Get notes, summary, questions and answers, MCQs, extras, competency-based questions and PDFs of Study of Compounds-Hydrogen Chloride: ICSE Class 10 Chemistry (Concise/Selina). However, the notes should only be treated as references, and changes should be made according to the needs of the students.
Summary
Hydrogen chloride is a compound made of hydrogen and chlorine, with the formula HCl. It usually exists as a gas. This gas can be prepared in a laboratory by heating common salt, which is sodium chloride, with concentrated sulphuric acid. When these substances react, hydrogen chloride gas is formed.
Hydrogen chloride gas is colourless and has a sharp, choking smell. It is a bit heavier than air. Because it’s heavier, it can be collected in a gas jar by letting it flow in and push the lighter air out from the top. A very noticeable property of hydrogen chloride gas is that it dissolves extremely well in water.
When hydrogen chloride gas dissolves in water, it forms a solution called hydrochloric acid. This acid is naturally present in our stomachs and helps in digesting food. To make hydrochloric acid in a laboratory, the gas is carefully passed into water. A special setup, often using an inverted funnel, is used for this. This funnel helps the gas dissolve efficiently and prevents water from being sucked back into the apparatus where the gas is being made, which could be dangerous. The “fountain experiment” is a popular demonstration that shows how readily hydrogen chloride gas dissolves in water. In this experiment, a small amount of water is introduced into a flask filled with hydrogen chloride gas. The gas dissolves so quickly that the pressure inside the flask drops, causing a solution (often coloured with an indicator like litmus) to spray up into the flask like a fountain. This fountain turns red if blue litmus is used, showing the acidic nature of the solution.
Hydrochloric acid, being an acid, has certain chemical behaviours. It can turn blue litmus solution red. It reacts with many metals, like zinc or magnesium, to produce hydrogen gas and a metal chloride. It also reacts with basic substances, such as metal oxides (like copper oxide) and metal hydroxides (like sodium hydroxide), to form a salt and water. When hydrochloric acid is added to carbonates, such as calcium carbonate (found in chalk or marble), it causes fizzing and releases carbon dioxide gas.
There are ways to test for the presence of hydrogen chloride gas or hydrochloric acid. If a glass rod dipped in ammonia solution is brought near hydrogen chloride gas, dense white fumes of a substance called ammonium chloride are formed. Another common test involves adding silver nitrate solution. If hydrogen chloride or a chloride is present, a white, curdy solid called silver chloride will form and settle down.
Hydrogen chloride and hydrochloric acid have several uses. They are used in industries to manufacture various chemicals, including other chlorides and dyes. They are also used for cleaning metal surfaces, a process sometimes called pickling. In medicine, dilute hydrochloric acid can be given to people who do not produce enough stomach acid.
Workbook solutions (Concise/Selina)
Exercise
MCQs
1. Hydrogen chloride molecule is:
(a) Polar covalent
(b) Non polar
(c) Ionic
(d) Co-ordinate
Answer: (a) Polar covalent
2. Hydrogen chloride gas being highly soluble in the water is dried by:
(a) Anhydrous calcium chloride
(b) Phosphorous pentoxide
(c) Quick lime
(d) Conc. Sulphuric acid
Answer: (d) Conc. Sulphuric acid
3. The aim of the Fountain experiment is to prove that :
(a) HCl turns blue litmus red
(b) HCl is denser than air
(c) HCl is highly soluble in water
(d) HCl fumes in moist air
Answer: (c) HCl is highly soluble in water
4. HCl dissolves in toluene (C₆H₅CH₃) and the solution :
P contains hydronium ions.
Q can produce carbon dioxide on reacting with sodium carbonate.
R does not show any acidic property.
Which of the following holds true ?
(a) Only P
(b) Only Q
(c) Only R
(d) Both P and Q
Answer: (c) Only R
5. Assertion (A) : HCl is produced by the reaction of hydrogen and chlorine in diffused sunlight.
Reason (R) : This reaction is explosive in sunlight.
1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
6. Assertion (A) : The Fountain experiment is used to demonstrate the high solubility of HCl gas.
Reason (R) : Red litmus solution turns blue in Fountain experiment.
1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
7. Assertion (A) : HCl gas is collected by downward delivery.
Reason (R) : HCl gas is heavier than air.
1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
8. Assertion (A) : HCl gas fumes in moist air.
Reason (R) : HCl gas is highly soluble.
1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
9. Assertion (A) : HCl gas dissolves in water as well as organic compounds like toluene.
Reason (R) : HCl is a polar covalent compound.
1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (a) (1)
10. Assertion (A) : HCl gas is used in the preparation of chlorine and chlorides.
Reason (R) : Lead chloride is not produced from HCl.
1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (c) (3)
11. Assertion (A) : HCl has sour taste.
Reason (R) : HCl is highly soluble in water.
1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true but R is false.
4. A is false but R is true.
(a) (1)
(b) (2)
(c) (3)
(d) (4)
Answer: (b) (2)
Very Short Answer Type
1. Fill in the blank from the choices in the bracket:
(a) Quicklime is not used to dry HCI gas because CaO is …………. (alkaline/acidic/neutral).
(b) When sodium chloride is heated with concentrated sulphuric acid below 200°C, one of the product formed is …………. (sodium hydrogen sulphate/sodium sulphate/chlorine).
Answer: (a) Quicklime is not used to dry HCl gas because CaO is alkaline.
(b) When sodium chloride is heated with concentrated sulphuric acid below 200°C, one of the product formed is sodium hydrogen sulphate.
2. Name
(a) a black metallic oxide which reacts with hydrochloric acid to give a coloured solution.
Answer: Copper oxide.
(b) two colourless gases, which when mixed produce a white solid.
Answer: Hydrogen chloride gas and ammonia gas are two gases which when mixed produce a white solid, ammonium chloride.
(c) two gases which chemically combine to form a liquid.
Answer: Hydrogen and Oxygen
(d) a chloride which is soluble in excess of ammonium hydroxide.
Answer: Silver chloride.
(e) the chemical in which gold can be dissolved.
Answer: Aqua-regia.
(f) the experiment which demonstrates that hydrogen chloride is soluble in water.
Answer: Fountain Experiment.
(g) the gas produced when chlorine water is exposed to sunlight.
Answer: Oxygen
(h) The acid which on mixing with silver nitrate solution produces a white precipitate which is soluble in excess of ammonium hydroxide.
Answer: Hydrochloric acid.
(i) The gas which produces dense white fumes with ammonia gas.
Answer: Hydrogen chloride gas.
(j) An element which reacts with hydrogen to form a compound which is strongly acidic in water.
Answer: Chlorine.
3. Identify the gas evolved when :
Answer: (a) Potassium sulphite is treated with dilute hydrochloric acid.
Sulphur dioxide (SO2).
(b) Concentrated hydrochloric acid is made to react with manganese dioxide.
Chlorine (Cl2).
4. Solution A reacts with an acid B (which gives greenish yellow gas on reacting with oxidising agents like Pb3O4) to give white precipitate C insoluble in nitric acid but soluble in ammonium hydroxide. Name A, B and C.
Answer: A is silver nitrate solution.
B is concentrated hydrochloric acid.
C is silver chloride.
Short Answer Type
1. A solution of hydrogen chloride in water is prepared. The following substances are added to separate portions of the solution: Complete the table by writing the gas evolved in each case and its odour.
Answer: The completed table is as follows:
2. Complete and balance the following reactions, state whether dilute or conc. acid is used.
Answer: (a) NH₄OH + HCl → NH₄Cl + H₂O
Dilute HCI is used.
(b) NaHSO₃ + HCl (dil) → NaCl + H₂O + SO₂↑
Dilute HCl is used.
(c) Pb(NO₃)₂ + 2HCl (dil) → PbCl₂↓ + 2HNO₃
Dilute HCl is used.
(d) Pb₃O₄ + 8HCl (Conc.) → 3PbCl₂ + 4H₂O + Cl₂↑
Concentrated HCl is used.
3. State the composition of aqua regia. State which component is the oxidising agent in aqua regia.
Answer: Aqua regia is a mixture having three parts of conc. hydrochloric acid and one part of conc. nitric acid.
In aqua regia, nitric acid (HNO₃) acts as the oxidising agent, which reacts with hydrochloric acid to produce nascent chlorine (3HCl Conc. + HNO₃ Conc. → NOCl + 2H₂O + 2[Cl]).
4. How will the action of dilute hydrochloric acid enable you to distinguish between the following:
Answer: (a) Sodium carbonate and sodium sulphite
When dilute hydrochloric acid is added:
- Sodium carbonate reacts to produce carbon dioxide gas (CO₂):
Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑ - Sodium sulphite reacts to produce sulphur dioxide gas (SO₂):
Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂↑
(b) Sodium thiosulphate and sodium sulphite.
When dilute hydrochloric acid is added:
- Sodium sulphite reacts to produce sulphur dioxide gas (SO₂):
Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂↑
No precipitate is formed. - Sodium thiosulphate reacts to produce sulphur dioxide gas (SO₂) and a yellow precipitate of sulphur (S):
Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂↑ + S↓
The formation of a yellow precipitate of sulphur in the case of sodium thiosulphate distinguishes it from sodium sulphite, as sulphur is not precipitated when sulphites are treated with dilute HCl.
5. MnO₂, PbO₂ and red lead react with conc. HCl acid liberate Cl₂. What is the common property being shown by these metal oxides?
Answer: The common property being shown by these metal oxides (MnO₂, PbO₂, and red lead) is that they are strong oxidising agents. Concentrated hydrochloric acid is oxidised to chlorine by these agents, and alternately, hydrochloric acid readily reduces these compounds.
6. Write an equation for the reactions of hydrochloric acid on:
Answer: (a) silver nitrate solution:
AgNO₃ + HCl → AgCl↓ + HNO₃
(b) magnesium foil:
Mg + 2HCl → MgCl₂ + H₂↑
(c) caustic soda solution (sodium hydroxide):
NaOH + HCl → NaCl + H₂O
(d) zinc carbonate:
ZnCO₃ + 2HCl → ZnCl₂ + H₂O + CO₂↑
(e) manganese (IV) oxide:
MnO₂ + 4HCl (Conc.) → MnCl₂ + 2H₂O + Cl₂↑
(f) copper oxide (assuming copper (II) oxide):
CuO + 2HCl → CuCl₂ + H₂O
7. Write balanced equations for the reactions of dilute hydrochloric acid with each of the following:
Answer: (a) iron:
Fe + 2HCl (dil) → FeCl₂ + H₂↑
(b) sodium hydrogen carbonate:
NaHCO₃ + HCl (dil) → NaCl + H₂O + CO₂↑
(c) iron (II) sulphide:
FeS + 2HCl (dil) → FeCl₂ + H₂S↑
(d) magnesium sulphite:
MgSO₃ + 2HCl (dil) → MgCl₂ + H₂O + SO₂↑
8. How would you distinguish between dilute HCl and dilute HNO₃, by addition of only one solution.
Answer: To distinguish between dilute HCl and dilute HNO₃, add silver nitrate solution to separate samples of each acid.
- With dilute HCl, a thick curdy white precipitate of silver chloride (AgCl) will be formed, which is insoluble in nitric acid.
HCl(aq) + AgNO₃(aq) → AgCl↓(s) + HNO₃(aq) - With dilute HNO₃, no precipitate will be formed when silver nitrate solution is added (as silver nitrate itself is a nitrate and will not react to form a precipitate with another nitrate solution).
9. State one appropriate observation when
Answer: (a) Copper sulphide is treated with dilute hydrochloric acid.
A colorless gas with a foul odor resembling that of rotten eggs is released. The gas produced is hydrogen sulfide (H₂S).
(b) A few drops of dil. HCl are added to AgNO₃ solution, followed by addition of NH₄OH solution.
When a few drops of dilute HCl are added to AgNO₃ solution, a thick curdy white precipitate of silver chloride is formed. On subsequent addition of NH₄OH solution, this white precipitate dissolves.
(c) Lead nitrate solution is mixed with dilute hydrochloric acid and heated.
When lead nitrate solution is mixed with dilute hydrochloric acid, a white precipitate of lead chloride is formed. On heating, this precipitate of lead chloride dissolves.
(d) A small piece of zinc is added to dilute hydrochloric acid.
When a small piece of zinc is added to dilute hydrochloric acid, brisk effervescence occurs due to the evolution of hydrogen gas, and the zinc metal dissolves.
(e) Dilute HCl is added to sodium carbonate crystals
When dilute HCl is added to sodium carbonate crystals, brisk effervescence occurs due to the evolution of carbon dioxide gas.
(f) Concentrated HCl is added to manganese dioxide.
When concentrated HCl is added to manganese dioxide and heated (heating is generally required for this reaction to proceed readily, as implied by tests for HCl), a greenish-yellow gas (chlorine) is evolved, and the black manganese dioxide dissolves to form a pale pink or near colorless solution of manganese (II) chloride.
10. Write balanced chemical equations for the following:
Answer: (a) Sodium thiosulphate is reacted with dilute hydrochloric acid.
Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂↑ + S↓
(b) Calcium bicarbonate reacts with dilute hydrochloric acid.
Ca(HCO₃)₂ + 2HCl → CaCl₂ + 2H₂O + 2CO₂↑
(c) Conc. hydrochloric acid reacts with potassium permanganate solution.
2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 8H₂O + 5Cl₂
(d) Action of dilute hydrochloric acid on sodium sulphide.
Na₂S + 2HCl (dil) → 2NaCl + H₂S↑
(e) Action of hydrochloric acid on sodium bicarbonate.
NaHCO₃ + HCl → NaCl + H₂O + CO₂↑
11. Give balanced equations with conditions, if any, for the following conversions.
Answer: (a) Sodium chloride → Hydrogen chloride
NaCl(s) + H₂SO₄(conc.) –(< 200°C, heat)–> NaHSO₄(s) + HCl↑(g)
Condition: Heating sodium chloride with concentrated sulphuric acid at a temperature below 200°C.
(b) Hydrogen chloride → Iron (II) chloride
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂↑(g)
Condition: Reaction of iron metal with aqueous hydrochloric acid.
Alternatively, if using HCl gas:
Fe(s) + 2HCl(g) –(heat)–> FeCl₂(s) + H₂(g) (Metals that come before hydrogen in the electrochemical series form their corresponding chlorides when heated with HCl and liberate hydrogen).
(c) Hydrogen chloride → Ammonium chloride
NH₃(g) + HCl(g) → NH₄Cl(s)
Condition: Reaction of ammonia gas with hydrogen chloride gas; two gases combine to form a solid.
(d) Hydrogen chloride → Lead chloride
Pb(NO₃)₂(aq) + 2HCl(aq) → PbCl₂↓(s) + 2HNO₃(aq)
Condition: Reaction of aqueous hydrochloric acid (dilute) with lead nitrate solution.
12. Explain why:
(a) anhydrous HCl is a poor conductor while aqueous HCl is an excellent conductor.
Answer: Anhydrous hydrogen chloride is a covalent compound and does not contain free ions, hence it is a poor conductor of electricity. Aqueous HCl is an excellent conductor because hydrogen chloride, being a polar covalent compound, ionises in water to produce mobile ions (H₃O⁺ and Cl⁻): HCl + H₂O ⇌ H₃O⁺ + Cl⁻. These mobile ions are responsible for conducting electricity.
(b) when the stopper of a bottle full of hydrogen chloride gas is opened there are fumes in the air.
Answer: When the stopper of a bottle full of hydrogen chloride gas is opened, the HCl gas comes into contact with atmospheric water vapour. HCl gas is highly soluble in water and dissolves in the atmospheric water vapour to form tiny droplets of hydrochloric acid. These tiny droplets appear as white fumes.
(c) a solution of hydrogen chloride in water turns blue litmus red, and conducts electricity, while a solution of the same gas in toluene : (i) has no effect on litmus, and (ii) does not conduct electricity.
Answer: In water, hydrogen chloride ionises to form hydronium ions (H₃O⁺) and chloride ions (Cl⁻). The H₃O⁺ ions are responsible for turning blue litmus red, indicating acidic properties. The presence of these mobile ions also allows the solution to conduct electricity.
In toluene, which is an organic (non-polar) solvent, hydrogen chloride dissolves but does not ionise to form H₃O⁺ ions. Since there are no H₃O⁺ ions, it (i) has no effect on litmus, and (ii) due to the absence of mobile ions, it does not conduct electricity. This indicates the absence of H₃O⁺ in toluene, showing that hydrogen chloride remains a covalent compound in toluene.
(d) thick white fumes are formed when a glass rod dipped in NH₄OH is brought near the mouth of a bottle full of HCl gas.
Answer: Ammonium hydroxide (NH₄OH) solution gives off ammonia gas (NH₃). When a glass rod dipped in NH₄OH is brought near the mouth of a bottle full of HCl gas, the ammonia gas reacts with hydrogen chloride gas to form dense white fumes of solid ammonium chloride (NH₄Cl).
NH₃(g) + HCl(g) → NH₄Cl(s)
(e) dry hydrogen chloride gas does not affect a dry strip of blue litmus paper but it turns red in the presence of a drop of water.
Answer: Dry hydrogen chloride gas does not produce H⁺ ions (or H₃O⁺ ions) as it is covalent. Therefore, it does not show acidic character and does not affect dry blue litmus paper. In the presence of a drop of water, hydrogen chloride dissolves and ionises to form hydronium ions (H₃O⁺), which are responsible for the acidic property, thus turning the blue litmus paper red.
(f) hydrogen chloride gas is not collected over water.
Answer: Hydrogen chloride gas is not collected over water because it is highly soluble in water. (1 volume of water dissolves 452 volumes of the gas at room temperature).
(g) dilute hydrochloric acid cannot be concentrated by boiling beyond 22.2%.
Answer: Dilute hydrochloric acid, when boiled, gets gradually concentrated until the concentration of the acid reaches 22.2% HCl by weight (and 77.8% water by weight). At this point, it forms a constant boiling mixture or azeotrope, which boils at 110°C without any change in its composition. No further increase in concentration is possible on boiling, as molecules of HCl (g) get mixed with water vapours and escape together.
(h) Hydrogen chloride gas cannot be dried over quick lime.
Answer:Quick lime (calcium oxide, CaO) is a basic substance. Hydrogen chloride gas is acidic. Therefore, quick lime cannot be used to dry HCl gas because it reacts chemically with hydrogen chloride: CaO + 2HCl → CaCl₂ + H₂O
Long Answer Type
1. Draw a labelled diagram for the laboratory preparation of hydrogen chloride gas and answer the following.
(a) Name the acid used. Why is this particular acid preferred to other acids?
(b) Give the balanced equation for the reaction.
(c) Name the drying agent used in drying hydrogen chloride gas.
(d) Phosphorus pentoxide and calcium oxide are good drying agents, but they cannot be used to dry hydrogen chloride gas. Why?
(e) Why is direct absorption of HCl gas in water not feasible?
(f) What arrangement is done to dissolve HCl gas in water.
Answer:
(a) The acid used is concentrated sulphuric acid. Concentrated sulphuric acid is preferred because, unlike an acid such as concentrated nitric acid, it is less volatile. Concentrated nitric acid is not used because it is volatile and may volatilize out along with hydrogen chloride. Sulphuric acid’s low volatility ensures that primarily HCl gas is evolved.
(b) The balanced equation for the reaction, when the temperature is maintained below 200°C, is:
NaCl + H₂SO₄ (conc.) –(< 200°C)–> NaHSO₄ + HCl↑
If the temperature is above 200°C, the reaction is:
2NaCl + H₂SO₄ (conc.) –(above 200°C)–> Na₂SO₄ + 2HCl↑
The precaution mentioned is to maintain the temperature at nearly 200°C, suggesting the first reaction is generally preferred.
(c) The drying agent used is concentrated sulphuric acid.
(d) Phosphorus pentoxide (P₂O₅) and calcium oxide (CaO, quick lime) cannot be used to dry hydrogen chloride gas because they react with hydrogen chloride.
The reactions are:
2P₂O₅ + 3HCl → POCl₃ + 3HPO₃
CaO + 2HCl → CaCl₂ + H₂O
(e) Direct absorption of HCl gas in water is not feasible due to ‘back suction’. Hydrogen chloride gas is highly soluble in water and is absorbed more quickly than it is generated. This causes the pressure in the delivery tube and flask to reduce, and the atmospheric pressure from outside forces water back up into the delivery tube and potentially into the generating flask. This could stop the reaction and might even result in an explosion if water contacts hot concentrated sulphuric acid.
(f) To dissolve HCl gas in water, an inverted funnel arrangement is used. The inverted funnel, connected to the hydrogen chloride gas supply, is placed in a beaker with its rim just touching the surface of the water. This arrangement prevents or minimizes back-suction and provides a large surface area for the absorption of HCl gas.
2. The given set up in the figure is for the preparation of an acid.
(a) Name the acid prepared by this method.
Answer: The acid prepared by this method is hydrochloric acid (aqueous solution of HCl gas).
(b) Name the reactants used.
Answer: The reactants used to prepare the acid in this setup (dissolving the gas) are hydrogen chloride gas and water.
(c) Why an empty flask is used?
Answer: An empty flask (anti-suction device) is used to prevent back suction from reaching the generating flask. If back suction occurs, the water from the trough will collect in this empty flask instead of going into the apparatus where the gas is being produced.
(d) What is the drying agent used? Why is this drying agent chosen?
Answer: If the HCl gas being dissolved needs to be dried prior to this setup, the drying agent used is concentrated sulphuric acid.
This drying agent is chosen because it effectively dries hydrogen chloride gas and does not react with it, unlike other drying agents such as phosphorus pentoxide or calcium oxide.
(e) What is the role of inverted funnel in the arrangement?
Answer: The role of the inverted funnel in the arrangement is:
(i) It prevents or minimizes back-suction of water into the gas supply line or generating flask.
(ii) It provides a large surface area for the absorption of HCl gas by the water.
3. How will you prove that Hydrochloric acid contains (i) hydrogen (ii) chlorine. Write equations for the reactions.
Answer: (i) To prove that hydrochloric acid contains hydrogen: React hydrochloric acid with an active metal (above hydrogen in the activity series), such as magnesium. Hydrogen gas will be evolved.
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂↑(g)
The evolution of hydrogen gas, which can be identified (e.g., it is a flammable gas), proves the presence of hydrogen in hydrochloric acid.
(ii) To prove that hydrochloric acid contains chlorine (as chloride ions): Add silver nitrate solution to hydrochloric acid. A thick curdy white precipitate of silver chloride will be formed, which is insoluble in nitric acid but soluble in ammonium hydroxide.
AgNO₃(aq) + HCl(aq) → AgCl↓(s) + HNO₃(aq)
The formation of silver chloride precipitate indicates the presence of chloride ions, and thus chlorine, in hydrochloric acid.
4. Give three distinct tests [apart from using an indicator] you would carry out with solution of HCl to illustrate the typical properties of an acid.
Answer: Three distinct tests to illustrate the typical properties of hydrochloric acid (apart from using an indicator) are:
(i) Reaction with an active metal: Add a piece of an active metal like magnesium or zinc to hydrochloric acid. Effervescence will be observed due to the evolution of hydrogen gas.
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂↑(g)
(ii) Reaction with a carbonate or bicarbonate: Add hydrochloric acid to a metal carbonate like sodium carbonate or calcium carbonate. Brisk effervescence will occur due to the evolution of carbon dioxide gas.
Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂↑(g)
(iii) Reaction with a metallic oxide (base): Add hydrochloric acid to a basic metallic oxide like copper (II) oxide (a black solid). The black solid will dissolve, and a coloured solution (e.g., blue-green for copper (II) chloride) will be formed, along with water. This demonstrates neutralization.
CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)
5. Convert:
(a) Two soluble metallic nitrates to insoluble metallic chlorides.
Answer: 1. Convert soluble lead nitrate to insoluble lead chloride:
Pb(NO₃)₂(aq) + 2HCl(aq) → PbCl₂↓(s) + 2HNO₃(aq)
(Lead chloride is a white precipitate, sparingly soluble in cold water but dissolves on heating).
2. Convert soluble silver nitrate to insoluble silver chloride:
AgNO₃(aq) + HCl(aq) → AgCl↓(s) + HNO₃(aq)
(Silver chloride is a white curdy precipitate).
(b) Insoluble iron (II) oxide to a soluble compound.
Answer: React insoluble iron (II) oxide (FeO) with hydrochloric acid to form soluble iron (II) chloride:
FeO(s) + 2HCl(aq) → FeCl₂(aq) + H₂O(l)
(c) Insoluble metal carbonate to a soluble chloride.
Answer: React an insoluble metal carbonate like calcium carbonate (CaCO₃) with hydrochloric acid to form soluble calcium chloride:
CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂↑(g)
(d) Metal sulphide to an offensive smelling gas.
Answer: React a metal sulphide like iron (II) sulphide (FeS) with dilute hydrochloric acid to produce hydrogen sulphide gas (H₂S), which has an offensive smell (like rotten eggs):
FeS(s) + 2HCl(aq) → FeCl₂(aq) + H₂S↑(g)
6. Study the flow chart and give balanced equations with conditions for the conversions A, B, D and E.
Answer: A: NaCl → HCl
NaCl(s) + H₂SO₄(conc.) –(< 200°C, heat)–> NaHSO₄(s) + HCl↑(g)
Condition: Heating sodium chloride with concentrated sulphuric acid at a temperature below 200°C.
B: HCl → ZnCl₂
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂↑(g)
Condition: Reaction of zinc metal with aqueous hydrochloric acid. (HCl gas can also react: Zn(s) + 2HCl(g) → ZnCl₂(s) + H₂(g))
D: HCl → AgCl
AgNO₃(aq) + HCl(aq) → AgCl↓(s) + HNO₃(aq)
Condition: Reaction of aqueous hydrochloric acid with silver nitrate solution.
E: HCl → MnCl₂
MnO₂(s) + 4HCl(conc.) –(heat)–> MnCl₂(aq) + 2H₂O(l) + Cl₂↑(g)
Condition: Heating manganese dioxide with concentrated hydrochloric acid.
7. In the laboratory preparation of hydrochloric acid, hydrogen chloride gas is dissolved in water.
(a) Draw a diagram to show the arrangement used for the absorption of HCl gas in water.
(b) State why such an arrangement is necessary. Give two reasons for the same.
(c) Write balanced chemical equations for the laboratory preparation of HCl gas when the reaction is: (A) below 200°C (B) above 200°C
Answer: (a)
(b) Such an arrangement (inverted funnel) is necessary because HCl gas is highly soluble in water.
Two reasons are:
- To prevent or minimize back-suction: Due to the high solubility of HCl, water tends to be drawn back into the gas supply or generating flask. The funnel breaks the direct path and provides a temporary space for the sucked-back water, preventing it from reaching the main apparatus.
- To provide a large surface area for absorption: The wide mouth of the funnel offers a larger contact area between the gas and the water, facilitating efficient dissolution of the HCl gas.
(c) A. Below 200°C:
NaCl(s) + H₂SO₄(conc.) –(< 200°C)–> NaHSO₄(s) + HCl↑(g)
B. Above 200°C:
2NaCl(s) + H₂SO₄(conc.) –(above 200°C)–> Na₂SO₄(s) + 2HCl↑(g)
8. Study the figure and answer the questions that follow:
(a) Identify the gas Y.
Answer: Gas Y is Hydrogen Chloride (HCl) gas.
(b) What property of gas Y does this experiment demonstrate?
Answer: It primarily demonstrates the high solubility of Gas Y (HCl) in water.
(c) Name another gas which has the same property and can be demonstrated through this experiment.
Answer: Ammonia (NH₃) gas is also highly soluble in water.
9. Refer to the diagram given below and write balanced equations with conditions, if any, for the following conversions P to S.
Answer: P: NaCl → HCl
NaCl(s) + H₂SO₄(conc.) –(< 200°C, heat)–> NaHSO₄(s) + HCl↑(g)
Condition: Heating sodium chloride with concentrated sulphuric acid, temperature below 200°C.
Q: HCl → Iron (II) chloride
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂↑(g)
Condition: Reaction of iron metal with dilute aqueous hydrochloric acid.
R: HCl → Ammonium chloride
NH₃(g) + HCl(g) → NH₄Cl(s)
Condition: Reaction of ammonia gas with hydrogen chloride gas.
S: HCl → Lead chloride
Pb(NO₃)₂(aq) + 2HCl(aq) → PbCl₂↓(s) + 2HNO₃(aq)
Condition: Reaction of aqueous hydrochloric acid (dilute) with lead nitrate solution. A white precipitate of lead chloride is formed.
10. State your observations:
(a) HCl gas is passed through lead nitrate solution and the mixture is heated.
Answer: Observation: When HCl gas is passed through lead nitrate solution, a white precipitate of lead chloride is formed. On heating the mixture, this white precipitate dissolves.
(b) Hydrochloric acid is added to silver nitrate solution.
Answer: Observation: When hydrochloric acid is added to silver nitrate solution, a thick curdy white precipitate of silver chloride is formed.
(c) Ammonium hydroxide solution is added to the resultant product of part (b).
Answer: Observation: When ammonium hydroxide solution is added to the white precipitate of silver chloride (resultant product of part b), the precipitate dissolves, forming a clear solution of diammine silver (I) chloride.
Distinguish by using HCl.
(a) Lead nitrate solution and silver nitrate solution.
(b) Potassium sulphite and potassium sulphide.
Answer: (a) To distinguish between lead nitrate solution and silver nitrate solution using hydrochloric acid:
When dilute hydrochloric acid is added to both solutions, a white precipitate is formed in each case.
- In the case of lead nitrate, a white precipitate of lead chloride is formed. The reaction is:
Pb(NO₃)₂ + 2HCl → PbCl₂↓ + 2HNO₃ - In the case of silver nitrate, a thick curdy white precipitate of silver chloride is formed. The reaction is:
AgNO₃ + HCl → AgCl↓ + HNO₃
Upon heating the mixtures, the precipitate of lead chloride dissolves, while the precipitate of silver chloride does not.
(b) To distinguish between potassium sulphite and potassium sulphide using hydrochloric acid:
When dilute hydrochloric acid is added to both substances, different gases are evolved.
When potassium sulphide is treated with dilute hydrochloric acid, hydrogen sulphide gas, which is an offensive smelling gas, is evolved. The reaction is:
K₂S + 2HCl → 2KCl + H₂S↑
When potassium sulphite is treated with dilute hydrochloric acid, sulphur dioxide gas is evolved. The reaction is:
K₂SO₃ + 2HCl → 2KCl + H₂O + SO₂↑
12. The following questions pertain to the laboratory preparation of hydrogen chloride gas.
(a) Write the equation for its preparation, mentioning the conditions required.
Answer: Equation: NaCl(s) + H₂SO₄(conc.) → NaHSO₄(s) + HCl↑(g)
Conditions required: Sodium chloride reacted with concentrated sulphuric acid, with heating, maintaining the temperature below 200°C (or nearly 200°C).
(b) Name the drying agent used in the above preparation and give a reason for the choice.
Answer: Drying agent: Concentrated sulphuric acid.
Reason for choice: Concentrated sulphuric acid is chosen because it effectively dries hydrogen chloride gas and, importantly, it does not react chemically with HCl. Other drying agents like phosphorus pentoxide or quicklime (CaO) are not suitable as they react with HCl.
(c) State a safety precaution taken during the preparation of hydrochloric acid.
Answer: A safety precaution is that the lower end of the thistle funnel (used for adding concentrated sulphuric acid to sodium chloride) must be dipped in the concentrated sulphuric acid in the flask. This prevents the HCl gas produced from escaping through the thistle funnel. Another precaution is to maintain the temperature at nearly 200°C to prevent the glass apparatus from cracking and to avoid the formation of a hard crust of sodium sulphate which is difficult to remove.
13. Identify the gas evolved and give chemical test in each of the following cases. Dilute hydrochloric acid reacts with:
(i) Iron (II) sulphide
(ii) Sodium sulphite.
Answer: (i) When dilute hydrochloric acid reacts with Iron (II) sulphide, the gas evolved is hydrogen sulphide (H₂S). The reaction is:
FeS + 2HCl → FeCl₂ + H₂S↑
Chemical Test: Hydrogen sulphide gas has a characteristic smell of rotten eggs. If a filter paper dipped in lead acetate solution is brought to the mouth of the test tube, it turns black.
(ii) When dilute hydrochloric acid reacts with Sodium sulphite, the gas evolved is sulphur dioxide (SO₂). The reaction is:
Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂↑
Chemical Test: Sulphur dioxide gas has a pungent smell like that of burning sulphur. If a filter paper dipped in acidified potassium dichromate solution is brought near the gas, the paper turns from orange to green.
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