Frank Brothers Solutions for ICSE Class 10 Chemistry Chapter 9a Ammonia

Chapter 9A. Ammonia

Solution 1:
Answer: Ammonia is found both in free state and in combined state. In free state, it is formed in traces amount by decaying urine and other organic matter. In combined state, ammonia is found as ammonium salts mainly as ammonium chloride and ammonium sulphate.
In simple words: Ammonia exists naturally in two ways - sometimes it's just by itself (free state) when things like urine break down, and sometimes it's joined with other chemicals to form salts like ammonium chloride.

📝 Teacher's Note: Use examples of decomposing organic matter that students can relate to, like compost or old leaves. This helps them understand the natural occurrence of ammonia.

🎯 Exam Tip: Always mention both states clearly and give specific examples like ammonium chloride and ammonium sulphate for full marks.

Solution 2:
Answer: (i) Liquid ammonia - Compressed ammonia gas at 6 atmospheric pressure. Chemical formula - \( NH_3 \)
(ii) Liquor ammonia - It is saturated solution of ammonia in water. It is very dilute solution of ammonium hydroxide (\( NH_4OH \)).
A saturated solution of ammonia in water is called liquor ammonia Fortis.
In simple words: Liquid ammonia is just ammonia gas squeezed under high pressure, while liquor ammonia is ammonia dissolved in water like sugar in tea.

📝 Teacher's Note: Emphasize the difference between physical compression (liquid ammonia) and chemical dissolution (liquor ammonia). Use the analogy of compressed air vs. sugar water.

🎯 Exam Tip: Remember the pressure (6 atmospheric) for liquid ammonia and mention NH₄OH formation for liquor ammonia to get full marks.

Solution 3:
Answer: (i) Ammonium chloride is heated with calcium hydroxide (an alkali), in ratio of 2:3 by weight, to produce ammonia.

Reactants should be dry and in grounded state. The reactants are heated to get the products.

Balanced reaction:
\[ 2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3 \]

Key: Laboratory Preparation of Ammonia

(ii) Ammonia gas is collected in an inverted dry gas jar by the downward displacement of air.

(iii) The substance used for drying ammonia gas is quicklime (calcium oxide). Concentrated sulphuric acid, anhydrous calcium chloride and phosphorus pentoxide cannot be used for drying ammonia gas because ammonia being basic reacts with them, and the following reaction takes place

Conc. \( H_2SO_4 \) reacts with ammonia to form ammonium sulphate.
\[ 2NH_3 + H_2SO_4(conc.) \longrightarrow (NH_4)_2SO_4 \]

Phosphorus pentoxide reacts with ammonia to form ammonium phosphate.
\[ 6NH_3 + P_2O_5 + 3H_2O \longrightarrow 2(NH_4)_3PO_4 \]

Anhydrous calcium chloride combines with ammonia to form addition compound.
\[ 4NH_3 + CaCl_2 \longrightarrow CaCl_2.4NH_3 \]
In simple words: To make ammonia in the lab, we heat ammonium chloride with calcium hydroxide. We collect it upside down because ammonia is lighter than air, and use quicklime to dry it because other drying agents react with ammonia.

📝 Teacher's Note: Demonstrate the 2:3 ratio calculation and explain why downward displacement works (ammonia's low density). Show why common drying agents fail with basic gases.

🎯 Exam Tip: Write the balanced equation clearly, mention the 2:3 ratio, and explain why CaO is used instead of common drying agents for full marks.

Solution 4:
Answer: (i) Nitrogen gas
(ii) Copper
(iii) Nitrogen and water vapour
(iv) Quick lime (calcium oxide)
(v) Warm water
(vi) Phenolphthalein
(vii) Nitrogen trichloride and hydrogen trichloride
(viii) Liquid ammonia
(ix) Ammonia solution
(x) Ammonium chloride
(xi) Ammonium chloride
(xii) Ammonium nitrate
(xiii) \( HCl + NH_3 \longrightarrow NH_4Cl \)
(xiv) \( Mg_3N_2 + 6H_2O \longrightarrow 3Mg(OH)_2 + 2NH_3 \)
In simple words: This is a list of various chemicals and compounds related to ammonia preparation and reactions.

📝 Teacher's Note: Use this as a comprehensive revision list. Group related items together (preparation materials, products, reactions) to help students organize their understanding.

🎯 Exam Tip: These are likely fill-in-the-blank answers. Learn the chemical formulas and balanced equations for maximum scoring.

Solution 5:
Answer: (a) Haber's Process
(b) The reactants nitrogen and hydrogen combine at low temperature and high pressure in presence of catalyst to form maximum yield of ammonia. The balanced equation for the reaction is:
\[ N_2 + 3H_2 \rightleftharpoons 2NH_3 + Heat \]

(c) The mixture of dry nitrogen and dry hydrogen is mixed in the ratio of 1:3 by volume to produce ammonia.
(d) Source of hydrogen: Hydrogen is generally obtained from water gas by Bosch process.

Source of nitrogen: Nitrogen is obtained from air by its liquefaction and fractional distillation.

(e) (i) Finely divided iron acts as catalyst which alters the rate of reaction, but itself does not undergo any chemical change.

(ii) Molybdenum acts as promoter which increase the efficiency of the catalyst.

(f) The gases leaving the catalyst chamber are cooled by passing through condensing pipes where ammonia gets liquefied and is collected in receiver.

Key: Haber's Process
In simple words: Haber's Process is the industrial method to make ammonia by combining nitrogen and hydrogen gases under high pressure and temperature with iron as a helper (catalyst).

📝 Teacher's Note: Emphasize the importance of Le Chatelier's principle in explaining the high pressure/low temperature conditions. Draw a simple flow diagram of the process.

🎯 Exam Tip: Remember the 1:3 ratio, mention iron as catalyst and molybdenum as promoter, and write the reversible equation with heat shown.

Solution 6:
Answer: Ammonia is highly soluble in water. This can be demonstrated by Fountain experiment.

• We take a round bottom flask filled with ammonia gas.
• The flask is closed with a two holed rubber stopper, one for the jet tube and the other for a dropper filled with water.
• The flask is fixed to the stand in an inverted position.
• The free end of the jet tube is dipped into a beaker containing red litmus solution.
• When the bulb of dropper is pressed, it is observed that red litmus solution rises up and strikes the wall of the flask and spreads in form of fountain, which is blue in colour.
In simple words: The fountain experiment shows that ammonia dissolves very easily in water - so easily that it creates a fountain effect when water rushes up to dissolve the ammonia gas.

📝 Teacher's Note: This is a classic demonstration. Emphasize the partial vacuum created by ammonia dissolving rapidly, causing the fountain effect. The color change confirms ammonia's basic nature.

🎯 Exam Tip: Describe the apparatus setup clearly, mention the inverted position, and explain both the physical (fountain) and chemical (blue litmus) observations.

Solution 7:
Answer: (i) Action of heat on ammonium chloride is reversible. When heated, it decomposes to form ammonia and hydrogen chloride. When the products are cooled, they recombine to form ammonium chloride.
\[ NH_4Cl \xrightleftharpoons{\Delta} NH_3 + HCl \]
This is thermal dissociation reaction.

(ii) Ammonium nitrate on heating completely disappears, escaping in the form of water vapour and nitrous oxide.
\[ NH_4NO_3 \xrightarrow{\Delta} N_2O + 2H_2O \]
This is thermal dissociation reaction.
In simple words: When you heat ammonium chloride, it breaks apart but can come back together when cooled. Ammonium nitrate completely disappears when heated, forming different gases.

📝 Teacher's Note: Demonstrate the reversible nature of NH₄Cl decomposition by showing the white smoke that reforms when the gases cool. Contrast this with the irreversible decomposition of NH₄NO₃.

🎯 Exam Tip: Use the double arrow for NH₄Cl (reversible) and single arrow for NH₄NO₃ (irreversible). Mention "thermal dissociation" for both.

Solution 8:
Answer: 1. Liquid ammonia is used as refrigerant as:
• It is highly volatile
• It has high specific latent heat of vaporization. 1 mole (17g) of liquid ammonia vaporises by absorbing 5.7 kcals of heat from the surroundings, which is there by cooled.
• It easily liquefies under pressure at room temperature.

2. Ammonia emulsifies fats and grease. Thus it is used to clean oils, fats and body grease etc. from clothes.

3. Liquid hydrogen is dangerous to transport as it is highly combustible. Thus, hydrogen is converted to liquid ammonia and transported in cylinders. Later it is catalytically converted to hydrogen.

Key: Uses of ammonia
In simple words: Ammonia is used in refrigerators because it absorbs lots of heat when it turns from liquid to gas, cooling things down. It's also good for cleaning greasy clothes and is a safer way to transport hydrogen.

📝 Teacher's Note: Connect the refrigeration principle to everyday experience with ACs and refrigerators. Explain how the phase change absorbs energy, creating the cooling effect.

🎯 Exam Tip: Mention the specific values (17g, 5.7 kcals) for the refrigeration use, and emphasize safety aspects for hydrogen transport.

Solution 9:
Answer: (a) Three uses of ammonium chloride:
1. For cleaning metal surfaces before soldering, tinning, etc.
2. In Leclanche cell and in dry cell.
3. In medicine and dyeing.

(b) Ammonium hydroxide precipitates hydroxides of metals when it is added to aqueous solution of their salts. The precipitate of metal hydroxides differ in colour and solubility and hence ammonium hydroxide is used in qualitative analysis.

(i) \( FeSO_4 + 2NH_4OH \rightarrow (NH_4)_2SO_4 + Fe(OH)_2 \)

The colour of precipitate is dirty green, which is insoluble in excess of ammonium hydroxide.

(ii) \( CuSO_4 + 2NH_4OH \rightarrow (NH_4)_2SO_4 + Cu(OH)_2 \)

The colour of precipitate is bluish white, which is soluble in excess of ammonium hydroxide.
In simple words: Ammonium chloride helps clean metals and is used in batteries. Ammonium hydroxide helps identify different metals because it forms colored precipitates that help us tell metals apart.

📝 Teacher's Note: Demonstrate the precipitation reactions with actual metal salt solutions. The color differences make this an excellent practical for qualitative analysis understanding.

🎯 Exam Tip: Learn the specific colors (dirty green for Fe, bluish white for Cu) and mention solubility behavior in excess NH₄OH for identification purposes.

Solution 10:
Answer:

In simple words: Liquid ammonia is made by squeezing ammonia gas under pressure, while liquid ammonia fortis is ammonia dissolved completely in water.

📝 Teacher's Note: Use physical demonstrations showing gas compression vs. dissolution to help students visualize the fundamental difference between these two forms.

🎯 Exam Tip: Always mention the 6 atmospheric pressure for liquid ammonia and emphasize the "saturated solution" aspect for fortis.

Solution 11:
Answer: Ammonia is a strong reducing agent. It is oxidised to nitrogen by removal of hydrogen, when it react with oxidizing agent.

(i) Reduction of chlorine
\[ 8NH_3(excess) + 3Cl_2 \rightarrow N_2 + 6NH_4Cl \]

(ii) Reduction of copper (II) oxide
\[ 3CuO + 2NH_3 \xrightarrow{\Delta} 3Cu + 3H_2O + N_2 \]
Black → Reddish brown

(iii) Reduction of lead (II) oxide
\[ 3PbO + 2NH_3 \xrightarrow{\Delta} 3Pb + 3H_2O + N_2 \]
Yellow → Grey

Key: Ammonia is a strong reducing agent.
In simple words: Ammonia acts like a helper that gives away hydrogen atoms to other chemicals, changing them from one form to another - like turning black copper oxide into reddish copper metal.

📝 Teacher's Note: Emphasize the color changes as visual proof of reduction. Students should understand that ammonia loses hydrogen while the other substance gains electrons (gets reduced).

🎯 Exam Tip: Always include the color changes (black→reddish brown, yellow→grey) and mention that ammonia gets oxidized to nitrogen in these reactions.

Solution 12:
Answer: Structure of ammonia

The presence of lone pair of electrons on the nitrogen atom makes ammonia basic in nature in its aqueous solution. Aqueous solution of ammonia dissociates partially to produce hydroxyl ion.
\[ NH_3 + H_2O \rightleftharpoons NH_4OH \]
\[ NH_4OH \rightleftharpoons NH_4^+ + OH^- \]
Key: Aqueous solution of ammonia is alkaline in nature.
In simple words: Ammonia has a pyramid shape with nitrogen at the center and three hydrogen atoms around it. It has an extra pair of electrons that make it basic when dissolved in water.

📝 Teacher's Note: Use molecular models to show the 3D pyramidal structure. Emphasize how the lone pair creates the basic nature by accepting protons from water.

🎯 Exam Tip: Draw the structure showing the lone pair clearly, and write both equilibrium equations to explain the basic nature.

Solution 13:
Answer: An aqueous solution of ammonia is prepared by dissolving ammonia in water. Ammonia is highly soluble in water.

The funnel arrangement is used to prepare aqueous solution of ammonia. In this method, ammonia gas is introduced through a funnel tube where the rate of dissolution of ammonia is faster than it is produced. This creates a partial vacuum, in delivery tube, resulting in low pressure. Thus, water is forced in to equalise the pressure. To overcome this problem of back suction the funnel is attached to outer end of the delivery tube and mouth of funnel just dips into the water in the trough.

Two advantages of this method:
(i) Prevents back suction of water into the flask.
(ii) Provides a larger surface area for the absorption of the gas.

Key: Ammonia is highly soluble in water.
In simple words: To make ammonia solution safely, we use a special funnel setup that prevents water from being sucked back into the gas-making apparatus while allowing maximum ammonia to dissolve.

📝 Teacher's Note: Draw the funnel arrangement clearly on the board. Explain the pressure difference concept and why back suction is dangerous for gas preparation apparatus.

🎯 Exam Tip: Sketch the funnel arrangement, mention both advantages clearly, and explain the pressure equalization principle for full marks.

Solution 14:
Answer:
(a) Two fertilizers manufactured from ammonia: Urea, ammonium sulphate, ammonium nitrate.
(b) Fertilizer from ammonia: Urea
\( 2NH_3 + CO_2 \xrightarrow{150°C,150atm} NH_2CONH_2 + H_2O \)
Key: Ammonia is used in manufacture of fertilizer.
In simple words: Ammonia is the main building block for making plant food (fertilizers) that helps crops grow better.

📝 Teacher's Note: Show students how ammonia's nitrogen content makes it perfect for plant nutrition. Connect this to why farmers use these fertilizers in fields.

🎯 Exam Tip: Always mention urea as the primary fertilizer and include the temperature/pressure conditions for the reaction.

Solution 15:
Answer:
(i) Once the reaction starts, further heating is not required because the reaction is exothermic. The heat evolved is sufficient to maintain the optimum temperature.
(ii) Ammonium nitrate is explosive in nature and it decomposes to give nitrous oxide and water. Thus, it is not used in the laboratory preparation of ammonia.
(iii) Calcium hydroxide is preferred as it is cheap and not deliquescent like NaOH or KOH.
(iv) Liquid ammonia is a polar covalent compound and neutral to litmus. Liquor ammonia is solution of ammonia in water, which is weakly basic in nature. It turns red litmus blue.
(v) Presence of moisture may result in formation of ammonium hydroxide. Thus dry \( N_2 \) and \( H_2 \) must be used in the Haber's process.
(vi) A promoter is a substance that is added to increase the efficiency of a catalyst.
(vii) Aqueous solution of ammonia that is ammonium hydroxide dissociates partially to hydroxyl ions. Thus, it conducts electricity.
\( NH_3 + H_2O \rightleftharpoons NH_4OH \)
\( NH_4OH \rightleftharpoons NH_4^+ + OH^- \)
(viii) Ammonia is highly soluble gas in water and so cannot be collected over water.
(ix) Ammonia is formed by decaying urine and other organic matter. Thus it is present in sewage water.
(x) Ammonium hydroxide precipitates hydroxides of metals, which differ in colour and solubility in excess of ammonium hydroxide. Thus it is used in identification of metal ions.
In simple words: These are practical reasons why ammonia behaves differently in various situations - like being explosive, dissolving in water, and helping identify metals.

📝 Teacher's Note: Use real examples - show students why we use lime instead of caustic soda (cost factor), and demonstrate the litmus test difference between liquid ammonia and ammonia solution.

🎯 Exam Tip: For (iv), clearly distinguish between "liquid ammonia" (pure) and "liquor ammonia" (aqueous solution) - this is a common confusion point.

Solution 16:
Answer:
(i) \( 8NH_3(excess) + 3Cl_2 \rightarrow N_2 + 6NH_4Cl \)
(ii) \( 3CuO + 2NH_3 \xrightarrow{\Delta} 3Cu + 3H_2O + N_2 \)
(iii) \( 4NH_3 + 3O_2 \rightarrow 2N_2 + 6H_2O + Heat \)
(iv) \( NH_3 + 3Cl_2(excess) \rightarrow NCl_3 + 3HCl \)
(v) \( 2NH_3 + H_2SO_4 \rightarrow (NH_4)_2SO_4 \)
(vi) \( NH_3 + H_2O \rightleftharpoons NH_4OH \)
\( NH_4OH \rightleftharpoons NH_4^+ + OH^- \)
(vii) \( 2NH_4Cl + CaCO_3 \rightarrow (NH_4)_2CO_3 + CaCl_2 \)
(viii) \( 3CuO + 2NH_3 \xrightarrow{\Delta} 3Cu + 3H_2O + N_2 \)
In simple words: These are chemical equations showing how ammonia reacts with different substances to form various products.

📝 Teacher's Note: Emphasize the difference between excess ammonia vs excess chlorine reactions. Practice balancing these equations step by step with students.

🎯 Exam Tip: Always check if the question specifies "excess" of any reactant - this changes the products formed, especially in ammonia-chlorine reactions.

Solution 17:
Answer:
(i) When excess ammonia is mixed with chlorine, ammonium chloride and nitrogen is formed.
\( 8NH_3(excess) + 3Cl_2 \rightarrow N_2 + 6NH_4Cl \)
(ii) When excess ammonia is mixed with chlorine, ammonium chloride and nitrogen is formed.
\( 8NH_3(excess) + 3Cl_2 \rightarrow N_2 + 6NH_4Cl \)
(iii) The filter paper turns into deep pink suggesting that ammonia solution is alkaline or basic in nature.
(iv) On passing ammonia over heated lead oxide yellow coloured lead monoxide is reduced to grey coloured lead metal.
\( 3PbO + 2NH_3 \xrightarrow{\Delta} 3Pb + 3H_2O + N_2 \)
Yellow → Grey
(v) On adding ammonium solution to ferric chloride solution we get ammonium chloride and reddish brown precipitate of ferric hydroxide which is insoluble in excess of ammonium solution.
\( FeCl_3 + 3NH_4OH \rightarrow 3NH_4Cl + Fe(OH)_3 \)
(vi) Initially when ammonium solution is added to aqueous solution of copper sulphate a bluish precipitate of copper hydroxide is obtained which dissolves in excess of ammonium hydroxide.
\( CuSO_4 + 2NH_4OH \rightarrow (NH_4)_2SO_4 + Cu(OH)_2 \)
\( Cu(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH \rightarrow [Cu(NH_3)_4]SO_4 + 4H_2O \)
(vii) Ammonia affects the lachrymal glands and brings tears to the eyes.
In simple words: These reactions show how ammonia behaves with different chemicals - forming salts, reducing metals, and creating colored precipitates.

📝 Teacher's Note: Demonstrate the color changes practically if possible. The copper complex formation is particularly striking for students to observe.

🎯 Exam Tip: For metal precipitation reactions, always mention whether the precipitate dissolves in excess ammonia - this distinguishes different metal hydroxides.

Solution 18:
Answer:
(a) \( NH_3 + H_2O \rightleftharpoons NH_4OH \)
\( NH_4OH \rightleftharpoons NH_4^+ + OH^- \)
(b) \( 8NH_3(excess) + 3Cl_2 \rightarrow N_2 + 6NH_4Cl \)
(c) \( 3CuO + 2NH_3 \xrightarrow{\Delta} 3Cu + 3H_2O + N_2 \)
In simple words: These are three important reactions of ammonia - dissolving in water, reacting with chlorine, and reducing copper oxide.

📝 Teacher's Note: These three reactions represent ammonia's basic nature, its reaction with halogens, and its reducing property respectively. Connect each to a real application.

🎯 Exam Tip: These are standard reactions that frequently appear in exams. Memorize the balanced equations and the specific conditions like heat (Δ) where required.

Solution 19:
Answer:

1. (i) Use of Ammonium Chloride
-Used in Leclanche cell and dry cell
(ii) Use of Ammonium Sulphate
-Used as a fertilizer
(iii) Use of Ammonium nitrate
-Used in fireworks
(iv) Use of Ammonium Carbonate
-Used in baking powder
2. Test of ammonia and ammonium ions:
Ammonia gas has a characteristic pungent smell
A glass rod dipped in concentrated hydrochloric acid and is introduced into the gas produces thick white fumes of ammonium chloride.
In simple words: This shows the differences between dry and wet ammonia, how it reacts differently with chlorine depending on amounts, and how it helps identify iron compounds.

📝 Teacher's Note: Use the glass rod test as a practical demonstration. Students can easily remember the white fumes formation as a key identification test.

🎯 Exam Tip: For iron compound tests, mention the specific colors - dirty green for Fe²⁺ and reddish brown for Fe³⁺. These color descriptions earn extra marks.

Solution 20:
Answer:
1. (i) Use of Ammonium Chloride
-Used in Leclanche cell and dry cell
(ii) Use of Ammonium Sulphate
-Used as a fertilizer
(iii) Use of Ammonium nitrate
-Used in fireworks
(iv) Use of Ammonium Carbonate
-Used in baking powder
2. Test of ammonia and ammonium ions:
° Ammonia gas has a characteristic pungent smell
° A glass rod dipped in concentrated hydrochloric acid and is introduced into the gas produces thick white fumes of ammonium chloride.
In simple words: Different ammonium compounds have specific uses in daily life, and ammonia can be easily identified by its smell and the white fumes test.

📝 Teacher's Note: Connect these uses to real-life examples - show students actual dry cells, fertilizer packets, and baking powder to make the learning practical.

🎯 Exam Tip: Remember the specific applications for each compound. Questions often ask for one use of each ammonium salt.

Solution 21:
Answer:
\( NH_3 + HNO_3 \rightarrow NH_4NO_3 \)
(A) (B)
\( NH_4NO_3 \xrightarrow{230°C} N_2O + 2H_2O \)
(C) (D)
In simple words: Ammonia reacts with nitric acid to form ammonium nitrate, which when heated produces nitrous oxide and water.

📝 Teacher's Note: This shows both acid-base neutralization and thermal decomposition. Emphasize the temperature requirement for decomposition.

🎯 Exam Tip: Always include the temperature (230°C) when writing the decomposition reaction of ammonium nitrate.

Solution 22:
Answer:
(i) Ammonia gas on reacting with aqueous solution of zinc chloride produces white gelatine like precipitate of zinc hydroxide.
\( ZnCl_2 + 2NH_4OH \rightarrow NH_4Cl + Zn(OH)_2 \)
(ii) Ammonia gas on reacting with aqueous solution of ferrous sulphate produces dirty green precipitate of ferrous hydroxide.
\( FeSO_4 + 2NH_4OH \rightarrow (NH_4)_2SO_4 + Fe(OH)_2 \)
(iii) Ammonia gas on reacting with aqueous solution of ferric chloride produces reddish brown precipitate of ferric hydroxide.
\( FeCl_3 + 3NH_4OH \rightarrow 3NH_4Cl + Fe(OH)_3 \)
(iv) Ammonia gas on reacting with aqueous solution of lead nitrate produces white precipitate of lead hydroxide.
\( Pb(NO_3)_2 + 2NH_4OH \rightarrow 2NH_4NO_3 + Pb(OH)_2 \)
(v) Ammonia gas on reacting with aqueous solution of copper sulphate produces bluish white precipitate of cupric hydroxide.
\( CuSO_4 + 2NH_4OH \rightarrow (NH_4)_2SO_4 + Cu(OH)_2 \)
(vi) Ammonia gas on reacting with aqueous solution of Chromium chloride produces green precipitate of chromium tri hydroxide.
\( CrCl_3 + 3NH_4OH \rightarrow 3NH_4Cl + Cr(OH)_3 \)
In simple words: Ammonia solution reacts with different metal salts to form colored precipitates that help identify which metal is present.

📝 Teacher's Note: Create a color chart showing all these precipitate colors. This visual aid helps students remember the qualitative analysis tests better.

🎯 Exam Tip: Learn the precipitate colors by heart - white (Zn, Pb), dirty green (Fe²⁺), reddish brown (Fe³⁺), bluish white (Cu), green (Cr). Colors are crucial for identification.

Solution 1992-1:
Answer: Silver chloride
In simple words: Silver chloride is the white precipitate formed when silver compounds react with chloride ions.

📝 Teacher's Note: This is likely from a precipitation reaction. Show students how AgCl precipitates immediately when AgNO₃ meets any chloride.

🎯 Exam Tip: Silver chloride is always the answer when asked about white precipitates formed with silver salts.

Solution 1992-2:
Answer: Ammonia is highly soluble gas in water and so cannot be collected over water.
In simple words: Ammonia dissolves completely in water, so we cannot use the water displacement method to collect it.

📝 Teacher's Note: Contrast this with gases like hydrogen or oxygen which are collected over water because they don't dissolve much.

🎯 Exam Tip: Always mention "highly soluble" when explaining why ammonia cannot be collected over water. This shows understanding of the reason.

Solution 1992-3:
Answer: Ammonia is the odd one out. Ammonia forms weakly basic solution when dissolved in water. The others give acidic solution when dissolved in water.
In simple words: Among a group of gases, ammonia is different because it makes water basic while others make it acidic.

📝 Teacher's Note: Use universal indicator to show students how ammonia turns water blue (basic) while other gases like CO₂, SO₂ turn it red (acidic).

🎯 Exam Tip: When identifying the odd one out, always explain both what makes it different AND what the others have in common.

Solution 1992-4:
Answer: Ammonium chloride on heating with an alkali produces ammonia with other products.
\( 2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3 \)
In simple words: When we heat ammonium chloride with a base like lime, ammonia gas is released along with salt and water.

📝 Teacher's Note: This is the standard laboratory preparation of ammonia. Emphasize that heat is essential for this reaction to occur.

🎯 Exam Tip: Always include the heat symbol (Δ) and balance the equation properly. This is a frequently asked preparation method.

Solution 1992-5:
Answer:
1. Ammonia is used in the manufacture of fertilisers such as ammonium sulphate, ammonium nitrate, etc.
2. It is used in the industrial preparation of nitric acid by Ostwald's process.
In simple words: Ammonia's main uses are making plant food (fertilizers) and producing nitric acid for industry.

📝 Teacher's Note: Connect fertilizer use to agriculture and nitric acid to explosive manufacturing. This shows ammonia's economic importance.

🎯 Exam Tip: These two uses are the most important industrial applications. Always mention them when asked about ammonia's uses.

Solution 1993-1:
Answer: Initially when ammonium solution is added to aqueous solution of copper sulphate a bluish precipitate of copper hydroxide is obtained which dissolves in excess of ammonium hydroxide.
\( CuSO_4 + 2NH_4OH \rightarrow (NH_4)_2SO_4 + Cu(OH)_2 \)
\( Cu(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH \rightarrow [Cu(NH_3)_4]SO_4 + 4H_2O \)
In simple words: Copper sulphate first forms a blue precipitate with ammonia, but this dissolves when more ammonia is added, forming a complex.

📝 Teacher's Note: This two-step reaction is excellent for demonstrating complex formation. The color change from blue precipitate to deep blue solution is very striking.

🎯 Exam Tip: Write both reactions to show the complete process. The complex formation in excess ammonia is a key distinguishing feature of copper.

Solution 1993-2:
Answer:
\( 3CuO + 2NH_3 \xrightarrow{\Delta} 3Cu + 3H_2O + N_2 \)
Black → Reddish brown
In simple words: Ammonia reduces black copper oxide to reddish brown copper metal when heated.

📝 Teacher's Note: This demonstrates ammonia's reducing properties. The color change from black to reddish brown is easily observable.

🎯 Exam Tip: Always mention the color change (black to reddish brown) and include the heat symbol. This shows ammonia acting as a reducing agent.

Solution 1994-1:
Answer: Magnesium on burning in air produces magnesium nitride.
\( Mg + N_2 \rightarrow Mg_3N_2 \)
When this magnesium nitride comes in contact with water ammonia gas is released.
\( Mg_3N_2 + 6H_2O \rightarrow 3Mg(OH)_2 + 2NH_3 \)
Key: Ammonia is formed from metal nitrides
In simple words: When magnesium burns in air, it forms a compound that releases ammonia when mixed with water.

📝 Teacher's Note: This shows an alternative source of ammonia from metal nitrides. Emphasize that this is why burning magnesium has a distinct smell.

🎯 Exam Tip: Write both equations to show the complete process. This demonstrates nitrogen fixation followed by hydrolysis to give ammonia.

Solution 1994-2:
Answer:

In simple words: The Haber process makes ammonia using iron as catalyst under high temperature and pressure conditions.

📝 Teacher's Note: Emphasize that these conditions represent a balance between reaction rate and equilibrium position. Too high temperature reduces yield but increases rate.

🎯 Exam Tip: Remember the approximate ranges for temperature (450-500°C) and pressure (200-900 atm). These specific values are often asked.

Solution 1994-3:
Answer: Ammonium chloride on heating with an alkali produces ammonia with other products.
\( 2NH_4Cl + Ca(OH)_2 \xrightarrow{\Delta} CaCl_2 + 2H_2O + 2NH_3 \)
In simple words: This is the standard laboratory method to prepare ammonia by heating ammonium chloride with lime.

📝 Teacher's Note: This is identical to Solution 1992-4. Reinforce that this is the most common lab preparation method for ammonia.

🎯 Exam Tip: This reaction appears frequently in exams. Practice balancing it and remember that heat is essential for the reaction to proceed.

Solution 1995-1:
Answer: The cation is \( \text{Cu}^{2+} \) ion. Solution B is copper sulphate. It is bright blue in colour.

📝 Teacher's Note: Demonstrate copper sulphate crystals in class - the vivid blue color is instantly recognizable. Students often confuse this with other blue compounds, so emphasize the specific blue shade of copper sulphate.

🎯 Exam Tip: Always mention both the ion formula (\( \text{Cu}^{2+} \)) and the compound name (copper sulphate) for complete marks. The blue color is a key identifying feature.

Solution 1995-2:
Answer: Three ways to identify ammonia gas:
1. It is a pungent smell gas.
2. It gives white precipitate when bubbles through a solution of lead nitrate.
3. It gives a brown colour or precipitate when treated with Nessler's reagent.

📝 Teacher's Note: The pungent smell test should be done carefully in a well-ventilated area. Demonstrate the lead nitrate test as it gives a clear visual confirmation that students can easily remember.

🎯 Exam Tip: Learn all three tests as questions often ask for multiple identification methods. The pungent smell is the easiest to remember and describe.

Solution 1995-3:
Answer: (i) \( \text{Mg}_3\text{N}_2 + 6\text{H}_2\text{O} \rightarrow 3\text{Mg}(\text{OH})_2 + 2\text{NH}_3 \)
(ii) \( 2\text{NH}_3 + 3\text{CuO} \rightarrow 3\text{Cu} + 3\text{H}_2\text{O} + \text{N}_2 \)
(iii) \( 8\text{NH}_3 + 3\text{Cl}_2 \rightarrow \text{N}_2 + 6\text{NH}_4\text{Cl} \)
(iv) \( 4\text{NH}_3 + 5\text{O}_2 \rightarrow 6\text{H}_2\text{O} + 4\text{NO} + \text{Heat} \)

📝 Teacher's Note: These equations show ammonia's versatility - as a base with water, as a reducing agent with metal oxides, and in industrial processes. Practice balancing these step by step.

🎯 Exam Tip: Check your balanced equations by counting atoms on both sides. These are standard reactions that appear frequently in exams.

Solution 1995-4:
Answer: This reaction shows that ammonia is a reducing agent.

📝 Teacher's Note: Explain that ammonia reduces metal oxides to metals by removing oxygen. Use the CuO + NH₃ reaction as a clear example of this reducing property.

🎯 Exam Tip: When asked about ammonia's role in reactions with metal oxides, always state "reducing agent" - this is the key term examiners look for.

Solution 1995-5:
Answer: This process is called as Ostwald's Process. The catalyst used is platinum.

📝 Teacher's Note: Emphasize that Ostwald's process is for making nitric acid from ammonia, not for making ammonia itself. Students often confuse this with Haber's process.

🎯 Exam Tip: Remember the distinction - Haber's process makes ammonia, Ostwald's process uses ammonia to make nitric acid. Both use specific catalysts.

Solution 1995-6:
Answer: During laboratory preparation of ammonia, it is passed through a drying tower containing quicklime (calcium oxide). Ammonia is collected in an inverted dry gas jar by the downward displacement of air.

📝 Teacher's Note: Demonstrate why quicklime is used - it removes moisture without reacting with ammonia. Show the inverted gas jar setup and explain why ammonia displaces air downward.

🎯 Exam Tip: Always mention both the drying agent (quicklime/CaO) and the collection method (downward displacement of air) for complete answers about ammonia preparation.

Solution 1995-7:
Answer: Ammonia gas

📝 Teacher's Note: This appears to be a simple identification answer. Ensure students can recognize ammonia from its properties and reactions in various contexts.

🎯 Exam Tip: When identifying gases, look for key clues like pungent smell, turning litmus blue, and collection method to confirm it's ammonia.

Solution 1996-1:
Answer: Ammonia forms ammonium hydroxide and turns red litmus blue as it is alkaline in nature.

📝 Teacher's Note: Demonstrate the litmus test with ammonia solution. Explain that dry ammonia gas doesn't affect litmus, but ammonia solution does because it forms OH⁻ ions.

🎯 Exam Tip: Be specific - mention that ammonia "turns red litmus blue" rather than just saying "affects litmus paper" for precise answers.

Solution 1996-2:
Answer: \( \text{NH}_4\text{Cl} + \text{NaOH} \xrightarrow{\Delta} \text{NaCl} + \text{H}_2\text{O} + \text{NH}_3 \)

📝 Teacher's Note: This is a classic lab preparation reaction. Heat the mixture gently and test for ammonia gas evolution using litmus paper or smell (carefully).

🎯 Exam Tip: Include the heat symbol (Δ) above the arrow - it's essential for this reaction and shows you understand the conditions needed.

Solution 1996-3:
Answer: Haber's process is used in industrial preparation of ammonia. Gaseous inputs in Haber's process are dry nitrogen and dry hydrogen gas. They are mixed in the ratio of 1:3 by volume. The following conditions favour maximum yield of ammonia:
1. Low temperature
2. High pressure
3. Use of catalyst
The gases after reaction pass through condensing pipes of cooling chamber where ammonia gets liquefied and is collected in receiver. Ammonia can also be collected by downward displacement of air.

📝 Teacher's Note: Explain Le Chatelier's principle to help students understand why these specific conditions are used. Use the analogy of squeezing more people into a smaller room (high pressure favors fewer gas molecules).

🎯 Exam Tip: Remember the 1:3 ratio of N₂:H₂ and the three key conditions. Always mention that the catalyst is iron for complete answers about Haber's process.

Solution 1997-1:
Answer: (i) \( 4\text{NH}_3 + 3\text{O}_2 \rightarrow 6\text{H}_2\text{O} + 2\text{N}_2 + \text{Heat} \)
(ii) \( 4\text{NH}_3 + 5\text{O}_2 \xrightarrow{\text{Platinum }900°\text{C}} 6\text{H}_2\text{O} + 4\text{NO} + \text{Heat} \)
\( 2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2 \)
(iii) (a) Catalyst used is Platinum
(b) The catalyst glows red hot as this reaction is an exothermic reaction.
(c) Ostwald's Process

📝 Teacher's Note: Show students how reaction conditions affect products - without catalyst gives N₂, with platinum catalyst gives NO. The red glow indicates high heat release.

🎯 Exam Tip: Pay attention to reaction conditions - temperature and catalyst determine the products formed when ammonia reacts with oxygen.

Solution 1997-2:
Answer: (i) When ammonia dissolve in water, it form ammonium hydroxide which dissociates into \( \text{NH}_4^+ \) and \( \text{OH}^- \) ion. Therefore it become soluble in water.
(ii) (a) Aqueous solution of ammonia turns red litmus blue stating that it is having basic ions. (b) It precipitates hydroxides of metals.
(iii) An aqueous solution of ammonia i.e. ammonium hydroxide reacts with acid to produce ammonium salt and water.
\( 2\text{NH}_4\text{OH} + \text{H}_2\text{SO}_4 \rightarrow (\text{NH}_4)_2\text{SO}_4 + \text{H}_2\text{O} \)

📝 Teacher's Note: Emphasize that ammonia's basic properties come from the OH⁻ ions formed when it dissolves in water. Show the metal hydroxide precipitation test with copper or iron salts.

🎯 Exam Tip: Always explain ammonia's basicity in terms of OH⁻ ion formation - this is the fundamental reason for all its basic properties.

Solution 1998-1:
Answer: Dry ammonia are neutral to litmus. An aqueous solution of ammonia turns red litmus blue stating that it is basic in nature.

📝 Teacher's Note: This is a crucial distinction that students often miss. Demonstrate both tests - dry ammonia gas vs ammonia dissolved in water. Only the aqueous solution shows basic properties.

🎯 Exam Tip: Always specify "dry ammonia" vs "aqueous ammonia" in your answers. The presence of water is what makes the difference in litmus tests.

Solution 1998-2:
Answer: Ammonium salts are used as fertilizers in fields.

📝 Teacher's Note: Connect this to agriculture - explain why plants need nitrogen and how ammonium salts provide readily available nitrogen for plant growth.

🎯 Exam Tip: When asked about uses of ammonia compounds, remember fertilizers as the major agricultural application - it's economically very important.

Solution 1998-3:
Answer: (a) Magnesium nitride (\( \text{Mg}_3\text{N}_2 \))
(b) \( \text{Mg}_3\text{N}_2 + 6\text{H}_2\text{O} \rightarrow 3\text{Mg}(\text{OH})_2 + 2\text{NH}_3 \)
(c) Ammonia is a good reducing agent.

📝 Teacher's Note: Explain how magnesium burns in air to form both the oxide and nitride. The reaction with water shows how metal nitrides can be sources of ammonia.

🎯 Exam Tip: Remember the formula Mg₃N₂ and that metal nitrides + water always produce the metal hydroxide + ammonia. This is a standard reaction pattern.

Solution 1998-4:
Answer: (i) The reactants nitrogen and hydrogen combine to form ammonia at low temperature, high pressure in presence of catalyst to for maximum yield of ammonia. The balanced equation for the reaction is:
\( \text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3 + \text{Heat} \)
(ii) Iron acts as catalyst.
(iii) Since the forward reaction occurs with decrease in volume, according to Le Chatelier's principle, high pressure favours the formation of ammonia.
Key: Haber's process

📝 Teacher's Note: Use the equilibrium arrow to show this is a reversible reaction. Explain Le Chatelier's principle with simple examples - like squeezing a sponge to reduce volume.

🎯 Exam Tip: Use the equilibrium arrow (⇌) for Haber's process, not a single arrow. Mention that high pressure favors the side with fewer gas molecules.

Solution 1998-5:
Answer: Ammonia (vapour density(V.D.)= 8.5) is less dense than air (vapour density (V.D.)= 14.4) and so is lighter than air. Fountain experiment proves that ammonia is highly soluble in water. The balanced equation for the reaction between ammonia and sulphuric acid is
\( 2\text{NH}_3 + \text{H}_2\text{SO}_4 \rightarrow (\text{NH}_4)_2\text{SO}_4 \)

📝 Teacher's Note: Demonstrate the fountain experiment if possible - it's a dramatic way to show ammonia's high solubility. Compare vapour densities to explain collection methods.

🎯 Exam Tip: Remember that vapour density less than 14.4 means the gas is lighter than air. This explains why ammonia is collected by downward displacement.

Solution 1991-1:
Answer: 1. Ammonia
2. Nitrogen

📝 Teacher's Note: This appears to be a simple identification question. Ensure students can distinguish between ammonia and nitrogen based on their properties and reactions.

🎯 Exam Tip: Learn the key differences - ammonia is basic and soluble in water, nitrogen is neutral and insoluble. These are fundamental distinctions.

Solution 2001-1:
Answer: Ammonia

📝 Teacher's Note: Simple identification answers require students to recognize ammonia from context clues in the question. Review all ammonia properties regularly.

🎯 Exam Tip: When identifying gases, look for multiple clues like smell, litmus behavior, and solubility to confirm your answer.

Solution 2001-2:
Answer: (i) \( 2\text{NH}_4\text{Cl} + \text{Ca}(\text{OH})_2 \xrightarrow{\Delta} \text{CaCl}_2 + 2\text{H}_2\text{O} + 2\text{NH}_3 \uparrow \)
(ii) \( \text{NH}_3 + \text{HCl} \xrightarrow{\Delta} \text{NH}_4\text{Cl} \)

📝 Teacher's Note: The first reaction is for lab preparation of ammonia, the second shows acid-base neutralization. Note the upward arrow showing gas evolution.

🎯 Exam Tip: Use the upward arrow (↑) to show gas evolution and heat symbol (Δ) for reactions requiring heating. These symbols convey important information.

Solution 2001-3:
Answer: \( 2\text{NH}_3 + 3\text{CuO} \rightarrow 3\text{Cu} + 3\text{H}_2\text{O} + \text{N}_2 \)

📝 Teacher's Note: This reaction demonstrates ammonia as a reducing agent. The copper oxide is reduced to metallic copper while ammonia is oxidized to nitrogen.

🎯 Exam Tip: In redox reactions, identify what's being reduced (CuO to Cu) and what's being oxidized (NH₃ to N₂) to understand the complete process.

Solution 2001-4:
Answer: Aqueous solution is acidic in nature due to presence of hydrogen ion. Aqueous solution of ammonia is weakly basic in nature due to presence of hydroxyl ion.

📝 Teacher's Note: Emphasize the difference between acids (H⁺ ions) and bases (OH⁻ ions). Ammonia is a weak base because it doesn't completely dissociate in water.

🎯 Exam Tip: Always specify "weakly basic" for ammonia - it's not a strong base like NaOH. The term "weakly" is important for accuracy.

Solution 2002-1:
Answer: Action of heat on ammonium chloride is reversible. When heated, it decomposes to form ammonia and hydrogen chloride. When the products are cooled, they recombine to form ammonium chloride.
\( \text{NH}_4\text{Cl} \rightleftharpoons \text{NH}_3 + \text{HCl} \)
Ammonium nitrate on heating completely disappears, escaping in the form of water vapour and nitrous oxide.
\( \text{NH}_4\text{NO}_3 \xrightarrow{\Delta} \text{N}_2\text{O} + 2\text{H}_2\text{O} \)

📝 Teacher's Note: Demonstrate the difference - NH₄Cl sublimes and recombines when cooled, while NH₄NO₃ completely decomposes. This shows different types of thermal decomposition.

🎯 Exam Tip: Use the equilibrium arrow (⇌) for reversible reactions and single arrow (→) for irreversible ones. This distinction is crucial for chemical accuracy.

Solution 2002-2:
Answer: Thermal dissociation.

📝 Teacher's Note: Thermal dissociation means breaking apart due to heat. It's different from thermal decomposition - dissociation implies the process can be reversed by cooling.

🎯 Exam Tip: Learn the distinction between dissociation (reversible) and decomposition (usually irreversible) when describing heat effects on compounds.

Solution 2003-1:
Answer: \( \text{Mg}_3\text{N}_2 + 6 \text{H}_2\text{O} \rightarrow 3 \text{Mg}(\text{OH})_2 + 2 \text{NH}_3 \)

📝 Teacher's Note: This is the standard reaction of metal nitrides with water. All Group 2 metal nitrides follow this pattern - nitride + water → metal hydroxide + ammonia.

🎯 Exam Tip: Remember the general pattern: Metal nitride + Water → Metal hydroxide + Ammonia. Apply this to any metal nitride question.

Solution 2003-2:
Answer: Ammonia is collected by downward displacement of air.

📝 Teacher's Note: This collection method is used because ammonia is lighter than air and highly soluble in water. Students should understand why each collection method is chosen.

🎯 Exam Tip: Always justify collection methods - ammonia uses downward displacement of air because it's less dense than air and cannot be collected over water due to high solubility.

Solution 2003-3:
Answer: Ammonia is highly soluble in water and so it is not collected over water.

📝 Teacher's Note: This explains why we can't use the standard gas collection method for ammonia. Connect this to the fountain experiment that demonstrates this high solubility.

🎯 Exam Tip: When explaining collection methods, always mention why certain methods cannot be used. For ammonia, high water solubility prevents collection over water.

Solution 2003-4:
Answer: Quick lime (calcium oxide) is used as a drying agent for ammonia.

📝 Teacher's Note: Quicklime is chosen because it removes water without reacting with ammonia gas. Other drying agents like concentrated sulfuric acid would react with ammonia.

🎯 Exam Tip: Remember that CaO is specifically used for basic gases like ammonia. For acidic gases, different drying agents would be used.

Solution 2004-1:
Answer: \( 8\text{NH}_3(excess) + 3\text{Cl}_2 \rightarrow \text{N}_2 + 6\text{NH}_4\text{Cl} \)

📝 Teacher's Note: This reaction shows what happens when excess ammonia reacts with chlorine. If chlorine were in excess, the products would be different - always check the stoichiometry.

🎯 Exam Tip: Pay attention to which reactant is in excess - this determines the products formed. The balanced equation must reflect the actual reaction conditions.

Solution 2004-4:
Answer: (i) The reactants nitrogen and hydrogen combine to form ammonia at low temperature, high pressure in presence of catalyst to for maximum yield of ammonia. The balanced equation for the reaction is:
\( \text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3 + \text{Heat} \)
(ii) The gases which leave the catalyst chamber contain ammonia and unreacted nitrogen and hydrogen gas. The gases leaving the catalyst chamber are cooled by passing through condensing pipes where ammonia gets liquefied and is collected in receiver.

📝 Teacher's Note: Explain that unreacted gases are recycled back to the reactor to improve efficiency. The separation by cooling takes advantage of ammonia's higher boiling point.

🎯 Exam Tip: In industrial processes, always mention recycling of unreacted materials and the separation method used to isolate the product.

Solution 2005-1:
Answer: (i) This shows that ammonia molecule accept \( \text{H}^+ \) ion from water to form ammonium ion.
\( \text{NH}_3 + \text{H}_2\text{O} \rightleftharpoons \text{NH}_4\text{OH} \)
\( \text{NH}_4\text{OH} \rightleftharpoons \text{NH}_4^+ + \text{OH}^- \)
(ii) When ammonia dissolves in water, the hydroxyl ion is also formed.
(iii) The precipitation of metal hydroxides from their aqueous solution confirms the presence of \( \text{OH}^- \) ion in the solution.

📝 Teacher's Note: Emphasize that ammonia acts as a Brønsted base by accepting protons (H⁺). The equilibrium arrows show this is a reversible process with limited ionization.

🎯 Exam Tip: Use equilibrium arrows for weak base ionization of ammonia. This shows it's not complete ionization like strong bases.

Solution 2005-2:
Answer: (i) Ammonium chloride on heating with an slaked lime produces ammonia with other products.
\( 2\text{NH}_4\text{Cl} + \text{Ca}(\text{OH})_2 \xrightarrow{\Delta} \text{CaCl}_2 + 2\text{H}_2\text{O} + 2\text{NH}_3 \)
(ii) \( \text{AlN} + 3\text{H}_2\text{O} \rightarrow \text{Al}(\text{OH})_3 + \text{NH}_3 \)

📝 Teacher's Note: These are two different ways to prepare ammonia - from ammonium salts with bases, and from metal nitrides with water. Both follow predictable patterns.

🎯 Exam Tip: Learn the general patterns: NH₄⁺ salt + Base → Salt + Water + NH₃, and Metal nitride + Water → Metal hydroxide + NH₃.

Solution 2006-1:
Answer: Ammonia gas on reacting with aqueous solution of lead nitrate produces white precipitate of lead hydroxide, which is insoluble in excess of ammonium hydroxide.
\( \text{Pb}(\text{NO}_3)_2 + 2\text{NH}_4\text{OH} \rightarrow 2\text{NH}_4\text{NO}_3 + \text{Pb}(\text{OH})_2 \)

📝 Teacher's Note: This is a classic confirmatory test for ammonia. The white precipitate that doesn't dissolve in excess ammonia distinguishes lead from other metals like zinc or aluminum.

🎯 Exam Tip: Specify that the precipitate is "insoluble in excess ammonium hydroxide" - this detail distinguishes it from other metal hydroxides that might dissolve.

Solution 2006-2:
Answer: Ammonia gas on reacting with aqueous solution of lead nitrate produces white precipitate of lead hydroxide, which is insoluble in excess of ammonium hydroxide.
\( \text{Pb}(\text{NO}_3)_2 + 2\text{NH}_4\text{OH} \rightarrow 2\text{NH}_4\text{NO}_3 + \text{Pb}(\text{OH})_2 \)

📝 Teacher's Note: This appears to be a repeat of the previous answer. Reinforce that this test is reliable and specific for identifying ammonia gas.

🎯 Exam Tip: This test appears frequently in exams. Remember both the chemical equation and the observation (white precipitate insoluble in excess NH₄OH).

Solution 2007-1:
Answer: By the pungent smell of ammonia gas.

📝 Teacher's Note: The pungent smell is the most immediate and obvious way to identify ammonia. However, emphasize safety - never inhale directly, always waft the gas toward your nose carefully.

🎯 Exam Tip: While smell is the easiest identification method, also know the chemical tests for complete answers when multiple identification methods are asked.

Solution 2007-2:
Answer: (i) \( 2\text{NH}_3 + 3\text{CuO} \rightarrow 3\text{Cu} + 3\text{H}_2\text{O} + \text{N}_2 \)
(ii) \( 8\text{NH}_3(excess) + 3\text{Cl}_2 \rightarrow \text{N}_2 + 6\text{NH}_4\text{Cl} \)

📝 Teacher's Note: Both reactions show ammonia acting as a reducing agent. In the first, it reduces copper oxide; in the second, it's oxidized by chlorine while some forms ammonium chloride.

🎯 Exam Tip: Note the "(excess)" specification for ammonia in the second reaction - this determines the products formed. Without excess ammonia, nitrogen trichloride would form instead.

Solution 2008-1:
Answer: (c) Magnesium nitride

📝 Teacher's Note: Magnesium nitride reacts with water to produce ammonia. This is a standard reaction of Group 2 metal nitrides and is commonly used for ammonia preparation in some contexts.

🎯 Exam Tip: Remember that metal nitrides (like Mg₃N₂) react with water to produce ammonia and the corresponding metal hydroxide. This is a reliable source of ammonia.