the lower-pressure interior of the flask.

Why is the absorbing funnel placed close to the water surface?

A shallow water seal can break when water rises into the funnel, admitting air and interrupting back suction.

What causes the white fumes when ammonia meets hydrogen chloride?

Fine solid ammonium chloride particles form and remain suspended in the air as dense white fumes.

How do the products of a carbonate reaction with hydrochloric acid differ from those of a sulphide reaction?

A carbonate releases carbon dioxide and forms water; a sulphide releases hydrogen sulphide instead.

How do the white precipitates from silver nitrate and lead nitrate differ in the stated tests?

Silver chloride dissolves in ammonia solution, whereas lead chloride dissolves in hot water.