Frank Brothers Solutions for ICSE Class 10 Chemistry Chapter 9b Nitric Acid

Chapter 9B. Nitric Acid

Solution 1: Answer: Nitric acid is formed in atmosphere during lightning discharge. Nitrogen in atmosphere combine with oxygen to form nitric acid.

📝 Teacher's Note: Use the analogy of nature's laboratory - lightning acts like a giant spark that forces nitrogen and oxygen to react, creating nitric acid naturally in the atmosphere.

🎯 Exam Tip: Remember the key phrase "during lightning discharge" - this is the natural formation method that examiners often ask about.

Solution 2: Answer:
(i) Gas produced in air during lightning is Nitric Oxide.
(ii) Hydrogen gas is obtained by treating manganese with 1% nitric acid.
(iii) Salt referred to as Chile saltpeter is \( NaNO_3 \).
(iv) Salt used in laboratory to prepare nitric acid is \( KNO_3/NaNO_3 \).
(v) Products obtained by catalytic oxidation of ammonia are NO, water & heat.
(vi) Products obtained by heating concentrated nitric acid are \( NO_2 \), water & \( O_2 \).
(vii) Copper nitrate is blue coloured, cobalt nitrate is coloured.

📝 Teacher's Note: Create a memory chart with students linking each substance to its color or properties. Visual associations help retain these facts better.

🎯 Exam Tip: Learn the colors mentioned - "blue for copper nitrate" is a common exam question. Also remember Chile saltpeter = sodium nitrate.

Solution 3: Answer:
(a) Aqua fortis: It is the other name for nitric acid, it is also known as 'strong water'. It was called so because earlier this was the only liquid which could dissolve many metals including silver.
(b) Aqua Regia: It is a mixture of concentrated nitric acid and concentrated hydrochloric acid in proportionof 1:3(by volume).
(c) Fuming nitric acid: It contains \( NO_2 \) dissolved in concentrated \( HNO_3 \). It is obtained by distilling concentrated \( HNO_3 \) with little starch.
(d) Decrepitation: It is the breaking of a substance usually accompanied by the emission of a crackling sound. An example for a substance which decrepitates on heating is lead nitrate (\( Pb(NO_3)_2 \)).

📝 Teacher's Note: Explain that "aqua" means water in Latin - so aqua fortis means "strong water" and aqua regia means "royal water" because it dissolves gold.

🎯 Exam Tip: Remember the 1:3 ratio for Aqua Regia (HNO₃:HCl) and that fuming nitric acid gets its color from dissolved NO₂ gas.

Solution 4: Answer:
(a) In laboratory, nitric acid is prepared by heating a mixture of concentrated sulphuric acid and potassium or sodium nitrate with conc. \( H_2SO_4 \) at 200°C.
\( KNO_3+H_2SO_4→KHSO_4+HNO_3 \)
(b) Concentrated sulphuric acid is non-volatile and produce volatile nitric acid.
(c) The temperature is maintained at 200°C in the above reaction to awoid following problems:
• Nitric acid would decompose at high temperature.
• Potassium or sodium sulphate is formed which will stick to the walls of glass and cannot be removed be easily.
• The glass apparatus may break at high temperature.
(d) No, concentrated HCl cannot be used in the place of concentrated \( H_2SO_4 \) because HCl is more volatile than \( HNO_3 \) and hence nitric acid vapours will carry HCl vapours.

📝 Teacher's Note: Emphasize the concept of volatility - the less volatile acid (H₂SO₄) can displace the more volatile acid (HNO₃) from its salt.

🎯 Exam Tip: Remember 200°C as the critical temperature and that H₂SO₄ is chosen because it's non-volatile, not because it's stronger.

Solution 5: Answer:
(a). Ostwald process is used for used for the manufacture of nitric acid.
(b). Reactants required for Ostwald process are ammonia and oxygen of air.
(c). In ostwald process ammonia gas and air are taken in 1:10 ratio.
(d). Pt gauge is used as a catalyst in ostwald process.
(e). Oxygen is the oxidizing agent which converts ammonia to nitric acid.
(f). \( 4NH_3 + 5O_2 \xrightarrow[\text{Pt gauge}]{\text{800°C}} 4NO + 6H_2O + \text{Heat} \)
\( 2NO + O_2 \xrightarrow{\text{50°C}} 2NO_2 \)
\( 4NO_2 + O_2 + 2H_2O → 4HNO_3 \)

📝 Teacher's Note: Break down the Ostwald process into three clear steps - oxidation of ammonia, formation of nitrogen dioxide, and absorption in water.

🎯 Exam Tip: Remember the 1:10 ratio (NH₃:air), platinum catalyst, and the three-step reaction sequence. Each step has different temperature conditions.

Solution 6: Answer: Ostwald process-
The nitric acid is manufactured by Ostwald process. In ostwald process ammonia gas and air are taken in 1:10 ratio. Pt gauge is used as a catalyst in ostwald process. Following steps are involved in Ostwald's process for the manufacture of nitric acid.
1. Oxidation of ammonia in catalytic chamber.
\( 4NH_3 + 5O_2 \xrightarrow[\text{Pt gauge}]{\text{800°C}} 4NO + 6H_2O + \text{Heat} \)
2. Oxidation of nitric acid in oxidation chamber.
\( 2NO + O_2 \xrightarrow{\text{50°C}} 2NO_2 \)
3. Absorption of nitrogen dioxide in water.
\( 4NO_2 + O_2 + 2H_2O → 4HNO_3 \)

📝 Teacher's Note: Use the provided diagram to explain how gases flow through different chambers - this visual representation helps students understand the industrial process better.

🎯 Exam Tip: If asked to draw the Ostwald process, include all three chambers with their specific reactions and temperature conditions clearly labeled.

Solution 7: Answer:
(a) Nitric acid cannot be concentrated beyond 68% by the distillation of a dilute solution of \( HNO_3 \) because it forms an azeotropic mixture i.e. at 121°C it boils without any change in its concentration of the mixture with water.
(b) 98% nitric acid is obtained by distilling 68% nitric acid with conc. \( H_2SO_4 \) under reduced pressure. The function of \( H_2SO_4 \) is to absorb water.

📝 Teacher's Note: Explain azeotropic mixtures as "stubborn mixtures" that resist separation by normal distillation - like oil and water that naturally want to stay mixed at certain ratios.

🎯 Exam Tip: Remember 68% and 121°C for the azeotropic mixture. The role of H₂SO₄ is water absorption, not acid strengthening.

Solution 8: Answer: Three equations to prove acidic nature of nitric acid are-
(a) Reaction with basic oxide-
\( CaO + 2HNO_3 → Ca(NO_3)_2 + H_2O \)
(b) Reaction with carbonates and hydrogen carbonates-
\( Na_2CO_3 + 2HNO_3 → 2NaNO_3 + H_2O(l) + CO_2 \)
\( NaHCO_3 + HNO_3 → NaNO_3 + H_2O + CO_2 \)
(c) Reaction with metallic sulphites-
\( Na_2SO_3 + 2HNO_3 → 2NaNO_3 + SO_2 + H_2O \)

📝 Teacher's Note: These are classic acid-base reactions that students should recognize as patterns - acids react with bases, carbonates (releasing CO₂), and sulphites (releasing SO₂).

🎯 Exam Tip: Remember the gas products: CO₂ from carbonates and SO₂ from sulphites. These effervescence reactions prove acidic nature.

Solution 9: Answer: Uses of nitric acid are-
(i) Nitric acid is used in the purification of silver, gold, platinum etc. because impurities of other metals are dissolved in it.
(ii) Nitric acid is used as an oxidiser in rocket fuel because it can supply large amount of oxygen.
(iii) Nitric acid is used in the manufacture of dyes, perfumes, drugs etc. from coal tar products since nitrobenzene is one of the raw materials of it which is manufactured from nitric acid.

📝 Teacher's Note: Connect these uses to everyday life - rocket fuel for space exploration, dyes for colorful clothes, and medicines that students might use.

🎯 Exam Tip: Remember the three main categories: metal purification, rocket fuel oxidizer, and manufacturing organic compounds (especially nitrobenzene for dyes).

Solution 10: Answer:
(a) Brown ring test-
Nitric acid in a test tube is taken and then and then freshly prepared ferrous sulphate solution is added. Concentrated \( H_2SO_4 \) is then added carefully down the sides of the test tube, dark brown ring is formed at the junction of two layers.
\( 6 FeSO_4 + 3H_2SO_4 + 2HNO_3 → 3Fe_2(SO_4)_3 + 2NO + 4H_2O \)
\( FeSO_4 + NO + 5H_2O → [Fe(NO)(H_2O)_5]SO_4 \)
Hydrated nitrosoferrous
Sulphate(brown ring)
(b) Freshly prepared ferrous sulphate solution is used in the ring test otherwise Ferrous sulphate undergoes aerial oxidation and converts to ferric sulphate
(c) If the test tube is disturbed brown ring will disappear because the complex formed will get dissolved in the layers of the liquid.

📝 Teacher's Note: Demonstrate this test carefully, emphasizing the slow addition of H₂SO₄ to form distinct layers. The brown ring appears only at the interface.

🎯 Exam Tip: Remember "freshly prepared" ferrous sulphate and the formation of the brown complex at the layer junction. Don't shake the test tube!

Solution 11: Answer:
(a) In laboratory preparation of nitric acid all the apparatus including cork should be made up of glass because nitric acid vapours are highly corrosive in nature and corrodes cork, rubber etc.
(b) Commercial nitric acid is yellow in colour because of presence of nitrogen dioxide but when it is diluted with water, it turns colourless because nitrogen dioxide get dissolved in water.
(c) Aluminum does not react with nitric acid of any concentration because of the formation of extremely thin, unreactive, protective layer of insoluble metallic oxide on the surface of aluminium which stops further reaction.
(d) Concentrated nitric acid renders iron passive because of the formation of extremely thin, unreactive, protective layer of insoluble metallic oxide on the surface which stops further reaction.
(e) Nitric acid is used in the purification of gold because impurities of other metals are dissolved in it.
(f) \( 3HCl + HNO_3 → 2H_2O + NOCl + 2[Cl] \)
Gold in Aqua regia:
\( Au + 4[Cl] → AuCl_3 \)
Platinum in Aqua regia:
\( Pt + 4[Cl] → PtCl_4 \)
(g) Nitric acid usually do not yield hydrogen gas with metals, instead it reacts with metals and form respective nitrates, nitric oxide and water.
(h) Lightning is a blessing as it is the natural source of synthesis of nitric acid.
(i) Concentrated nitric acid is brown in colour due to the presence of nitrogen dioxide formed by heating the acid.
(j) Concentrated nitric acid is a stronger oxidizing agent due to nascent oxygen which it gives on decomposition.
\( 2HNO_3 → H_2O + 2NO_2 + [O] \)
(Conc.) nascent oxygen
\( 2HNO_3 → H_2O + 2NO + 3[O] \)
(Dilute) nascent oxygen

📝 Teacher's Note: Emphasize the protective oxide layer concept for aluminum and iron - like a natural shield that prevents further reaction. This is an important concept in chemistry.

🎯 Exam Tip: Remember that nitric acid doesn't produce hydrogen with metals (unlike other acids), and the brown color comes from dissolved NO₂ gas.

Solution 12:
Answer: Two tests for nitric acid are-
(i) Brown ring test-
Nitric acid in a test tube is taken and then and then freshly prepared ferrous sulphate solution is added. Concentrated \( H_2SO_4 \) is then added carefully down the sides of the test tube, dark brown ring is formed at the junction of two layers.

\( 6 FeSO_4 + 3H_2SO_4 + 2HNO_3 → 3Fe_2(SO_4)_3 + 2NO + 4H_2O \)

\( FeSO_4 + NO + 5H_2O → [Fe(NO)(H_2O)_5]SO_4 \)
Hydrated nitrosoferrous
Sulphate(brown ring)

(ii) Nitric acid on heating gives brown fumes of nitrogen dioxide.

\( 4HNO_3 \xrightarrow{Δ} 2H_2O + 4NO_2 + O_2 \)
Brown fumes
In simple words: Nitric acid can be identified by two tests - the brown ring test where a brown ring forms when mixed with iron sulphate, and when heated it produces brown gas fumes.

📝 Teacher's Note: Demonstrate the brown ring test practically as it's very visual. Emphasize the careful layering technique to avoid mixing the acids too quickly.

🎯 Exam Tip: Always mention the brown ring forms "at the junction" of the two layers - this specific detail shows you understand the test mechanism.

Solution 13:
Answer:
(a) Action of concentrated nitric acid on copper-

\( Cu + 4HNO_3 → Cu(NO_3)_2 + 2NO_2 + 2H_2O \)
(Conc.)

(b) \( 2 AgNO_3 \xrightarrow{Δ} 2 Ag + O_2 + 2 NO_2 \)
In simple words: When nitric acid reacts with copper, it produces copper nitrate and brown nitrogen dioxide gas. Silver nitrate when heated breaks down into silver metal and gases.

📝 Teacher's Note: Show students how the brown fumes are produced when copper reacts with concentrated nitric acid - it's a classic demonstration of nitric acid's oxidizing power.

🎯 Exam Tip: Remember that concentrated nitric acid produces NO₂ (brown gas) while dilute nitric acid produces NO (colorless gas that turns brown in air).

Solution 14:
Answer:
(a) When sodium hydrogen carbonate is added to nitric acid, formation of sodium nitrate, water and carbon dioxide will occur.

\( NaHCO_3 + HNO_3 → NaNO_3 + H_2O + CO_2 \)

(b) When cupric oxide reacts with nitric acid formation of copper nitrate and water will occur.

\( CuO + 2HNO_3 → Cu(NO_3)_2 + H_2O \)

(c) When zinc reacts with dilute nitric acid formation of zinc nitrate, water and nitric oxide will occur.

\( 3Zn + 8HNO_3 → 3Zn(NO_3)_2 + 4H_2O + 2NO \)

(d) When concentrated nitric acid is heated formation of nitrogen dioxide will occur-

\( 4HNO_3 \xrightarrow{Δ} 2H_2O + 4NO_2 + O_2 \)
In simple words: These reactions show nitric acid acting as both an acid (with bases and carbonates) and as an oxidizing agent (with metals), producing different nitrogen compounds depending on concentration.

📝 Teacher's Note: Emphasize the difference between dilute and concentrated nitric acid reactions - dilute gives NO while concentrated gives NO₂. This is a common exam point.

🎯 Exam Tip: When writing equations with nitric acid, always check if it's dilute or concentrated as this determines the nitrogen product formed.

Solution 15:
Answer: Aqua-regia is a mixture of Conc. Nitric acid and conc. Hydrochloric acid in 1:3.

\( 3HCl + HNO_3 → 2H_2O + NOCl + 2[Cl] \)
In simple words: Aqua-regia is a powerful acid mixture that can dissolve even gold because it produces very reactive chlorine atoms that can attack noble metals.

📝 Teacher's Note: Explain that the name "aqua-regia" means "royal water" because it can dissolve gold, the "king of metals." The ratio 3:1 (HCl:HNO₃) is crucial.

🎯 Exam Tip: Always mention the exact ratio 1:3 (HNO₃:HCl) and that it produces nascent chlorine [Cl] which is responsible for dissolving noble metals.

Solution 16:
Answer:
(i) Reddish brown fumes of nitrogen dioxide are observed

(ii) When hydrogen sulphide is bubbled through conc.HNO₃ formation of sulphur, nitric oxide and water will occur.

\( 3 H_2S + 2HNO_3 → 3S + 2NO + 4H_2O \)

(iii) When conc. HNO₃ drops on the skin of a person it reacts with the protein of the skin and forms a yellow compound called xanthoprotein acid. Hence skin becomes yellow. Excess of conc. HNO₃ causes blisters on the skin and is highly corrosive.

(iv) When scrap zinc is heated with conc. HNO₃ formation of zinc nitrate, water and nitrogen dioxide will occur.

\( Zn + 4HNO_3 → Zn(NO_3)_2 + 2H_2O + 2NO_2 \)
In simple words: Concentrated nitric acid is very reactive - it produces brown gas when heated, turns proteins yellow on skin, and reacts with metals and non-metals producing different nitrogen compounds.

📝 Teacher's Note: The xanthoprotein test is a classic way to identify proteins. Emphasize safety when discussing skin contact with concentrated acids.

🎯 Exam Tip: Remember that concentrated HNO₃ produces NO₂ (brown gas) while dilute HNO₃ typically produces NO (colorless gas). The xanthoprotein reaction is specific to concentrated nitric acid.

Solution 17:
Answer:
(a) When sulphur is added to hot and conc. HNO₃ formation of oxide or oxy-acid will occur.

\( S + 6HNO_3 → H_2SO_4 + 2H_2O + 6NO_2 \)

(b) Nitric acid act as oxidizing agent and oxidizes iodide to iodine.

\( HNO_3 + H_2O → H_3O^+ + NO_3^- \)

\( KI → I^- + K^+ \)

\( 2I^- \xrightarrow{-2e^-} I_2 \)

\( 2HNO_3 + H_2O + 2KI → NO_3^- + I_2 + 2 KNO_3 + H_3O^+ \)

(c) Nitric acid ionizes in water to form free hydronium ions and nitrate ions.

\( HNO_3 + H_2O \rightleftharpoons H_3O^+ + NO_3^- \)

(d) When nitric acid is added to washing soda carbon dioxide will form which when passed through a freshly prepared lime water, turns lime water milky.

\( Na_2CO_3 + 2HNO_3 → 2NaNO_3 + H_2O(l) + CO_2 \)

\( Ca(OH)_2 + CO_2 → CaCO_3 + H_2O \)

(e) When limestone reacts with nitric acid-

\( CaCO_3 + 2HNO_3 → Ca(NO_3)_2 + H_2O(l) + CO_2 \)
In simple words: Nitric acid shows different behaviors - it oxidizes elements like sulphur and iodine, acts as an acid with carbonates producing CO₂, and ionizes in water to give H₃O⁺ ions.

📝 Teacher's Note: Connect the lime water test to carbonate identification - this links nitric acid reactions to practical analytical chemistry that students encounter in labs.

🎯 Exam Tip: When nitric acid reacts with carbonates, always write the CO₂ formation and mention the lime water test if asked about identification.

Solution 18:
Answer:
(i) \( C + 4HNO_3 → 2H_2O + 4NO_2 + CO_2 \)
(ii) \( 3SO_2 + 2HNO_3 + 2H_2O → 3H_2SO_4 + 2NO \)
(iii) \( 3Fe + 8HNO_3 (conc.) → 3Fe(NO_3)_3 + 4H_2O + 2NO \)
(iv) \( FeSO_4 + NO + 5H_2O → [Fe(NO)(H_2O)_5]SO_4 \)
(v) \( Pt + HNO_3 + HCl → PtCl_4 \)
(vi) \( 2HNO_3 + 6FeSO_4 + 3H_2SO_4 → 3Fe_2(SO_4)_3 + 2NO + 4H_2O \)
In simple words: These equations show nitric acid reacting with various substances - carbon, sulfur dioxide, iron, and in complex formation reactions, always acting as a strong oxidizing agent.

📝 Teacher's Note: Point out that equation (iv) is the brown ring test reaction and equation (v) shows how aqua-regia dissolves platinum. These are important practical applications.

🎯 Exam Tip: Memorize the brown ring test equation (iv) as it's frequently asked. Also remember that platinum requires aqua-regia, not just nitric acid alone.

Solution 19:
Answer:

In simple words: Dilute nitric acid reacts with copper producing brown gas and gives a brown ring test, while dilute hydrochloric acid shows no reaction with copper and no brown ring test.

📝 Teacher's Note: Use this comparison to emphasize that nitric acid is an oxidizing acid while hydrochloric acid is a non-oxidizing acid. This explains why HCl cannot dissolve copper.

🎯 Exam Tip: When comparing acids, always mention oxidizing vs non-oxidizing nature - this explains most differences in their reactions with metals.

Solution 1991-1:
Answer: Gold will dissolve only in aqua regia i.e., a mixture of conc. hydrochloric acid and conc. nitric acid in 1:3 but copper will dissolve in nitric acid.

To separate gold from a mixture of gold and platinum add some nitric acid to the mixture, gold will remain undissolved hence can be filtered.
In simple words: Gold is so unreactive that only aqua-regia can dissolve it, while copper dissolves in simple nitric acid, so we can use nitric acid to separate them.

📝 Teacher's Note: Explain that this is actually how gold is refined industrially - the principle of selective dissolution based on reactivity differences.

🎯 Exam Tip: Remember the ratio for aqua-regia is 1:3 (HNO₃:HCl) and that gold's unreactivity is key to its separation from other metals.

Solution 1991-2:
Answer: \( KNO_3 + H_2SO_4 \xrightarrow{200°C} KHSO_4 + HNO_3 \)
In simple words: Potassium nitrate reacts with sulfuric acid when heated to produce nitric acid - this is one way to make nitric acid in the lab.

📝 Teacher's Note: This is the classic laboratory method for preparing nitric acid. Emphasize that concentrated H₂SO₄ is used because it has a higher boiling point than HNO₃.

🎯 Exam Tip: Remember the temperature (200°C) and that this method works because H₂SO₄ is less volatile than HNO₃.

Solution 1991-3:
Answer: Balanced equation: \( Cu + 4HNO_3 \xrightarrow{Δ} Cu(NO_3)_2 + 2NO_2 + 2H_2O \)

Word equation: copper + nitric acid \( \xrightarrow{Δ} \) copper nitrate + nitrogen dioxide + water
In simple words: When copper metal is heated with nitric acid, it forms copper nitrate salt, brown nitrogen dioxide gas, and water.

📝 Teacher's Note: This is a classic redox reaction where nitric acid acts as an oxidizing agent. Point out that copper goes from 0 to +2 oxidation state.

🎯 Exam Tip: Always balance the equation carefully - 1 Cu needs 4 HNO₃ to produce 2 NO₂. The brown gas formation is a key observation to mention.

Solution 1992-1:
Answer: In the laboratory preparation of nitric acid, the mixture of concentrated sulphuric acid and sodium nitrate should not be heated very strongly, above 200°C because a higher temperature can cause following problems:

1. Nitric acid would decompose to form nitrogen dioxide.
2. Sodium sulphate is formed which may stick to the glass and cannot be removed easily
3. The glass apparatus may break.
In simple words: If we heat too strongly when making nitric acid, the acid breaks down, unwanted salts form that stick to glass, and the equipment might crack from high temperature.

📝 Teacher's Note: Connect this to practical laboratory safety and equipment care. Students should understand that controlled heating is crucial in chemical preparations.

🎯 Exam Tip: Mention all three problems when asked why gentle heating is required - decomposition, salt formation, and equipment damage.

Solution 1992-2:
Answer: \( Cu + 4HNO_3 → Cu(NO_3)_2 + 2NO_2 + 2H_2O \)
In simple words: Copper reacts with nitric acid to form copper nitrate, nitrogen dioxide gas, and water.

📝 Teacher's Note: This is the same reaction as 1991-3, showing how fundamental reactions appear repeatedly in exams from different years.

🎯 Exam Tip: This Cu + HNO₃ reaction is extremely common in exams - memorize the balanced equation and products.

Solution 1992-3:
Answer: Commercial nitric acid is yellow in colour because of presence of nitrogen dioxide but when it is diluted with water, it turns colourless because nitrogen dioxide gets dissolved in water.

Brown ring test can be used to test nitric acid:

Brown ring test: Nitric acid in a test tube is taken and then and then freshly prepared ferrous sulphate solution is added. Concentrated \( H_2SO_4 \) is then added carefully down the sides of the test tube, dark brown ring is formed at the junction of two layers.

\( 6 FeSO_4 + 3H_2SO_4 + 2HNO_3 → 3Fe_2(SO_4)_3 + 2NO_2 + 4H_2O \)

\( FeSO_4 + NO + 5H_2O → [Fe(NO)(H_2O)_5]SO_4 \)
Hydrated nitrosoferrous
Sulphate(brown ring)
In simple words: Commercial nitric acid looks yellow because it contains nitrogen dioxide gas dissolved in it. When diluted, this gas escapes and the acid becomes colorless. The brown ring test confirms the presence of nitric acid.

📝 Teacher's Note: Explain that the yellow color comes from partial decomposition during storage. This is why fresh nitric acid is colorless while old samples are yellow.

🎯 Exam Tip: Always connect the yellow color to NO₂ presence and mention that dilution removes this color by dissolving the gas.

Solution 1992-4:
Answer: Oxidation of ammonia in catalytic chamber.

\( 4NH_3 + 5O_2 \xrightarrow{\text{Rhodium gauze}_{900°C}} 4NO + 6H_2O + Heat \)
In simple words: Ammonia gas is mixed with oxygen and passed over a hot rhodium catalyst to produce nitric oxide, which is then further processed to make nitric acid.

📝 Teacher's Note: This is the Ostwald process step 1. Emphasize the industrial importance and the specific conditions - rhodium catalyst and 900°C.

🎯 Exam Tip: Remember the catalyst (rhodium gauze) and temperature (900°C) for full marks. This is the first step in the Ostwald process.

Solution 1994-1:
Answer: Copper is heated with nitric acid, they react together to produce nitrogen dioxide.

\( Cu + 4HNO_3 \xrightarrow{Δ} Cu(NO_3)_2 + 2NO_2 + 2H_2O \)
In simple words: When copper metal is heated with nitric acid, brown nitrogen dioxide gas is produced along with copper nitrate and water.

📝 Teacher's Note: Again the same fundamental reaction - this shows students how important it is to master key reactions as they appear repeatedly.

🎯 Exam Tip: The word "heated" in the question is key - make sure to include the heat symbol (Δ) in your equation.

Solution 1994-2:
Answer: \( Cu + 4HNO_3 \xrightarrow{Δ} Cu(NO_3)_2 + 2NO_2 + 2H_2O \)
In simple words: The balanced equation for copper reacting with nitric acid when heated.

📝 Teacher's Note: Students should notice this is the fourth time this exact reaction has appeared - emphasize mastering fundamental reactions.

🎯 Exam Tip: This reaction is clearly a favorite in exams - ensure perfect balancing and include the heat symbol.

Solution 1994-3:
Answer: The nitric acid is manufactured by Ostwald process. In ostwald process ammonia gas and air are taken in 1:10 ratio. Temperature is maintained at 800°C I the catalytic chamber, as the reaction is exothermic so the heat evolved maintains the temperature in the catalytic chamber.
In simple words: Nitric acid is made industrially by the Ostwald process where ammonia and air react at high temperature with a catalyst. The reaction produces its own heat to keep going.

📝 Teacher's Note: Emphasize the 1:10 ratio of NH₃:air and that the reaction is self-sustaining due to being exothermic. This is brilliant industrial chemistry.

🎯 Exam Tip: Remember the ratio (1:10), temperature (800°C), and that it's exothermic. These specific details earn full marks.

Solution 1994-4:
Answer: \( KNO_3 + H_2SO_4 \xrightarrow{200°C} KHSO_4 + HNO_3 \)
In simple words: Potassium nitrate reacts with sulfuric acid when heated to 200°C to produce nitric acid and potassium hydrogen sulfate.

📝 Teacher's Note: This is the second time this laboratory preparation method appears, showing its importance in nitric acid chemistry.

🎯 Exam Tip: Always include the temperature (200°C) and remember that this method works because H₂SO₄ is less volatile than HNO₃.

Solution 1995-1:

Answer: During a thunderstorm, the rainwater contains nitric acid. The nitric acid is formed as a result of three chemical reactions which is described as follows:

1. During lightning discharge, nitrogen in atmosphere combines with oxygen to form nitric oxide.

\( N_2 + O_2 \xrightarrow{lightning} 2NO \)

2. Nitric oxide is further oxidized to nitrogen dioxide.

\( 2NO + O_2 \rightarrow 2NO_2 \)

3. Nitrogen dioxide dissolves in moisture or rain water to from nitric acid.

\( 4NO_2 + 2H_2O + O_2 \rightarrow 4HNO_3 \)

In simple words: During lightning, nitrogen and oxygen from air combine step by step to eventually form nitric acid, which mixes with rainwater. This is why rain during thunderstorms is slightly acidic.

📝 Teacher's Note: Use the analogy of a chemical factory in the sky - lightning provides the energy to start reactions that normally wouldn't happen at room temperature. Emphasize the three-step process clearly.

🎯 Exam Tip: Always write all three balanced equations in sequence and mention that lightning provides the activation energy for the first step.

Solution 1997-1:

Answer: Lead nitrate is a soluble salt. On heating lead nitrate the following reaction occurs:

\( 2Pb(NO_3)_2 (s) \xrightarrow{\Delta} 2PbO (s) + 4NO_2 (g) + O_2 (g) \)

Preparation of nitric acid from potassium nitrate:

\( KNO_3 + H_2SO_4 \xrightarrow{200°C} KHSO_4 + HNO_3 \)

In simple words: When you heat lead nitrate, it breaks down into lead oxide and releases brown nitrogen dioxide gas and oxygen. Similarly, potassium nitrate reacts with sulfuric acid to produce nitric acid.

📝 Teacher's Note: Demonstrate the thermal decomposition with a test tube - students can observe the brown fumes. Explain that nitrates of heavy metals decompose differently than alkali metal nitrates.

🎯 Exam Tip: Remember that lead nitrate gives PbO + NO₂ + O₂, while the preparation method uses concentrated H₂SO₄ at 200°C - mention the temperature.

Solution 1998-1:

Answer: \( N_2 + O_2 \xrightarrow{lightning} 2NO \)

In simple words: Nitrogen and oxygen from the air combine during lightning to form nitric oxide gas - this is the first step in making nitric acid naturally.

📝 Teacher's Note: Emphasize that this reaction needs very high energy (lightning) because nitrogen has a very strong triple bond that's hard to break under normal conditions.

🎯 Exam Tip: Always mention "lightning" as the condition above the arrow - it's the key energy source for this reaction.

Solution 1999-1:

Answer: When concentrated nitric acid is added to copper brown fumes of nitrogen dioxide are observed.

In simple words: When you add strong nitric acid to copper metal, it produces brown-colored gas (nitrogen dioxide) that you can see rising from the reaction mixture.

📝 Teacher's Note: This is a classic demonstration reaction. The brown fumes are very distinctive and help students identify NO₂ gas. Always do this in a fume hood.

🎯 Exam Tip: The key observation is "brown fumes" - this is the characteristic test for nitrogen dioxide gas formation.

Solution 2000-1:

Answer: When concentrated nitric acid is added to copper brown fumes of nitrogen dioxide are observed.

In simple words: This is the same reaction as above - copper reacts with concentrated nitric acid to produce visible brown gas (nitrogen dioxide).

📝 Teacher's Note: Connect this to the previous answer - it's the same concept but repeated to reinforce the important observation of brown fumes.

🎯 Exam Tip: This reaction also produces Cu(NO₃)₂ and water, but the brown fumes are the most important observation to mention.

Solution 2001-1:

Answer: 1. Nitrogen dioxide
2. Ammonia gas

In simple words: These are two important nitrogen-containing gases - nitrogen dioxide is brown and toxic, while ammonia gas has a sharp smell and is basic in nature.

📝 Teacher's Note: Compare the properties of these gases - NO₂ is acidic and brown, while NH₃ is basic and colorless with a pungent smell. Good for discussing acid-base properties.

🎯 Exam Tip: Remember the key identifying features: NO₂ = brown gas, NH₃ = sharp smell and turns red litmus blue.

Solution 2001-2:

Answer: (i) concentrated
(ii) \( S + 6HNO_3 \rightarrow H_2SO_4 + 2H_2O + 6NO_2 \)

In simple words: The first part asks for concentrated acid, and the second shows how sulfur reacts with nitric acid to form sulfuric acid, water, and nitrogen dioxide gas.

📝 Teacher's Note: Emphasize that concentrated HNO₃ is needed for this oxidation reaction. Show how sulfur (oxidation state 0) gets oxidized to +6 in H₂SO₄.

🎯 Exam Tip: Balance the equation carefully - 6 HNO₃ molecules are needed, producing 6 NO₂ molecules and 2 H₂O molecules.

Solution 2001-3:

Answer: (i) \( Na_2CO_3 + 2HNO_3 \rightarrow 2NaNO_3 + H_2O + CO_2 \)
(ii) \( Cu + 4HNO_3 \xrightarrow{\Delta} Cu(NO_3)_2 + 2NO_2 + 2H_2O \)

In simple words: The first reaction shows an acid-base reaction where sodium carbonate neutralizes nitric acid. The second shows copper metal being dissolved by nitric acid with heat, producing brown gas.

📝 Teacher's Note: Contrast these two reactions - one is simple acid-base neutralization, the other is oxidation of a metal. Point out the different products formed.

🎯 Exam Tip: For the copper reaction, always include the delta symbol for heat and remember that concentrated HNO₃ is needed to produce NO₂.

Solution 2002-1:

Answer: For laboratory preparation of nitric acid potassium nitrate or sodium nitrate and conc. Sulphuric acid is required.

In simple words: To make nitric acid in the lab, you mix either potassium nitrate or sodium nitrate with concentrated sulfuric acid and heat the mixture.

📝 Teacher's Note: Explain that this is a displacement reaction - the stronger acid (H₂SO₄) displaces the weaker acid (HNO₃) from its salt. Use the general principle of strong acids displacing weak ones.

🎯 Exam Tip: Mention both possible nitrates (KNO₃ or NaNO₃) and emphasize that concentrated H₂SO₄ is essential - dilute won't work.

Solution 2002-2:

Answer: Yellowish brown colour of nitric acid is due to the presence of nitrogen dioxide formed due to thermal decomposition of nitric acid.

In simple words: Pure nitric acid should be colorless, but it often looks yellowish-brown because some of it breaks down to form nitrogen dioxide, which is brown.

📝 Teacher's Note: Connect this to storage conditions - nitric acid should be kept in dark, cool places to prevent decomposition. Light and heat speed up the breakdown reaction.

🎯 Exam Tip: Always mention that pure HNO₃ is colorless, and the brown color comes from NO₂ impurity due to decomposition.

Solution 2002-3:

Answer: (i) \( Cu + 4HNO_3 \xrightarrow{\Delta} Cu(NO_3)_2 + 2NO_2 + 2H_2O \)
(ii) \( CuO + 2HNO_3 \rightarrow Cu(NO_3)_2 + H_2O \)

In simple words: Copper metal reacts with nitric acid to produce copper nitrate, brown gas, and water. Copper oxide simply neutralizes nitric acid to form copper nitrate and water.

📝 Teacher's Note: Compare these reactions - the metal needs heating and produces gas, while the oxide reacts easily at room temperature without gas formation. Good example of how oxidation state affects reactivity.

🎯 Exam Tip: The metal reaction needs heat (Δ) and produces NO₂, while the oxide reaction is simple acid-base neutralization with no gas evolved.

Solution 2002-4:

Answer: Platinum glaze is used as catalyst in the manufacture for nitric acid

In simple words: In the industrial production of nitric acid, platinum is used as a catalyst to speed up the chemical reactions without getting used up in the process.

📝 Teacher's Note: Explain the Ostwald process where platinum gauze catalyzes the oxidation of ammonia to nitric oxide. Platinum works because it's unreactive but speeds up the reaction.

🎯 Exam Tip: Mention that this is part of the Ostwald process for industrial nitric acid production - platinum catalyzes NH₃ oxidation to NO.

Solution 2003-1:

Answer: The apparatus used is made up of glass as nitric acid is highly corrosive in nature.

In simple words: Glass equipment is used for nitric acid preparation because nitric acid can eat through (corrode) most metals, but glass is safe and doesn't react.

📝 Teacher's Note: Emphasize safety - nitric acid attacks most metals and many other materials. Glass is chemically inert and transparent, making it ideal for laboratory work.

🎯 Exam Tip: Always mention that HNO₃ is "highly corrosive" and attacks metals - this explains why glass apparatus is essential.

Solution 2003-2:

Answer: \( KNO_3 + H_2SO_4 \xrightarrow{200°C} KHSO_4 + HNO_3 \)

In simple words: Potassium nitrate reacts with sulfuric acid at 200°C to produce potassium hydrogen sulfate and nitric acid, which can be distilled off.

📝 Teacher's Note: This is the standard laboratory method. Point out that the temperature is important - too low won't work, too high might decompose the HNO₃ formed.

🎯 Exam Tip: Always include the temperature (200°C) above the arrow - it's essential for this reaction to proceed.

Solution 2005-1:

Answer: \( S + 6HNO_3 \rightarrow H_2SO_4 + 2H_2O + 6NO_2 \)

In simple words: Sulfur reacts with nitric acid to form sulfuric acid, water, and brown nitrogen dioxide gas - this shows the oxidizing power of nitric acid.

📝 Teacher's Note: This is a good example of nitric acid acting as an oxidizing agent. Sulfur goes from oxidation state 0 to +6, while nitrogen goes from +5 to +4.

🎯 Exam Tip: Balance carefully - 6 HNO₃ needed for 1 sulfur atom, producing 6 NO₂ molecules. Remember this shows HNO₃ as an oxidizing agent.

Solution 2005-2:

Answer: (i) Dilute nitric acid is generally considered to be a typical acid except for its reaction with metals, nitric acid did not evolve hydrogen gas on reacting with any metal, other than manganese and magnesium.
(ii) \( Cu + 4HNO_3 \xrightarrow{\Delta} Cu(NO_3)_2 + 2NO_2 + 2H_2O \)
(iii) In a glass apparatus nitric acid gets decomposed by sunlight. Yellowish brown colour of nitric acid is due to the presence of nitrogen dioxide formed due to thermal decomposition of nitric acid.

In simple words: Nitric acid is special because unlike other acids, it doesn't produce hydrogen gas with most metals. It also breaks down in sunlight forming brown nitrogen dioxide, which gives it a yellowish color.

📝 Teacher's Note: This highlights the unique properties of HNO₃ - its oxidizing nature prevents H₂ evolution, and its instability to light and heat. Good for comparing with other acids.

🎯 Exam Tip: Key points: no H₂ gas with metals (except Mn and Mg), decomposes in sunlight, brown color due to NO₂ impurity.

Solution 2006-1:

Answer: (i) Nitric acid is highly corrosive in nature and corrodes cork, rubber, etc.
(ii) Nitric acid ionizes in water to form free hydronium ions and nitrate ions. This shows that nitric acid is acidic in nature.

In simple words: Nitric acid is very reactive and can damage materials like cork and rubber. When it dissolves in water, it breaks apart to form ions, which is why it acts as an acid.

📝 Teacher's Note: Connect part (i) to practical laboratory safety and part (ii) to the theory of acids. Show the ionization equation: HNO₃ + H₂O → H₃O⁺ + NO₃⁻.

🎯 Exam Tip: For (i) mention specific materials it corrodes, for (ii) explain ionization produces H₃O⁺ ions which make it acidic.

Solution 2007-1:

Answer: (i) A = sulphuric acid, B = potassium nitrate, C = nitric acid
(ii) \( 4HNO_3 \rightarrow 4NO_2 + 2H_2O + O_2 \)
(iii) \( Cu + 4HNO_3 \xrightarrow{\Delta} Cu(NO_3)_2 + 2NO_2 + 2H_2O \)

In simple words: The chemicals are identified as sulfuric acid, potassium nitrate, and nitric acid. Nitric acid decomposes to form nitrogen dioxide, water and oxygen. Copper reacts with nitric acid to produce copper nitrate, brown gas and water.

📝 Teacher's Note: This tests understanding of the laboratory preparation of HNO₃ and its decomposition. Link the decomposition reaction to why HNO₃ develops a brown color on storage.

🎯 Exam Tip: For the decomposition, balance carefully: 4 HNO₃ gives 4 NO₂ + 2 H₂O + 1 O₂. Include heat symbol (Δ) for the copper reaction.

Solution 2007-2:

Answer: If nitric acid is kept in a bottle for long time then it will become brown in colour.

In simple words: When nitric acid sits in a bottle for a long time, it slowly breaks down and turns brown because it forms nitrogen dioxide gas which dissolves back into the acid.

📝 Teacher's Note: This demonstrates the instability of HNO₃. Emphasize that pure HNO₃ is colorless, and proper storage (dark, cool) minimizes decomposition.

🎯 Exam Tip: Mention that the brown color is due to NO₂ formed from decomposition - this connects to the thermal decomposition equation.

Solution 2008-1:

Answer: \( AlN + 3H_2O \rightarrow Al(OH)_3 + NH_3 \)

In simple words: Aluminum nitride reacts with water to form aluminum hydroxide and ammonia gas - this is a hydrolysis reaction.

📝 Teacher's Note: This is an example of hydrolysis of a metal nitride. Compare with other nitrides like Mg₃N₂ which behave similarly. Good for showing how non-metal nitrides differ from metal nitrides.

🎯 Exam Tip: Balance the equation correctly - 1 AlN needs 3 H₂O to produce 1 Al(OH)₃ and 1 NH₃.

Solution 2008-2:

In simple words: The Haber process uses iron catalyst to make ammonia from nitrogen and hydrogen. The Ostwald process uses platinum catalyst to convert ammonia and air into nitric acid.

📝 Teacher's Note: These are the two most important industrial processes for nitrogen compounds. Emphasize the connection - ammonia from Haber process is used as input for Ostwald process.

🎯 Exam Tip: Remember the catalysts: Iron for Haber process (N₂ + H₂ → NH₃), Platinum for Ostwald process (NH₃ + O₂ → HNO₃).

Solution 2009-1:

Answer: oxygen gas is evolved on heating sodium nitrate.

In simple words: When you heat sodium nitrate, it breaks down and releases oxygen gas along with forming sodium nitrite.

📝 Teacher's Note: This is thermal decomposition of an alkali metal nitrate. Compare with thermal decomposition of other metal nitrates to show the pattern based on metal reactivity.

🎯 Exam Tip: The complete reaction is: 2NaNO₃ → 2NaNO₂ + O₂. Alkali metal nitrates give nitrites + O₂, unlike heavy metal nitrates.