Frank Brothers Solutions for ICSE Class 10 Chemistry Chapter 8 Study Of Compounds I Hydrogen Chloride

Chapter 8. Study of Compounds-I: Hydrogen Chloride

Solution 1:
(i) Acid present in the stomach of mammals is hydrochloric acid.
(ii) Concentrated sulphuric acid is used to dry hydrogen chloride gas.
(iii) Salt obtained by heating sodium chloride with concentrated sulphuric acid below 200 degree Celsius is sodium hydrogen sulphate
NaCl + \( H_2SO_4 \) → NaHSO₄ + HCl
Sodium chloride + Sulphuric Acid(conc.) → Sodium hydrogen sulphate + Hydrogen gas
(iv) When ammonia gas is bubbled through hydrogen chloride white precipitate of ammonium chloride is obtained.
HCl(l) + NH₃ (g) → NH₄Cl(s)
(White precipitate)
(v) Two solutions that yields white precipitates, when treated with hydrogen chloride or hydrochloric acid are-
(a)Silver nitrate
(b)Lead nitrate
(vi) Gas obtained by treating metals with hydrochloric acid is hydrogen gas.
(vii) Gas obtained by treating ferrous sulphide with hydrochloric acid is hydrogen sulphide (\( H_2S \)).
(viii) Five oxidizing agents that liberated chlorine from concentrated hydrochloric acid are-
(a)\( MnO_2 \)
(b)\( PbO_2 \)
(c)\( KMnO_4 \)
(d)\( K_2Cr_2O_7 \)
(e)\( CaOCl_2 \)
(ix) Hydrochloric acid is used to extract glue from bones.
(x) Silver chloride is soluble in excess of ammonium hydroxide.
(xi) Chlorine is a greenish yellow gas.
(xii) Gold can be dissolved in aqua regia which is mixture of three parts of concentrated hydrochloric acid and one part of concentrated nitric acid.
(xiii) \( K_2Cr_2O_7 \) reacts with hydrochloric acid to give a coloured solution.
(xiv) HCl and NH₃ are two colourless gases which when mixed produce a white solid.

📝 Teacher's Note: This is a comprehensive list of HCl properties and reactions. Have students create flashcards for each point and practice writing balanced equations. Focus on the industrial and biological applications to make the chemistry more relatable.

🎯 Exam Tip: Remember the key reactions: NaCl + H₂SO₄ for HCl preparation, and HCl + NH₃ for white fumes. These equations frequently appear in exams.

Solution 2:
Answer: Calcium oxide and phosphorous pentoxide are very good drying agents but they are not used to dry HCl gas because they react with hydrogen chloride.
CaO +2 HCl → CaCl₂ +H₂O
2P₂O₅ +3HCl → POCl₃ +3HPO₃

📝 Teacher's Note: Demonstrate this concept by showing that drying agents must be chemically inert to the substance being dried. Use the analogy of trying to dry your clothes with something that would stain them - it defeats the purpose.

🎯 Exam Tip: Always check if the drying agent reacts with the gas. Write the reaction equation to prove your point - examiners love to see chemical evidence.

Solution 3:
Answer: (i) Hydrogen chloride gas is prepared by the reaction of sodium chloride and sulphuric acid at the temperature below 200°C. Sodium chloride is preferred for laboratory preparation of HCl gas since it is cheapest and most easily available chloride. Lower temperature is preferred because at higher temperature sodium sulphate forms a hard crust and sticks to the bottom of the flask and is difficult to remove.

NaCl + H₂SO₄ → NaHSO₄ + HCl
Sodium chloride + Sulphuric acid (conc.) → Hydrogen chloride gas

(ii) A glass rod dipped in ammonia solution is brought near the mouth of the gas jar. Dense white fumes of ammonium chloride will be produced. It proves the presence of HCl gas in the gas jar.

(iii) Concentrated H₂SO₄ is used to dry HCl gas.

(iv) The gas is collected by upward displacement of air because-
(i) It is heavier than air.
(ii) It is highly soluble in water.

(v) Important precautions are-
(i)The lower end of thistle funnel must be dipped below the concentrated H₂SO₄.
(ii)Temperature should be maintained nearly 200°C because at higher temperature -
(a)The apparatus may break
(b)Fuel is wasted.
(c) Sodium sulphate formed in the reaction forms a crust on the surface which is difficult to remove.

📝 Teacher's Note: Set up the actual apparatus if possible. Students learn better when they see the white fumes test in action. Emphasize safety precautions when handling concentrated acids.

🎯 Exam Tip: Always mention the white fumes test with NH₃ to identify HCl gas. Draw a neat labeled diagram of the apparatus - it's worth 2-3 marks.

Solution 4:
Answer: Hydrochloric acid cannot be prepared by mixing hydrochloride gas directly in water as it leads to back suctions.

A complete set up of apparatus for the preparation of hydrochloric acid in laboratory is shown in the given figure.

Hydrogen chloride gas prepared by the reaction of sodium chloride and sulphuric acid, is passed to inverted funnel arrangement through empty flask. An inverted funnel connected to the HCl gas supply is placed in a beaker in such a way that the rim of the funnel just touches the surface of water in the beaker. HCl gas coming through delivery tube fills the mouth of the funnel and then dissolves in water. By this way we get hydrochloric acid in the beaker.

OR

Hydrogen chloride gas is dissolved in water by inverted funnel arrangement. HCl gas cannot be dissolved in water directly as it lead to back suction.

The purpose of funnel while preparing hydrochloric acid from HCl gas is :
1. To prevent back suction of water.
2. It provides a larger surface for dissolution of hydrogen chloride gas.

📝 Teacher's Note: Explain the concept of back suction using a simple demonstration with a straw and colored water. This helps students understand why we can't just bubble HCl gas directly into water.

🎯 Exam Tip: Remember the two main purposes of the inverted funnel: preventing back suction and providing larger surface area. These are standard marking points in exams.

Solution 5:
Answer: (i) Lower temperature is preferred because at higher temperature sodium sulphate forms a hard crust and sticks to the bottom of the flask and is difficult to remove.
(ii) Hydrogen chloride is not collected over water because it is highly soluble in water
(iii) Hydrochloric acid cannot be concentrated over 22.2 % by boiling because this mixture(77.8 % water +22.2 % HCl) boils at a constant temperature of 110°C without any change in concentration. Such a mixture is called as azeotrope
(iv) HCl gas does not conduct electricity, but hydrochloric acid conducts electricity because in hydrochloric acid(aq), hydrogen ions are present to facilitate the movement of electrons whereas no such medium is present in gas
(v) Dilute hydrochloric acid cannot be concentrated beyond 22.2%(by weight) by distillation. This is due to the fact that this mixture(77.8% water +22.2% HCl) boils at a constant temperature of 110°C without any change in concentration. So if its concentration is high it will fall back at a lower concentration
(vi) When the stopper of a bottle full of HCl gas is opened there are fumes in the air due to high solubility of HCl gas in water. It fumes in moist air forming a cloud of tiny droplets of hydrochloric acid.
(vii) A solution of HCl gas in water turns blue litmus red and conducts electricity, while HCl gas dissolves in toluene and it has no effect because hydrochloric acid is a polar covalent compound. In polar solvent like water it ionizes to give hydrogen ions and also facilitates the movement of electrons to conduct electricity while it does not ionize in organic solvents like toluene.
(viii) Dissolved chlorine reacts slowly with water to give hydrochloric acid and hypochlorous acid:
Cl₂+ H₂O → HCl +HClO
(ix) Formation of Hydrochloric acid in the reaction is responsible for the acidic nature.

📝 Teacher's Note: The azeotrope concept is tricky - use the analogy of a stubborn mixture that refuses to separate further. Demonstrate conductivity with a simple circuit if possible.

🎯 Exam Tip: Remember 22.2% is the maximum concentration of HCl by boiling - this specific number often appears in exams. Always explain azeotrope formation when asked about concentration limits.

Solution 6:
Answer: (i) If calcium oxide is used to dry HCl, then it will react with HCl gas and result in the formation of calcium chloride and water.
CaO +2 HCl → CaCl₂ +H₂O
(ii) When concentrated hydrochloric acid is kept open, it fumes in moist air forming a cloud of tiny droplets of hydrochloric acid.
(iii) Hydrochloric acid is formed when Hydrogen chloride gas prepared in laboratory, is passed through water, using a delivery tube.

📝 Teacher's Note: Emphasize that drying agents must not react with the substance being dried. This is a fundamental principle in chemistry that students often overlook.

🎯 Exam Tip: For drying agent questions, always write the reaction equation showing why it can't be used. This demonstrates chemical understanding beyond just memorization.

Solution 7:
Answer: In the following reaction, hydrochloric acid acts as an acid.
Zn + 2HCl → ZnCl₂ +H₂
CuO +2HCl → CuCl₂ +H₂O
FeS +2HCl → FeCl₂ +H₂S

📝 Teacher's Note: Help students identify the acid behavior by looking at what HCl produces in each reaction - it donates H⁺ ions and forms salts. Use these reactions to reinforce the definition of acids.

🎯 Exam Tip: When asked about HCl acting as an acid, always show reactions where it forms salts and liberates H⁺ ions. These three reactions are classic examples that examiners love to test.

Solution 8.
Answer:
(i) Two gas jars A and B are taken. A is filled with air and B with dry HCl gas. B is inverted over A, kept for few minutes and then jar is removed. Now a rod dipped in ammonium hydroxide is introduced into the jar A, white fumes are seen. This shows that HCl displaces the air since it is heavier than air.

(ii) The high solubility of HCl gas can be demonstrated by "Fountain experiment". In it, a well dried round bottom flask is filled with dry HCl gas and fitted with a two holed rubber stopper. A jet tube is inserted into the flask through one of the holes. A dropper filled with water is fitted into the other hole. Closing the outer end of the jet tube, the flask is inverted and the tube is dipped into a blue litmus solution taken in a beaker, then dropper is pressed. It is seen that the blue litmus solution enters the jet tube with a great force, forming a red fountain.

It happens because water introduced from dropper completely absorbs HCl gas, thereby creating a very low pressure within the flask. To make up for this loss in pressure, blue litmus solution rises in the jet tube and colour of litmus changes to red.

(iii) When HCl gas is heated above \( 500°C \), it dissociates into hydrogen and chlorine.
\[ 2HCl \xrightarrow{>500°C} H_2 + Cl_2 \]
In simple words: These experiments show the basic properties of hydrogen chloride gas - it's denser than air, very soluble in water, and breaks down when heated strongly.

📝 Teacher's Note: Use the fountain experiment as a visual demonstration to show HCl's extreme solubility. Emphasize safety when handling HCl gas and always use proper ventilation in the lab.

🎯 Exam Tip: For the fountain experiment, mention the key points - dry HCl gas, complete absorption by water, low pressure creation, and litmus color change for full marks.

Solution 9.
Answer:
a)
(i) Hydrogen
(ii) Carbon dioxide
(iii) Sulphur dioxide
(iv) Hydrogen sulphide
(v) Carbon dioxide
(vi) sulphur dioxide

b)
(i) \( Zn + 2HCl \rightarrow ZnCl_2 + H_2 \)
(ii) \( CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2 \)
(iii) \( Na_2SO_3 + 2HCl \rightarrow 2NaCl + SO_2 + H_2O \)
(iv) \( PbS + 2HCl \rightarrow PbCl_2 + H_2S \)
(v) \( Mg(HCO_3)_2 + 2HCl \rightarrow MgCl_2 + 2H_2O + 2CO_2 \)
(vi) \( K_2SO_3 + 2HCl \rightarrow 2KCl + SO_2 + H_2O \)
In simple words: HCl reacts with different compounds to produce various gases - metals give hydrogen, carbonates give carbon dioxide, sulphites give sulphur dioxide, and sulphides give hydrogen sulphide.

📝 Teacher's Note: Ask students to identify the pattern - HCl always forms a chloride salt plus water and the characteristic gas from the other reactant. This helps them predict products systematically.

🎯 Exam Tip: Balance equations carefully and remember that HCl displaces weaker acids from their salts to form the corresponding gases.

Solution 10.
Answer:
(i) Aqua regia is a mixture is 3 parts of concentrated HCl and 1 part of concentrated \( HNO_3 \).
(ii) It is used to dissolve noble metals like gold and platinum.
(iii) HCl dissolves noble metals like gold and platinum. HCl with \( HNO_3 \) reacts to produce nascent chlorine which reacts with gold, platinum etc. to form their respective chlorides.
In simple words: Aqua regia is a special acid mixture that can dissolve even gold and platinum, which normal acids cannot do.

📝 Teacher's Note: Explain that neither HCl nor HNO₃ alone can dissolve gold, but their mixture creates nascent chlorine which is powerful enough to attack noble metals.

🎯 Exam Tip: Remember the 3:1 ratio of HCl to HNO₃ and mention that it produces nascent chlorine for dissolving noble metals.

Solution 11.
Answer:
Two tests for hydrochloric acid are-
(i) When silver nitrate solution is added to hydrochloric acid, it gives white precipitate of silver chloride.
(ii) When lead nitrate solution is added to it, it gives white precipitate of lead chloride.
In simple words: You can identify HCl by adding silver nitrate or lead nitrate - both will form white precipitates that confirm the presence of chloride ions.

📝 Teacher's Note: Demonstrate both tests in class. Point out that the silver chloride test is more specific and commonly used in qualitative analysis.

🎯 Exam Tip: Write the complete chemical equations for both tests to show the formation of AgCl and PbCl₂ precipitates.

Solution 12.
Answer:
Uses of hydrochloric acid are-
(i) It is used in the production of dyes, drugs, paints, photographic chemicals etc.
(ii) It is used in the extraction of bone charcoal, by dissolving away calcium phosphate.
(iii) It is used in the extraction of glue from bones.
(iv) It is used in the production of glucose from starch.
In simple words: HCl is widely used in industries to make medicines, dyes, clean bones for glue-making, and convert starch into sugar.

📝 Teacher's Note: Connect these uses to students' daily life - medicines they take, colors in their clothes, and food processing all involve HCl at some stage.

🎯 Exam Tip: List at least four different uses from different industries to show the versatility of hydrochloric acid.

Solution 13.
Answer:
(i) \( 2HCl \xrightarrow{>500°C} H_2 + Cl_2 \)
(ii) \( MnO_2 + 4HCl \rightarrow MnCl_2(aq) + 2H_2O + Cl_2 ↑ \)
Manganese dioxide → Manganese chloride
(iii) \( NaCl + H_2SO_4 \xrightarrow{<200°C} NaHSO_4 + HCl \)
Sodium chloride → Sodium hydrogen sulphate
In simple words: HCl can be decomposed by heat or can react with oxidizing agents to release chlorine gas, and can be prepared from salt and sulfuric acid.

📝 Teacher's Note: Emphasize the temperature conditions in these reactions - below 200°C for preparation and above 500°C for decomposition.

🎯 Exam Tip: Include temperature conditions and states of products (especially noting which gases escape) for complete answers.

Solution 14.
Answer:
(i) On reaction with metals, hydrochloric acid forms respective chlorides and liberates hydrogen.
\( Zn + 2HCl \rightarrow ZnCl_2 + H_2 ↑ \)

(ii) On reaction with oxidizing agents, hydrochloric acid forms respective salts water and pungent smelling, greenish yellow chlorine gas.
\( MnO_2 + 4HCl \rightarrow MnCl_2(aq) + 2H_2O + Cl_2 ↑ \)
Manganese dioxide → Manganese chloride
In simple words: HCl behaves differently with metals (gives hydrogen gas) versus oxidizing agents (gives chlorine gas).

📝 Teacher's Note: Use the activity series to explain why some metals react with HCl while others don't. Demonstrate the different colored gases produced.

🎯 Exam Tip: Mention the physical properties of the gases formed - hydrogen is colorless, chlorine is greenish-yellow and pungent.

Solution 15.
Answer:
(i) 3:1 conc. HCl and conc. \( HNO_3 \)
(ii) Azeotropes
(iii) Oxidising
(iv) diffused sunlight
(v) silver chloride
(vi) hot water
In simple words: These are key properties and characteristics of HCl and its reactions under different conditions.

📝 Teacher's Note: Explain what azeotropes are - mixtures that boil at constant temperature without change in composition, making concentration beyond a point impossible.

🎯 Exam Tip: Learn these specific terms and ratios as they frequently appear in short answer questions about HCl properties.

Solution 1991-1.
Answer:
Silver nitrate crystals are dissolved in distilled water to prepare its solution and not in tap water. This is due to the reason that tap water contains chloride ions which gives white precipitate with \( AgNO_3 \) solution.

Dilute hydrochloric acid decomposes metallic carbonates, hydrogen carbonates to give the corresponding metal chloride and carbon dioxide. The gas \( CO_2 \) liberated with brisk effervescence while sulphuric acid does not give \( CO_2 \).
In simple words: Tap water has chlorides that interfere with silver nitrate tests, and HCl produces carbon dioxide bubbles when it reacts with carbonates, which sulfuric acid doesn't do.

📝 Teacher's Note: Always emphasize using distilled water in analytical chemistry to avoid interference from impurities. Demonstrate the effervescence test with marble chips.

🎯 Exam Tip: Mention "brisk effervescence" when describing CO₂ evolution and explain why distilled water is essential for accurate tests.

Solution 1992-1.
Answer:
(a) HCl and \( NH_3 \) combine chemically to form a solid.
(b) Silver chloride is soluble in excess of \( NH_4OH \).
(c) On reaction with metals, hydrochloric acid forms respective chlorides and liberates hydrogen.
\( Zn + 2HCl \rightarrow ZnCl_2 + H_2 ↑ \)

On reaction with oxidizing agents, hydrochloric acid forms respective salts water and pungent smelling, greenish yellow chlorine gas.
\( MnO_2 + 4HCl \rightarrow MnCl_2(aq) + 2H_2O + Cl_2 ↑ \)
Manganese dioxide → Manganese chloride
(d) \( 2KMnO_4 + 16HCl \rightarrow 2KCl + 2MnCl_2 + 5Cl_2 + 8H_2O \)
In simple words: HCl shows various chemical behaviors - forming solids with ammonia, dissolving silver chloride in excess ammonia, and producing different gases with different reactants.

📝 Teacher's Note: The white smoke formation between HCl and NH₃ is a classic demonstration. Explain how excess NH₄OH forms a complex with AgCl, making it soluble.

🎯 Exam Tip: For balanced equations with KMnO₄, use the half-reaction method and ensure you balance electrons transferred correctly.

Solution 1992-2.
Answer:
An inverted funnel connected to the HCl gas supply is placed in a beaker in such a way that the rim of the funnel just touches the surface of water in the beaker. HCl gas coming through the delivery tube fills the mouth of the funnel and then dissolves in water. Hence, a low pressure is created and back suction occurs. As a result of it, water level rises in funnel and level of water in the beaker is lowered. Thus, the rim of the funnel loses contact with water and the outside air rushes in through the gap to equalize the pressure on either side of the funnel. The solution falls back into the beaker due to force of gravity. The water level then falls and the funnel drops back to its original position.
In simple words: This setup prevents backflow by using an inverted funnel that automatically breaks contact with water when suction occurs, allowing air to enter and equalize pressure.

📝 Teacher's Note: This is a clever engineering solution to prevent water from being sucked back into the gas generator due to HCl's high solubility.

🎯 Exam Tip: Explain the sequence clearly - gas dissolves → pressure drops → water rises → contact breaks → air enters → pressure equalizes.

Solution 1992-3.
Answer:
Dilute hydrochloric acid cannot be concentrated beyond 22.2% (by weight) by distillation. This is due to the fact that this mixture (77.8% water + 22.2% HCl) is an azeotropic mixture, it boils at a constant temperature of \( 110°C \) without any change in concentration.
In simple words: You can't make HCl stronger than 22.2% by boiling because at that concentration, water and HCl evaporate together in the same ratio.

📝 Teacher's Note: Relate this to other azeotropic mixtures like ethanol-water. Explain why this property makes it impossible to get 100% pure HCl by simple distillation.

🎯 Exam Tip: Remember the exact percentage (22.2%) and temperature (110°C) for the azeotropic mixture of HCl-water.

Solution 1994-1.
Answer:
When dilute hydrochloric acid is added to a mixture of iron and sulphur, then hydrogen gas will evolve. While, when dilute hydrochloric acid is added in turn to the compound formed between iron and sulphur the gas formed will be \( H_2S \).
In simple words: A mixture gives hydrogen gas with HCl, but when iron and sulfur are chemically combined (as iron sulfide), they give hydrogen sulfide gas instead.

📝 Teacher's Note: Use this example to illustrate the difference between mixtures and compounds - the chemical properties change completely when elements combine chemically.

🎯 Exam Tip: Write the equations: Fe + 2HCl → FeCl₂ + H₂ (mixture) vs FeS + 2HCl → FeCl₂ + H₂S (compound).

Solution 1995-1.
Answer:
Chlorine gas is evolved when an oxide and concentrated hydrochloric acid are heated.
In simple words: When you heat metal oxides with concentrated HCl, chlorine gas is produced through an oxidation reaction.

📝 Teacher's Note: Specify that this works with oxidizing oxides like MnO₂, PbO₂, etc. Not all oxides will produce this reaction.

🎯 Exam Tip: Give a specific example like MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂ to support your answer.

Solution 1996-1.
Answer:
Lead chloride cannot be prepared by adding dilute HCl to lead sulphate solution because lead sulphate is insoluble salt. Infact it can be prepared by adding dilute HCl to lead nitrate solution since lead nitrate is soluble compound.
In simple words: You can't make lead chloride from lead sulfate because lead sulfate doesn't dissolve in water, but you can make it from lead nitrate which does dissolve.

📝 Teacher's Note: Use solubility rules to explain why sulfates of heavy metals are generally insoluble while nitrates are always soluble.

🎯 Exam Tip: Remember the general rule - all nitrates are soluble, but sulfates of Pb, Ba, Sr are insoluble.

Solution 1997-1.
Answer:
Hydrogen chloride to anhydrous \( FeCl_3 \) by adding hydrochloric acid to \( Fe_2O_3 \)
\( Fe_2O_3 + 6HCl \rightarrow 2FeCl_3 + 3H_2O \)
In simple words: Iron oxide reacts with hydrochloric acid to form iron chloride and water.

📝 Teacher's Note: Point out that this is an acid-base reaction where the oxide acts as a base and HCl as an acid.

🎯 Exam Tip: Balance the equation carefully - you need 6 HCl molecules to neutralize one Fe₂O₃ formula unit.

Solution 1998-1.
Answer:
When silver nitrate solution is added to dilute HCl, white precipitate of AgCl is formed and nitric acid is formed.
\( AgNO_3 + HCl \longrightarrow AgCl(s) + HNO_3 \)
In simple words: This is the standard test for chloride ions - silver nitrate produces a white precipitate of silver chloride.

📝 Teacher's Note: This is one of the most important confirmatory tests in qualitative analysis. The white precipitate is characteristic of chloride ions.

🎯 Exam Tip: Mention that the precipitate is white and insoluble, which confirms the presence of chloride ions.

Solution 2000-1.
Answer:
(a) Sulphuric acid must be added to sodium chloride to prepare hydrogen chloride.
(b) \( NaCl + H_2SO_4 \xrightarrow{<200°C} NaHSO_4 + HCl \)
Sodium chloride + Sulphuric acid(conc.) → Sodium hydrogen sulphate + Hydrogen chloride gas
(c) When hydrogen chloride mixes with ammonia white fumes of ammonium chloride will be observed.
\( HCl(g) + NH_3(g) \rightarrow NH_4Cl(s) \)
In simple words: HCl is made by heating salt with sulfuric acid, and when it meets ammonia gas, it forms white smoke of ammonium chloride.

📝 Teacher's Note: The white smoke test is dramatic and memorable. Explain that this happens because both gases are highly soluble and react instantly in air.

🎯 Exam Tip: Remember that concentrated H₂SO₄ is used and the temperature should be below 200°C to prevent formation of bisulfate.

Solution 2000-2.
Answer:
(a) Fountain experiment demonstrates that hydrogen chloride is very soluble in water.
(b) Tests for hydrogen chloride are-
(i) When silver nitrate solution is added to hydrochloric acid, it gives white precipitate of silver chloride.
(ii) When lead nitrate solution is added to it, it gives white precipitate of lead chloride.
(iii) Concentrated hydrochloric acid when added to oxidizing agents like manganese dioxide, lead dioxide etc., it liberates chlorine gas which is a greenish yellow, pungent smelling gas.
In simple words: The fountain shows HCl dissolves completely in water, and we can test for it using silver or lead salts that form white precipitates, or by making chlorine gas with oxidizing agents.

📝 Teacher's Note: Demonstrate the fountain experiment if possible, as it's one of the most striking ways to show gas solubility. Safety precautions are essential.

🎯 Exam Tip: For identification tests, always mention the color and nature of precipitates formed, and describe the physical properties of gases evolved.

Solution 2000-3:

Answer:
(a) \( CaOCl_2 + 2HCl \rightarrow CaCl_2 + H_2O + Cl_2 \)
(b) \( Pb(NO_3)_2 + 2HCl \rightarrow PbCl_2 \downarrow + 2HNO_3 \)
White
(c) \( MnO_2 + 4HCl \rightarrow MnCl_2(aq) + 2H_2O + Cl_2 \uparrow \)
Manganese dioxide → Manganese chloride
(d) \( PbO_2 + 4HCl \rightarrow PbCl_2(aq) + 2H_2O + Cl_2 \uparrow \)
Lead dioxide → Lead chloride

📝 Teacher's Note: These reactions demonstrate different ways to prepare chlorine gas using concentrated HCl with various oxidizing agents. Show students the color changes and gas evolution to make these reactions memorable.

🎯 Exam Tip: Remember to balance equations carefully and indicate physical states (aq), (s), gas evolution (↑) and precipitate formation (↓) for complete answers.

Solution 2000-4:

Answer: When concentrated HCl is added to lead (IV) oxide and warmed, formation of metal chloride, water and greenish yellow chlorine gas will occur.

📝 Teacher's Note: Emphasize the greenish-yellow color of chlorine gas as a key identifying feature. This reaction is useful for demonstrating oxidation-reduction in action.

🎯 Exam Tip: Always mention the characteristic color of chlorine gas (greenish-yellow) when describing its preparation reactions.

Solution 2001-1:

Answer: Reaction between sodium sulphite and dilute hydrochloric acid is:
\( Na_2SO_3 + 2HCl \rightarrow 2NaCl + H_2O + SO_2 \uparrow \)

📝 Teacher's Note: This reaction produces sulfur dioxide gas which has a characteristic choking smell. Demonstrate in a well-ventilated area and explain the test for SO₂ gas.

🎯 Exam Tip: Don't forget to show gas evolution with the upward arrow (↑) and ensure the equation is properly balanced.

Solution 2001-2:

Answer:
(a) \( NaCl + H_2SO_4 \xrightarrow{<300°C} NaHSO_4 + HCl \)
Sodium chloride + Sulphuric acid (conc.) → Sodium hydrogen sulphate + Hydrogen chloride gas
Sulphuric acid should be concentrated.
(b) When hydrogen chloride mixes with ammonia white fumes of ammonium chloride will be observed.
\( HCl(g) + NH_3(g) \rightarrow NH_4Cl(s) \)

📝 Teacher's Note: The white fumes test is classic for identifying HCl gas. Show students how dense white fumes form immediately when ammonia and HCl gas meet.

🎯 Exam Tip: Specify that concentrated H₂SO₄ is needed and mention the temperature condition (<300°C) for complete marks.

Solution 2001-3:

Answer: Lead dioxide can be used to oxidize hydrogen chloride to chlorine

📝 Teacher's Note: This is a simple oxidation reaction where PbO₂ acts as the oxidizing agent. Connect this to the electron transfer concept in redox reactions.

🎯 Exam Tip: Keep answers concise but complete - mentioning the role of PbO₂ as an oxidizing agent shows deeper understanding.

Solution 2002-1:

Answer: When dilute hydrochloric acid is added to lead nitrate solution, white precipitate of lead chloride forms.
\( Pb(NO_3)_2 + 2HCl \rightarrow PbCl_2 \downarrow + 2HNO_3 \)
White

📝 Teacher's Note: This is an excellent example of a precipitation reaction. The white precipitate of PbCl₂ is characteristic and helps students understand ionic reactions.

🎯 Exam Tip: Always mention the color of the precipitate and use the downward arrow (↓) to indicate precipitation clearly.

Solution 2002-2:

Answer:
(a) All are oxidizing agent which is used to convert HCl into chlorine gas.
(b) \( Pb_3O_4 + 8HCl \rightarrow 3PbCl_2(aq) + 4H_2O + Cl_2 \uparrow \)
Trilead tetroxide (Purple) → Lead chloride

📝 Teacher's Note: Explain how oxidizing agents remove electrons from HCl, causing it to form chlorine gas. The purple color of Pb₃O₄ is distinctive.

🎯 Exam Tip: When listing oxidizing agents, explain their common function rather than just naming them - this shows conceptual understanding.

Solution 2004-1:

Answer:
\( Pb_3O_4 + 8HCl \rightarrow 3PbCl_2(aq) + 4H_2O + Cl_2 \uparrow \)
Trilead tetroxide (Red) → Lead chloride

📝 Teacher's Note: Point out that Pb₃O₄ can appear red or purple depending on particle size and lighting. This reaction is commonly used in laboratory preparation of chlorine.

🎯 Exam Tip: Check the stoichiometry carefully - 8 HCl molecules are needed for every Pb₃O₄ molecule to balance the equation correctly.

Solution 2004-2:

📝 Teacher's Note: This table helps students connect different substances with their characteristic gas products and odors. Practice identifying gases by their smells safely.

🎯 Exam Tip: Learn the characteristic odors of common gases - CO₂ and H₂ are odorless, Cl₂ is pungent, and H₂S smells like rotten eggs.

Solution 2004-3:

Answer: Silver nitrate crystals are dissolved in distilled water to prepare its solution and not in tap water. This is due to the reason that tap water contains chloride ions which gives white precipitate with \( AgNO_3 \) solution.

📝 Teacher's Note: This is an important laboratory practice. Tap water contains various ions that can interfere with chemical tests, especially halide ion tests with silver nitrate.

🎯 Exam Tip: Always mention that distilled water is pure and free from interfering ions when explaining why it's used instead of tap water.

Solution 2005-1:

Answer:
\( CuO + 2HCl \rightarrow CuCl_2 + H_2O \)
\( MnO_2 + 4HCl \rightarrow MnCl_2(aq) + 2H_2O + Cl_2 \uparrow \)
Manganese dioxide → Manganese chloride

📝 Teacher's Note: Show the difference between these two reactions - CuO gives a simple acid-base reaction while MnO₂ acts as an oxidizing agent producing chlorine gas.

🎯 Exam Tip: Notice that only the second reaction produces gas (Cl₂) - this shows MnO₂ is acting as an oxidizing agent, not just a base.

Solution 2005-2:

Answer:
(a) Fountain experiment.
(b) This experiment demonstrates that hydrogen chloride is very soluble in water.
(c) Red

📝 Teacher's Note: The fountain experiment beautifully demonstrates gas solubility. As HCl dissolves rapidly, it creates a vacuum that draws water up in a fountain-like effect.

🎯 Exam Tip: Remember that the fountain turns red due to the indicator showing the acidic nature of the dissolved HCl gas.

Solution 2006-1:

Answer:
(a) \( Zn + 2HCl \rightarrow ZnCl_2 + H_2 \)
(b) When silver nitrate solution is added to hydrochloric acid, it gives white precipitate of silver chloride

📝 Teacher's Note: These are classic reactions - the first shows metal-acid reaction producing hydrogen gas, the second is the standard test for chloride ions.

🎯 Exam Tip: For part (b), always mention the white color of AgCl precipitate as it's a key identifying feature in halide ion tests.

Solution 2007-1:

Answer:
(a) Hydrogen chloride is more dense.
(b) The gas is collected by upward displacement of water.

📝 Teacher's Note: HCl is denser than air, so it can be collected by upward displacement. However, since it's very soluble in water, this method is actually problematic in practice.

🎯 Exam Tip: Be careful - while HCl is denser than air, its high solubility in water makes upward displacement of water unsuitable for collection.

Solution 2008-1:

Answer:
(a) Sulphuric acid is although weaker than hydrochloric acid hydrochloric acid is more volatile than sulphuric acid this property makes it useful in the preparation of hydrochloric acid.
(b) It is heavier than air so it is collected by downward delivery.
(c) It is soluble in water so it is not collected over water.

📝 Teacher's Note: Emphasize that volatility, not acid strength, determines usefulness in acid preparation. HCl's high volatility allows it to be driven off as gas.

🎯 Exam Tip: Don't confuse acid strength with volatility - H₂SO₄ is stronger but less volatile than HCl, making HCl easier to prepare as a gas.

Solution 2009-1:

Answer:
(a) \( Pb_3O_4 + 8HCl \rightarrow 3PbCl_2(aq) + 4H_2O + Cl_2 \uparrow \)
Trilead tetroxide (Red) → Lead chloride
(b) \( Mg + 2HCl \rightarrow MgCl_2 + H_2 \)

📝 Teacher's Note: Compare these two reactions - the first is redox producing chlorine gas, the second is simple metal-acid reaction producing hydrogen gas.

🎯 Exam Tip: Notice the different products - oxidizing agents with HCl give Cl₂ gas, while reactive metals with HCl give H₂ gas.

Solution 2009-2:

Answer: Hydrochloric acid is not prepared directly dissolving hydrochloride gas in water because it may lead to back suction. When the delivery tube carrying HCl gas is directly immersed in water, the reverse rise of water in delivery tube is known as back suction. To prepare hydrochloric acid from hydrogen gas, it is essential to avoid back suction and it is done by using 'inverted funnel arrangement'.

📝 Teacher's Note: Back suction is a common laboratory hazard. The inverted funnel keeps the gas inlet just above water surface, preventing water from being sucked back into the apparatus.

🎯 Exam Tip: Always explain both the problem (back suction) and the solution (inverted funnel) when discussing HCl gas absorption in water.