Selina Concise Solutions for ICSE Class 10 Chemistry Chapter 8 Study of Compounds Hydrogen Chloride

Exercise

Question 1. Draw a labelled diagram for the laboratory preparation of hydrogen chloride gas. Give the balanced equation for the reaction.
Answer: Labelled Diagram for laboratory preparation of Hydrogen chloride is shown in the text with the apparatus setup including sodium chloride and concentrated sulphuric acid in the generating flask, with concentrated sulphuric acid as drying agent, and collection setup.
Balanced equation: \( \text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{<200°C} \text{NaHSO}_4 + \text{HCl} \uparrow \)
In simple words: When we heat common salt with concentrated sulphuric acid, it produces hydrogen chloride gas that we can collect in a bottle.

📝 Teacher's Note: Show students the actual apparatus setup and emphasize safety with concentrated acid handling. Point out why we use an inverted funnel and empty flask to prevent back-suction.

🎯 Exam Tip: Always draw the complete setup with proper labels and write the balanced equation with temperature condition for full marks.

Question 2. Name the drying agents:
(a) used in drying hydrogen chloride gas.
(b) phosphorus pentoxide and calcium oxide are good drying agent but they cannot be used to dry hydrogen chloride gas. Why?
Answer: (a) Hydrogen chloride is dried by passing through concentrated sulphuric acid.
(b) Phosphorous pentoxide and CaO cannot be used to dry HCl because they react with HCl.
\( 2\text{P}_2\text{O}_5 + 3\text{HCl} \rightarrow \text{POCl}_3 + 3\text{HPO}_3 \)
\( \text{CaO} + 2\text{HCl} \rightarrow \text{CaCl}_2 + \text{H}_2\text{O} \)
In simple words: We use concentrated sulphuric acid to dry HCl gas because it absorbs water without reacting with the gas, while other drying agents would react and spoil our gas.

📝 Teacher's Note: Demonstrate to students that a good drying agent must not react with the substance being dried. Use this as an example to teach selectivity in chemistry.

🎯 Exam Tip: For any drying agent question, always mention that it should not react with the gas being dried and write the reaction equations to show why unsuitable agents fail.

Question 3. Explain why:
(a) Anhydrous HCl is poor conductor while aqueous HCl is excellent conductor.
(b) When the stopper of a bottle full of hydrogen chloride gas is opened there are fumes in the air.
(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.
(d) thick white fumes are formed when a glass rod dipped in NH₂OH is brought near the mouth of a bottle full of HCl gas.
(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.
(f) hydrogen chloride gas is not collected over water.
Answer: (a) Anhydrous HCl is poor conductor due to the absence of ions in it whereas aqueous HCl is excellent conductor since it contains ions.
(b) When the stopper is opened HCl gas comes in contact with water vapors of air and gives white fumes due to the formation of hydrochloric acid.
(c) A solution of HCl in water gives hydronium ions and conducts electricity, but HCl is also soluble in dry toluene, but in that case it neither (i) turns blue litmus red (ii) nor does conducts electricity. This indicates the absence of H⁺ ions in toluene showing thereby that hydrogen chloride is a covalent compound.
(d) When ammonium hydroxide is brought near the mouth of HCl, dense white fumes are formed due to the formation of ammonium chloride.
\( \text{HCl} + \text{NH}_4\text{OH} \rightarrow \text{NH}_4\text{Cl} + \text{H}_2\text{O} \)
(e) Dry hydrogen chloride is not acidic whereas moist Hydrogen chloride is acidic. In presence of a drop of water HCl gas dissolves in water and forms hydrochloric acid which turns blue litmus paper red.
(f) Hydrogen chloride is not collected over water as it is highly soluble in water.
In simple words: HCl gas needs water to become acidic and conduct electricity - without water it behaves like a simple molecule, but with water it breaks into ions and becomes acidic.

📝 Teacher's Note: Use the analogy of sugar dissolving in water vs oil to explain how HCl behaves differently in water vs toluene. Emphasize that water is crucial for ionization.

🎯 Exam Tip: Always mention "ionization" and "formation of ions" when explaining conductivity differences. For collection methods, state the solubility reason clearly.

Question 4. Write the main difference in hydrogen chloride gas and hydrochloric acid.
Answer: Difference between Hydrogen chloride gas and Hydrochloric acid is:

In simple words: HCl gas is just a gas that doesn't show acidic properties, but when dissolved in water it becomes hydrochloric acid which is strongly acidic.

📝 Teacher's Note: Use physical samples if available to show students that the same chemical compound can behave very differently in different states or solvents.

🎯 Exam Tip: Present differences in a clear table format and always mention the role of water in making HCl acidic.

Question 5. The given set up in the figure is for the preparation of an acid.
(a) Name the acid prepared by this method.
(b) name the reactants used.
(c) why empty flask is used
(d) what is the drying agent used? Why is this drying agent chosen?
(e) what is the role of inverted funnel in the arrangement
Answer: Hydrochloric acid is prepared by this method.
(a) The reactants are sodium chloride and Sulphuric acid.
(b) The empty flask acts as Anti-Suction device. In case the back suction occurs the water will collect in it and will not reach the generating flask.
(c) The drying agent is Concentrated Sulphuric acid. Sulphuric acid is chosen as drying agent because it does not react with HCl.
(d) The Inverted funnel:
Prevents or minimizes back suction of water.
Provides a large surface area for absorption of HCl gas.
In simple words: This setup safely makes hydrochloric acid by mixing salt with acid, and has safety features to prevent water from going back into the reaction flask.

📝 Teacher's Note: Demonstrate the fountain experiment separately to show HCl's high solubility. Explain each safety feature's importance in preventing accidents.

🎯 Exam Tip: Identify each part of the apparatus and explain its specific function - this shows complete understanding of the experimental setup.

Question 6. Write an equation for the reactions of aqueous hydrochloric acid on:
(a) silver nitrate solution (b) magnesium foil
(c) caustic soda solution (d) zinc carbonate
(e) lead nitrate solution (f) copper oxide
Answer: Equations are:
(a) \( \text{AgNO}_3 + \text{HCl} \rightarrow \text{AgCl} + \text{HNO}_3 \)
(b) \( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \)
(c) \( \text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O} \)
(d) \( \text{ZnCO}_3 + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2\text{O} + \text{CO}_2 \)
(e) \( \text{Pb(NO}_3\text{)}_2 + 2\text{HCl} \rightarrow \text{PbCl}_2 + 2\text{HNO}_3 \)
(f) \( \text{CuO} + 2\text{HCl} \rightarrow \text{CuCl}_2 + \text{H}_2\text{O} \)
In simple words: Hydrochloric acid reacts with different substances to form chloride salts plus other products like water, hydrogen gas, or carbon dioxide.

📝 Teacher's Note: Group these reactions by type - precipitation, acid-base, metal-acid, and carbonate-acid reactions. This helps students recognize patterns.

🎯 Exam Tip: Balance all equations carefully and remember that carbonates always produce CO₂, metals produce H₂, and bases produce water.

Question 7.
(a) Name an element which reacts with hydrogen to form a compound which is strongly acidic in water.
(b) Explain why dilute hydrochloric acid cannot be concentrated by boiling beyond 22.2%.
Answer: (a) Chlorine. The compound formed which is strongly acidic in water, is HCl.
\( \text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl} \)
(b) A dilute aqueous solution of hydrochloric acid gets gradually concentrated on distillation, till the concentration of the acid reaches 22.2% HCl by weight which boils at 110°C. When this concentration is reached, no further increase in concentration of the acid becomes possible by boiling. This is because vapours evolved before 110°C are vapours of water but at temperature above 110°C vapours consist mostly of molecules of HCl.
In simple words: Chlorine gas combines with hydrogen to make HCl. When you try to concentrate HCl acid by boiling, it reaches a maximum of 22.2% because after that point, HCl itself starts evaporating instead of just water.

📝 Teacher's Note: Explain azeotropic mixtures using this example - when two liquids have the same boiling point, they can't be separated by simple distillation.

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

Question 8. Hydrochloric acid contains (i) hydrogen (ii) chlorine. Prove it. Write equations for the reactions.
Answer: We can prove that hydrochloric acid contains both hydrogen and chlorine by the following experiment. Take a voltameter used for electrolysis of water, fitted with platinum cathode and graphite anode. Into the voltameter pour 4 molar HCl and pass direct current. It is seen that a colourless gas is evolved at cathode and a greenish gas is evolved at anode. When a burning splinter is brought near a colourless gas, it bursts into flame thereby proving that it is hydrogen gas. When moist starch iodide paper is held in the greenish yellow gas, it turns blue black, thereby proving that the gas is chlorine.
\( 2\text{HCl} \rightarrow \text{H}_2 + \text{Cl}_2 \)
This experiment proves that hydrochloric acid contains both hydrogen and chlorine.
In simple words: When we pass electricity through HCl solution, it breaks apart into hydrogen gas (which burns) and chlorine gas (which turns starch-iodide paper blue-black).

📝 Teacher's Note: Perform this electrolysis demonstration carefully with proper ventilation due to chlorine gas production. Emphasize the importance of chemical tests for gas identification.

🎯 Exam Tip: Describe both the physical observations (colorless gas at cathode, greenish gas at anode) and the chemical tests used to identify each gas.

Question 9. Name:
(a) black metallic oxide which reacts with hydrochloric acid to give a coloured solution.
(b) two colourless gases, which when mixed produce a white solid.
(c) two gases which chemically combine to form a liquid
(d) a chloride which is soluble in excess of ammonium hydroxide
(e) the chemical in which gold can be dissolved
(f) the experiment which demonstrates that hydrogen chloride is soluble in water.
(g) the gas produced when chlorine water is exposed to sunlight
Answer: (a) Manganese dioxide
(b) Hydrogen chloride and ammonia
(c) Hydrogen and oxygen
(d) AgCl (Silver chloride)
(e) Aqua regia
(f) Fountain experiment
(g) Hydrogen chloride gas
In simple words: These are specific examples of chemical reactions and phenomena involving HCl and related compounds, each showing a unique property or behavior.

📝 Teacher's Note: Connect each answer to the underlying chemical principle - solubility, acid-base reactions, complex formation, etc. This builds conceptual understanding.

🎯 Exam Tip: Learn these specific examples as they frequently appear in name-based questions. Each represents an important chemical concept.

Question 10. Give reasons for the following:
(a) An aqueous solution of chlorine in acidic in nature
Hint: \( \text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{HClO} \)
(b) silver nitrate solution can be used to distinguish HCl from HNO₃
Answer: (a) An aqueous solution of chlorine is acidic as it dissolves in water to form hydrochloric and hypochlorous acids.
\( \text{Cl}_2 + \text{H}_2\text{O} \rightarrow \text{HCl} + \text{HClO} \)
(b) Silver nitrate solution gives a white precipitate of AgCl with HCl but no precipitate with HNO₃, thus it can distinguish between them.
In simple words: Chlorine water becomes acidic because it forms acids when mixed with water. Silver nitrate helps tell HCl from nitric acid because only HCl forms a white precipitate with it.

📝 Teacher's Note: Demonstrate the AgNO₃ test practically to show the immediate white precipitate formation. This visual test is very important for identification.

🎯 Exam Tip: For distinguishing tests, always mention the specific observation (white precipitate) and write the chemical equation for the positive test.

Question. Solution A reacts with an acid B (which gives greenish yellow gas on reacting with oxidizing agents like \( Pb_3O_4 \)) to give white precipitate C insoluble in nitric acid but soluble in ammonium hydroxide. Name A, B and C.
Answer: A is Silver nitrate, B is Hydrochloric acid, C is Silver chloride
In simple words: When silver nitrate meets hydrochloric acid, they form a white solid called silver chloride that doesn't dissolve in nitric acid but dissolves in ammonium hydroxide.

📝 Teacher's Note: Use actual silver nitrate and HCl solutions to demonstrate this classic precipitation reaction in class. Students can observe the immediate white precipitate formation and test its solubility properties.

🎯 Exam Tip: Remember the key clue "white precipitate insoluble in nitric acid but soluble in ammonium hydroxide" - this specifically identifies silver chloride among all chlorides.

Question. Complete the following reactions and balance them.
(a) \( NH_4OH + HCl → \)
(b) \( NaHSO_3 + HCl → \)
(c) \( Pb(NO_3)_2 + HCl → \)
(d) \( Pb_3O_4 + HCl → \)
(e) \( Zn + 2HCl → \)
(f) \( Ca(HCO_3)_2 + 2HCl → \)
Answer:
(a) \( NH_4OH + HCl → NH_4Cl + H_2O \)
(b) \( NaHSO_3 + HCl → NaCl + H_2O + SO_2 \)
(c) \( Pb(NO_3)_2 + 2HCl → PbCl_2 + 2HNO_3 \)
(d) \( Pb_3O_4 + 8HCl → 3PbCl_2 + 4H_2O + Cl_2 \)
(e) \( Zn + 2HCl → ZnCl_2 + H_2 \)
(f) \( Ca(HCO_3)_2 + 2HCl → CaCl_2 + 2H_2O + 2CO_2 \)
In simple words: These reactions show how hydrochloric acid reacts with different substances to form salts, water, and sometimes gases like hydrogen, sulfur dioxide, or carbon dioxide.

📝 Teacher's Note: Practice balancing these equations step by step, emphasizing conservation of atoms. Use color coding for different elements to help students track atom balance.

🎯 Exam Tip: Always check that the same number of atoms of each element appears on both sides of the equation. Start with the most complex molecule first when balancing.

Question. How will the action of dilute hydrochloric acid enable you to distinguish between the following:
(a) Sodium carbonate and sodium sulphite
(b) sodium thiosulphate and sodium sulphite
Answer:
(a) Sodium carbonate on treating with dil. HCl results in the formation of sodium chloride with the liberation of carbon dioxide gas.
\( Na_2CO_3 + 2HCl → 2NaCl + H_2O + CO_2 ↑ \)
Sodium sulphite on treating with dil. HCl results in the formation of sodium chloride with the liberation of sulphur dioxide gas.
\( Na_2SO_3 + 2HCl → 2NaCl + H_2O + SO_2 ↑ \)
(b) Sodium thiosulphate reacts with dil. HCl to produce sulphur dioxide gas and precipitates yellow sulphur.
\( Na_2S_2O_3 + 2HCl → 2NaCl + H_2O + SO_2 + S ↓ \)
Sulphur is not precipitated when sulphites are treated with dil. HCl.
In simple words: Different compounds produce different gases when they react with dilute HCl - carbonates give CO₂, sulphites give SO₂, and thiosulphates give SO₂ plus yellow sulfur powder.

📝 Teacher's Note: Demonstrate these reactions with actual samples. The yellow sulfur precipitate from thiosulfate is visually striking and helps students remember the distinction.

🎯 Exam Tip: Focus on the unique products: CO₂ from carbonates, SO₂ from sulphites, and SO₂ plus yellow sulfur from thiosulfates.

Question. Give three distinct test [apart from using an indicator] you would carry out with solution of HCl to illustrate the typical properties of an acid.
Answer: Three tests are:
1. HCl gas gives thick white fumes of ammonium chloride when glass rod dipped in ammonia solution is held near the vapours of the acid.
\( NH_3 + HCl → NH_4Cl \)
2. With silver nitrate HCl gives white precipitate of silver chloride. The precipitate is insoluble in nitric acid but soluble in ammonium hydroxide.
\( AgNO_3 + HCl → AgCl + HNO_3 \)
3. A greenish yellow gas is liberated when concentrated hydrochloric acid is heated with oxidizing agent like manganese dioxide.
\( MnO_2 + 4HCl → MnCl_2 + 2H_2O + Cl_2 \)
In simple words: These tests show HCl's unique behaviors - forming white smoke with ammonia, creating white precipitate with silver nitrate, and releasing greenish gas with oxidizing agents.

📝 Teacher's Note: The ammonia test with white fumes is spectacular and safe for classroom demonstration. Always use proper ventilation when showing the chlorine gas evolution.

🎯 Exam Tip: Remember these specific visual cues - white fumes, white precipitate, and greenish-yellow gas. These are classic identification tests for HCl.

Question. \( MnO_2 \), \( PbO_2 \) and red lead react with conc. HCl acid liberates \( Cl_2 \). What is the common property being shown by these metal oxides?
Answer: \( MnO_2 \), \( PbO_2 \) and red lead react with conc. HCl acid to liberate \( Cl_2 \). This shows that hydrochloric acid is oxidized to chlorine by oxidizing agents. These metal oxides are acting as oxidizing agents.
In simple words: All these compounds have the ability to steal electrons from HCl, converting it into chlorine gas - this special ability is called being an oxidizing agent.

📝 Teacher's Note: Explain oxidation as "loss of electrons" and relate it to the conversion of Cl⁻ ions in HCl to Cl₂ gas. Use electron transfer diagrams to clarify the concept.

🎯 Exam Tip: When you see chlorine gas being evolved from HCl, immediately think "oxidation" and identify the other substance as an oxidizing agent.

Question. Giving reasons state which of the two- a solution of HCl in water or in toluene is an electrolyte.
Answer: HCl dissolves both in water and toluene, when HCl dissolves in water it ionizes and forms hydronium and chloride ions. Whereas this ionization is not observed in toluene hence a solution of HCl in water can be used as an electrolyte.
In simple words: Only the water solution conducts electricity because HCl breaks into charged particles (ions) in water, but stays as whole molecules in toluene.

📝 Teacher's Note: Demonstrate conductivity using a simple circuit with LED bulb. Show how HCl in water lights the bulb but HCl in toluene doesn't.

🎯 Exam Tip: Remember: electrolytes must have ions, and HCl only forms ions in polar solvents like water, not in non-polar solvents like toluene.

Question. Convert two soluble metallic nitrates to insoluble metallic chlorides using dil. HCl.
Answer: Conversion of metallic nitrates to insoluble metallic chlorides using dil. HCl:
(i) \( Pb(NO_3)_2 + 2HCl → PbCl_2 + 2HNO_3 \)
(ii) \( Hg_2(NO_3)_2 + 2HCl → Hg_2Cl_2 + 2HNO_3 \)
In simple words: Lead and mercury nitrates are soluble in water, but when they react with HCl, they form chlorides that don't dissolve and appear as solid precipitates.

📝 Teacher's Note: Emphasize that most chlorides are soluble except for lead chloride, silver chloride, and mercury(I) chloride. This is a key solubility rule.

🎯 Exam Tip: Learn the exceptions to chloride solubility: AgCl, PbCl₂, and Hg₂Cl₂ are the main insoluble chlorides to remember.

Question. State the composition of aqua regia. State which component is the oxidizing agent in aqua regia.
Answer: A mixture having three parts of conc. Hydrochloric acid and one part of conc. Nitric acid is called aqua-regia. Nitric acid acts as oxidizing agent.
In simple words: Aqua regia is a powerful acid mixture made by combining 3 parts concentrated HCl with 1 part concentrated HNO₃, where nitric acid provides the oxidizing power.

📝 Teacher's Note: Explain that aqua regia can dissolve gold and platinum, which individual acids cannot. The name means "royal water" because it dissolves the "royal metals."

🎯 Exam Tip: Remember the 3:1 ratio of HCl:HNO₃ and that HNO₃ is always the oxidizing component in acid mixtures.

Question. Convert hydrochloric acid to nascent chlorine.
Answer: Hydrochloric acid can be converted to nascent chlorine by:
\( 3HCl + HNO_3 → NOCl + 2H_2O + 2[Cl] \)
Conc. HCl + Conc. HNO₃ → Nascent chlorine
In simple words: When concentrated HCl reacts with concentrated nitric acid, it produces highly reactive single chlorine atoms called nascent chlorine.

📝 Teacher's Note: Explain that nascent chlorine is much more reactive than ordinary Cl₂ gas because the chlorine atoms are just formed and haven't paired up yet.

🎯 Exam Tip: Nascent chlorine is represented as [Cl] and is formed when HCl is oxidized by strong oxidizing agents like HNO₃.

Question. Study the flow chart and give balanced equations with conditions for the conversions A, B, C and D.
Answer:
A: \( NaCl + H_2SO_4 \xrightarrow{<200°C} NaHSO_4 + HCl \)
B: \( Fe + 2HCl → FeCl_2 + H_2 \)
C: \( HCl + NH_3 → NH_4Cl \)
D: \( PbO_2 + 4HCl → PbCl_2 + 2H_2O + Cl_2 \)
In simple words: This shows how sodium chloride can be converted to HCl, which then reacts with different substances to form various products including salts and gases.

📝 Teacher's Note: Use this flowchart to show students how one compound can be the starting point for many different reactions. Emphasize the importance of reaction conditions.

🎯 Exam Tip: Pay attention to temperature conditions and states of reactants (concentrated vs dilute) as these affect the products formed.

Question. 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:

In simple words: Different substances produce different gases with HCl - carbonates give odorless CO₂, metals give odorless H₂, oxidizing agents give pungent Cl₂, and sulfides give rotten-egg smelling H₂S.

📝 Teacher's Note: Use actual samples to demonstrate these reactions safely. The rotten egg smell of H₂S is memorable but use minimal amounts due to toxicity.

🎯 Exam Tip: Remember the characteristic odours: chlorine (pungent), hydrogen sulfide (rotten eggs), while CO₂ and H₂ are odourless.

Question. Write balanced equations.
(i) copper oxide and dilute hydrochloric acid.
(ii) Manganese (iv) oxide and concentrated hydrochloric acid.
(b) (i) Name the experiment illustrated below.
(ii) Which property of hydrogen chloride is demonstrated by this experiment?
(iii) State the colour of the water that has entered the round bottomed flask.
Answer:
(a)
(i) \( CuO + 2HCl → CuCl_2 + H_2O \)
(ii) \( MnO_2 + 4HCl → MnCl_2 + 2H_2O + Cl_2 \)
(b)
(i) The experiment is called Fountain Experiment.
(ii) This experiment shows that hydrogen chloride is highly soluble in water.
(iii) Red
In simple words: The fountain experiment shows how quickly HCl gas dissolves in water, creating a vacuum that sucks water up like a fountain, and the red color comes from the indicator in the water.

📝 Teacher's Note: The fountain experiment is visually spectacular. Explain how the rapid dissolution of HCl creates a pressure difference that causes the water fountain effect.

🎯 Exam Tip: The fountain experiment specifically demonstrates high solubility, and the red color indicates the acidic nature of the dissolved HCl.

Question. Write balanced equations for the reaction of dilute hydrochloric acid with each of the following:
(a) iron
(b) sodium hydrogencarbonate,
(c) iron (II) sulphide
(d) sodium sulphite
(e) sodium thiosulphate solution
Answer:
(i) \( Fe + 2HCl → FeCl_2 + H_2 \)
(ii) \( NaHCO_3 + HCl → NaCl + H_2O + CO_2 \)
(iii) \( FeS + 2HCl → FeCl_2 + H_2S \)
(iv) \( Na_2SO_3 + 2HCl → 2NaCl + H_2O + SO_2 \)
(v) \( Na_2S_2O_3 + 2HCl → 2NaCl + H_2O + SO_2 + S \)
In simple words: Dilute HCl reacts with metals to give hydrogen gas, with carbonates to give CO₂, with sulfides to give H₂S, with sulphites to give SO₂, and with thiosulfates to give SO₂ plus sulfur.

📝 Teacher's Note: Group these reactions by the type of gas evolved - this helps students predict products. Emphasize the pattern of acid + metal/salt reactions.

🎯 Exam Tip: Learn the gas patterns: metals → H₂, carbonates → CO₂, sulfides → H₂S, sulphites → SO₂, thiosulfates → SO₂ + S.

Question. What property of hydrogen chloride is demonstrated when it is collected by downward delivery (upward displacement)?
Answer: When hydrogen chloride is collected by downward delivery or upward displacement, it shows that it is heavier than air.
In simple words: Hydrogen chloride gas sinks down because it weighs more than the air around it, just like a heavy ball falls down when dropped.

📝 Teacher's Note: Demonstrate this with a visual aid showing gas collection methods. Students often confuse "downward delivery" with the gas moving down - clarify that it's the collection method, not gas movement direction.

🎯 Exam Tip: Always mention "heavier than air" as the key property. This is the specific answer examiners look for in gas collection questions.

Question. Why is hydrogen chloride not collected over water?
Answer: Hydrogen chloride is not collected over water as it is soluble in water.
In simple words: Hydrogen chloride dissolves completely in water, so if we try to collect it over water, it will just mix with the water and disappear into the solution.

📝 Teacher's Note: Relate this to everyday examples like sugar dissolving in tea. Students should understand that soluble gases cannot be collected over the solvent they dissolve in.

🎯 Exam Tip: Write "highly soluble in water" for full marks. The word "highly" shows complete understanding of the concept.

Question. Write the equations for the following reactions:
(a) Dilute hydrochloric acid and sodium thiosulphate,
(b) Dilute hydrochloric acid and lead nitrate solution.
Answer:
(i) \( \text{Na}_2\text{S}_2\text{O}_3 + 2\text{HCl} \rightarrow 2\text{NaCl} + \text{H}_2\text{O} + \text{SO}_2 + \text{S} \)
(ii) \( \text{Pb(NO}_3\text{)}_2 + 2\text{HCl} \rightarrow \text{PbCl}_2 + 2\text{HNO}_3 \)
In simple words: These are chemical reactions where hydrochloric acid reacts with different compounds to form new products. The first reaction produces sulfur gas and solid sulfur, while the second forms a white precipitate of lead chloride.

📝 Teacher's Note: Emphasize balancing equations step by step. Point out that reaction (i) produces both a gas and a precipitate, making it visually interesting for demonstrations.

🎯 Exam Tip: Always balance the equations and include physical states if asked. Double-check that the number of atoms of each element is equal on both sides.