Selina Concise Solutions for ICSE Class 10 Chemistry Chapter 4 Analytical Chemistry

Exercise 1(A)

Question 1. Write the probable colour of the following salts.
(a) Ferrous salts
(b) Ammonium salts
(c) Cupric salts
(d) Calcium salts
(e) Aluminium Salts
Answer:
(a) Ferrous salts: Light green
(b) Ammonium salts: Colourless
(c) Cupric salts: Blue
(d) Calcium salts: Colourless
(e) Aluminium salts: Colourless
In simple words: Different metal salts have characteristic colours that help identify them - ferrous salts are light green like fresh leaves, cupric salts are blue like the sky, while ammonium, calcium and aluminium salts are colourless like water.

📝 Teacher's Note: Use actual salt samples or pictures to show students these colours. Create a colour chart that students can refer to during practicals. Common mistake: students confuse ferric (reddish-brown) with ferrous (light green) salts.

🎯 Exam Tip: Remember the mnemonic "Fresh Copper Blue" for ferrous-green, copper-blue. Always specify the exact colour term given in the syllabus - "light green" not just "green".

Question 2. Name:
(a) a metallic hydroxide soluble in excess of \( NH_4OH \).
(b) a metallic oxide soluble in excess of caustic soda solution.
(c) a strong alkali
(d) a weak alkali
(e) two coloured metal ions
(f) two coloured metal ions
(g) a metal that evolves a gas which burns with a pop sound when boiled with alkali solutions.
(h) two bases which are not alkalis but dissolves in alkalis to yield colourless solutions.
(j) a coloured cation not a representative element.
Answer:
(a) \( Cu(OH)_2 \)
(b) \( ZnO \)
(c) \( NaOH \)
(d) \( NH_4OH \)
(e) \( Na^+ \), \( Ca^{2+} \)
(f) \( Fe^{2+} \), \( Mn^{2+} \)
(g) Aluminium
(h) \( Zn(OH)_2 \) and \( Al(OH)_3 \)
(i) \( PbO \)
(j) Ammonium ion
In simple words: This question tests knowledge of different chemical behaviours - some substances dissolve in excess reagents (amphoteric behaviour), some are strong or weak bases, and some metals produce hydrogen gas that makes a 'pop' sound when it burns.

📝 Teacher's Note: Demonstrate the 'pop' test for hydrogen gas in class. Explain amphoteric behaviour using the analogy of substances that can act like both acids and bases depending on what they react with.

🎯 Exam Tip: For part (g), always mention the 'pop' sound test specifically - it's the characteristic test for hydrogen gas. Remember that amphoteric oxides and hydroxides dissolve in both acids and bases.

Question 3. Write balanced equations for Q.2 (g) and (i)
Answer:
\( 2Al + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2 \)
(Hot and conc.) Sodium meta aluminate
(colourless)

\( PbO + 2NaOH \rightarrow Na_2PbO_2 + H_2O \)
(Yellow) sodium plumbate
(colourless, soluble)
In simple words: When aluminium reacts with hot concentrated sodium hydroxide, it produces hydrogen gas (which gives the pop sound) and forms a colourless sodium compound. Lead oxide dissolves in sodium hydroxide to form a colourless solution.

📝 Teacher's Note: Emphasize that aluminium needs hot and concentrated alkali to react. Show students how to balance equations by counting atoms on both sides. Point out the colour changes during reactions.

🎯 Exam Tip: Always mention the conditions (hot and concentrated) for aluminium reaction. Include the colour of products in brackets - examiners look for these observational details.

Question 4. What happens when ammonia solution is added first dropwise and then in excess to the following solution:
(i) \( CuSO_4 \) (ii) \( ZnSO_4 \) (iii) \( FeCl_3 \)
Write balanced equations for these reactions.
Answer:
(i)
\( CuSO_4 + 2NH_4OH \rightarrow Cu(OH)_2 \downarrow + (NH_4)_2SO_4 \)
Blue pale blue ppt. colourless solution
With excess of \( NH_4OH \), ppt dissolves
\( Cu(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH \rightarrow [Cu(NH_3)_4]SO_4 + 4H_2O \)
Excess Tetrammine Copper(II) Sulphate

(ii)
\( ZnSO_4 + 2NH_4OH \rightarrow Zn(OH)_2 + (NH_4)_2SO_4 \)
Colourless white, gelatinous ppt colourless
With excess of \( NH_4OH \), ppt dissolves
\( Zn(OH)_2 + (NH_4)_2SO_4 + 2NH_4OH \rightarrow [Zn(NH_3)_4]SO_4 + 4H_2O \)
(excess) Tetramminezinc(II) Sulphate
(colourless)

(iii)
\( FeCl_3 + 3NH_4OH \rightarrow Fe(OH)_3 \downarrow + 3NH_4Cl \)
Yellow solution reddish brown ppt. colourless solution
In simple words: Adding ammonia solution first forms coloured precipitates (solid particles), but when you add more ammonia, some precipitates dissolve to form complex compounds while others remain as precipitates.

📝 Teacher's Note: Demonstrate this step-by-step in lab - students need to see the colour changes and precipitate formation/dissolution. Explain complex ion formation using simple analogies like molecules holding hands.

🎯 Exam Tip: Always describe both steps - initial precipitate formation and then what happens with excess reagent. Use arrows (↓) to show precipitate formation and mention all colour changes.

Question 5. What do you observe when caustic soda solution is added to the following solution, first a little and then in excess:
(a) \( FeCl_3 \)
(b) \( ZnSO_4 \)
(c) \( Pb(NO_3)_2 \)
(d) \( CuSO_4 \)
Write balanced equations for these reactions.
Answer:
(i) \( FeCl_3 + 3NaOH \rightarrow Fe(OH)_3 \downarrow + 3NaCl \)
Yellow reddish brown ppt colourless solution
In excess of alkali, the reddish brown ppt. of \( Fe(OH)_3 \) remains insoluble

(ii) \( ZnSO_4 + 2NaOH \rightarrow Zn(OH)_2 \downarrow + Na_2SO_4 \)
Colourless white gelatinous ppt. colourless
In excess of alkali, white gelatinous ppt. of \( Zn(OH)_2 \) becomes soluble
\( Zn(OH)_2 + 2NaOH \text{ (Excess)} \rightarrow Na_2ZnO_2 + 2H_2O \)
Sodium zincate (colourless)

(iii) \( Pb(NO_3)_2 + 2NaOH \rightarrow Pb(OH)_2 \downarrow + 2NaNO_3 \)
White ppt (colourless)
In excess of alkali, white precipitate of \( Pb(OH)_2 \) becomes soluble:
\( Pb(OH)_2 + 2NaOH \text{(excess)} \rightarrow Na_2PbO_2 + 2H_2O \)
Sodium plumbate (colourless)

\( CuSO_4 + 2NaOH \rightarrow Cu(OH)_2 \downarrow + Na_2SO_4 \)
Blue colourless pale blue ppt. (colourless)
In excess of alkali, pale blue precipitate of \( Cu(OH)_2 \) is insoluble
In simple words: When you add a little caustic soda, precipitates form. But when you add more caustic soda, some precipitates dissolve (showing amphoteric behaviour) while others like iron and copper hydroxides stay as precipitates.

📝 Teacher's Note: Use this to teach amphoteric behaviour - zinc and lead hydroxides dissolve in excess alkali, but iron and copper hydroxides don't. Create a comparison table showing which hydroxides are amphoteric.

🎯 Exam Tip: Remember that only zinc and lead hydroxides dissolve in excess alkali among these examples. Always state whether excess alkali dissolves the precipitate or not - this is a key marking point.

Question 6. Name the chloride of a metal which is soluble in excess of ammonium hydroxide. Write equation for the same.
Answer:
Zinc chloride (\( ZnCl_2 \)) is soluble in excess of ammonium hydroxide.
\( ZnCl_2 + 2NH_4OH \rightarrow Zn(OH)_2 \downarrow + 2NH_4Cl \)
Colourless White gelatinous ppt.
With excess of \( NH_4OH \) ppt dissolves
\( Zn(OH)_2 + 2NH_4Cl + 2NH_4OH \text{ (excess)} \rightarrow [Zn(NH_3)_4]Cl_2 + 4H_2O \)
Tetrammine zinc (II) Chloride
Colourless
In simple words: Zinc chloride first forms a white precipitate with ammonia solution, but when you add more ammonia, the precipitate dissolves to form a colourless complex compound.

📝 Teacher's Note: This demonstrates complex ion formation - zinc ions bind with ammonia molecules to form a soluble complex. Draw the structure of the tetrammine complex to show students how four ammonia molecules surround the zinc ion.

🎯 Exam Tip: Write both equations - precipitate formation first, then dissolution in excess reagent. Always name the complex compound formed (tetrammine zinc chloride).

Question 7. On adding dilute ammonia solution to a colourless solution of a salt, a white gelatinous precipitate appears. This precipitate however dissolves on addition of excess of ammonia solution identify (choose from Na, Al, Zn, Pb, Fe)
(a) Which metal salt solution was used?
(b) what is the formula of the white gelatinous precipitate obtained?
Answer:
(a) \( ZnCl_2 \)
(b) \( Zn(OH)_2 \)
In simple words: The clues are: colourless solution, white gelatinous precipitate that dissolves in excess ammonia. This pattern uniquely identifies zinc salts and zinc hydroxide precipitate.

📝 Teacher's Note: Teach students to use observation clues systematically - colourless solution eliminates copper and iron salts, gelatinous precipitate suggests zinc, and dissolution in excess ammonia confirms zinc's amphoteric nature.

🎯 Exam Tip: Look for key words like "gelatinous" (suggests zinc) and "dissolves in excess" (confirms amphoteric behaviour). Process of elimination works well for such identification questions.

Question 8. Name:
(a) a yellow monoxide that dissolves in hot and concentrated caustic alkali
(b) a white, insoluble oxide that dissolves when fused with caustic soda or caustic potash
(c) a compound containing zinc in the anion
Answer:
(a) \( PbO \)
(b) \( ZnO \)
(c) \( K_2ZnO_2 \)
In simple words: Lead oxide is yellow and dissolves in hot alkali, zinc oxide is white and dissolves when heated strongly with alkali, and potassium zincate has zinc as part of the negative ion.

📝 Teacher's Note: Explain the concept of anions and cations clearly. In \( K_2ZnO_2 \), the \( ZnO_2^{2-} \) is the anion containing zinc. This is unusual since zinc is typically found in cations.

🎯 Exam Tip: Remember colour clues - yellow monoxide is always lead oxide. For part (c), think of compounds like sodium zincate or potassium zincate where zinc forms part of the complex anion.

Question 9. What do you observe when freshly precipitated aluminium hydroxide reacts with caustic soda solution? Give balanced equation.
Answer:
When freshly precipitated aluminum hydroxide reacts with caustic soda solution, white salt of sodium meta aluminate is obtained.
\( Al(OH)_3 + NaOH \rightarrow NaAlO_2 + 2H_2O \)
Sodium meta aluminate
In simple words: The white precipitate of aluminium hydroxide dissolves in sodium hydroxide solution to form a colourless solution of sodium meta aluminate, showing aluminium's amphoteric nature.

📝 Teacher's Note: This demonstrates amphoteric behaviour of aluminium - it acts as an acid when reacting with a base. Compare this with the reaction of aluminium with acids to show its dual nature.

🎯 Exam Tip: Always mention that the precipitate dissolves and specify the product name (sodium meta aluminate). This reaction shows why aluminium hydroxide is called amphoteric.

Question 10. What do you understand by amphoteric oxide Give the balanced equations for the reaction with three different amphoteric oxides with a caustic alkali. Write you observation if any.
Answer:
Amphoteric oxides are oxides that can react with both acids and bases to form salts. They show dual behaviour - acting as acids when reacting with bases and as bases when reacting with acids. Examples include zinc oxide, aluminium oxide, and lead oxide.

Three equations with caustic alkali:
1. \( ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O \) (Sodium zincate - colourless)
2. \( Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O \) (Sodium aluminate - colourless)
3. \( PbO + 2NaOH \rightarrow Na_2PbO_2 + H_2O \) (Sodium plumbate - colourless)
In simple words: Amphoteric oxides are like substances that can behave as both acids and bases depending on what they react with - like a person who can speak both languages depending on who they're talking to.

📝 Teacher's Note: Use the analogy of a bilingual person to explain amphoteric behaviour. Show students how these oxides dissolve in both acids and bases, unlike normal oxides that only dissolve in one type.

🎯 Exam Tip: Define amphoteric first, then give three clear examples with balanced equations. Always mention that all products are colourless solutions, showing complete dissolution.

Question 11. Distinguish by adding:
(a) sodium hydroxide solution and
(b) Ammonium hydroxide solution to
(i) Calcium salt solution and lead salt solution
(ii) Lead salt solution and ferrous salt solution
(iii) copper salt solution and ferrous salt solution
(iv) Fe (II) salt solution and Fe (III) Salt solution
(v) Ferrous nitrate and lead nitrate
Answer:
(i) Calcium salt vs Lead salt:
With NaOH: Calcium forms white ppt that doesn't dissolve in excess; Lead forms white ppt that dissolves in excess NaOH
With \( NH_4OH \): Calcium forms white ppt insoluble in excess; Lead forms white ppt insoluble in excess \( NH_4OH \)

(ii) Lead salt vs Ferrous salt:
With NaOH: Lead gives white ppt soluble in excess; Ferrous gives dirty green ppt insoluble in excess
With \( NH_4OH \): Lead gives white ppt insoluble in excess; Ferrous gives dirty green ppt insoluble in excess

(iii) Copper salt vs Ferrous salt:
With NaOH: Copper gives pale blue ppt insoluble in excess; Ferrous gives dirty green ppt insoluble in excess
With \( NH_4OH \): Copper gives pale blue ppt that dissolves in excess to form deep blue solution; Ferrous gives dirty green ppt insoluble in excess

(iv) Fe(II) vs Fe(III):
With NaOH: Fe(II) gives dirty green ppt; Fe(III) gives reddish brown ppt
With \( NH_4OH \): Fe(II) gives dirty green ppt; Fe(III) gives reddish brown ppt

(v) Ferrous nitrate vs Lead nitrate:
With NaOH: Ferrous gives dirty green ppt insoluble in excess; Lead gives white ppt soluble in excess
With \( NH_4OH \): Ferrous gives dirty green ppt insoluble in excess; Lead gives white ppt insoluble in excess
In simple words: Different metal salts give different coloured precipitates and show different behaviour with excess reagents - this helps us identify which metal is present in an unknown solution.

📝 Teacher's Note: Create a comparison chart showing all these reactions. Emphasize the importance of colour observation and behaviour with excess reagent. Practice with actual salt solutions to reinforce learning.

🎯 Exam Tip: Always mention both the colour of precipitate AND its behaviour with excess reagent. This double identification makes your answer complete and scores full marks.

Solution 11:

(a) Distinguish by adding Sodium hydroxide solution:

(i) \( \text{Ca(NO}_3\text{)}_2 + 2\text{NaOH} \rightarrow \text{Ca(OH)}_2 + 2\text{NaNO}_3 \)
On adding excess of NaOH, ppt. of \( \text{Ca(OH)}_2 \) is sparingly soluble.
\( \text{Pb(NO}_3\text{)}_2 + 2\text{NaOH} \rightarrow \text{Pb(OH)}_2 + 2\text{NaNO}_3 \)
On adding excess of NaOH, ppt of \( \text{Pb(OH)}_2 \) is soluble.

(ii) \( \text{Pb(NO}_3\text{)}_2 + 2\text{NaOH} \rightarrow \text{Pb(OH)}_2 + 2\text{NaNO}_3 \)
On adding excess of NaOH, ppt of \( \text{Pb(OH)}_2 \) is soluble.
\( \text{ZnSO}_4 + 2\text{NaOH} \rightarrow \text{Zn(OH)}_2 + \text{Na}_2\text{SO}_4 \)
With excess of NaOH, white gelatinous ppt. of \( \text{Zn(OH)}_2 \) is soluble. So, these two cannot be distinguished by NaOH alone. However white ppt. of \( \text{Pb(OH)}_2 \) is readily soluble in acetic acid also.

(iii) \( \text{CuSO}_4 + 2\text{NaOH} \rightarrow \text{Cu(OH)}_2 + \text{Na}_2\text{SO}_4 \)
With excess of NaOH, alkali pale blue ppt of \( \text{Cu(OH)}_2 \) is insoluble.
\( \text{FeSO}_4 + 2\text{NaOH} \rightarrow \text{Fe(OH)}_2 + \text{Na}_2\text{SO}_4 \)
With excess of NaOH, dirty green ppt. of \( \text{Fe(OH)}_2 \) is insoluble.

(iv) \( \text{FeSO}_4 + 2\text{NaOH} \rightarrow \text{Fe(OH)}_2 + \text{Na}_2\text{SO}_4 \)
With excess of NaOH, dirty green ppt of \( \text{Fe(OH)}_2 \) is insoluble.
\( \text{FeCl}_3 + 3\text{NaOH} \rightarrow \text{Fe(OH)}_3 + 3\text{NaCl} \)
With excess of NaOH, reddish brown ppt of \( \text{Fe(OH)}_3 \) is insoluble.

(b) Distinguish by adding Ammonium hydroxide solution:

(i) On addition of \( \text{NH}_4\text{OH} \) to calcium salts no precipitation of \( \text{Ca(OH)}_2 \) occurs even with addition of excess of \( \text{NH}_4\text{OH} \) because the concentration of OH⁻ ions from ionization of \( \text{NH}_4\text{OH} \) is so low that it cannot precipitate the hydroxide of calcium.
\( \text{Pb(NO}_3\text{)}_2 + 2\text{NH}_4\text{OH} \rightarrow \text{Pb(OH)}_2 + 2\text{NH}_4\text{NO}_3 \)
On adding excess of \( \text{NH}_4\text{OH} \), chalky white ppt. of \( \text{Pb(OH)}_2 \) is insoluble.

(ii) \( \text{Pb(NO}_3\text{)}_2 + 2\text{NH}_4\text{OH} \rightarrow \text{Pb(OH)}_2 + 2\text{NH}_4\text{NO}_3 \)
On adding excess of \( \text{NH}_4\text{OH} \), chalky white ppt. of \( \text{Pb(OH)}_2 \) is insoluble.
\( \text{ZnSO}_4 + 2\text{NH}_4\text{OH} \rightarrow \text{Zn(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 \)
With excess of \( \text{NH}_4\text{OH} \), white gelatinous ppt. of \( \text{Zn(OH)}_2 \) is soluble.

(iii) \( \text{CuSO}_4 + 2\text{NH}_4\text{OH} \rightarrow \text{Cu(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 \)
With excess of \( \text{NH}_4\text{OH} \), pale blue ppt. of \( \text{Cu(OH)}_2 \) is soluble.
\( \text{FeSO}_4 + 2\text{NH}_4\text{OH} \rightarrow \text{Fe(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 \)
With excess of \( \text{NH}_4\text{OH} \), dirty green ppt. of \( \text{Fe(OH)}_2 \) is insoluble.

(iv) \( \text{FeSO}_4 + 2\text{NH}_4\text{OH} \rightarrow \text{Fe(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 \)
With excess of \( \text{NH}_4\text{OH} \), dirty green ppt. of \( \text{Fe(OH)}_2 \) is insoluble.
\( \text{FeCl}_3 + 3\text{NH}_4\text{OH} \rightarrow \text{Fe(OH)}_3 + 3\text{NH}_4\text{Cl} \)
With excess of \( \text{NH}_4\text{OH} \), reddish brown ppt of \( \text{Fe(OH)}_3 \) is insoluble.

📝 Teacher's Note: Use actual test tubes to demonstrate the color changes and solubility differences - visual memory helps students retain these analytical tests better than just reading equations.

🎯 Exam Tip: Always mention both the precipitate formation AND its behavior with excess reagent - this shows complete understanding of the analytical process.

Question 12. You are provided with two reagent bottles marked A and B. One of which contains \( \text{NH}_4\text{OH} \) solution and the other contains NaOH solution. How will you identify them by a chemical test?
Answer: Reagent bottles A and B can identified by using calcium salts such as \( \text{Ca(NO}_3\text{)}_2 \).
On adding NaOH to \( \text{Ca(NO}_3\text{)}_2 \), \( \text{Ca(OH)}_2 \) is precipitated as white precipitate which is sparingly soluble in excess of NaOH.
\( \text{Ca(NO}_3\text{)}_2 + 2\text{NaOH} \rightarrow \text{Ca(OH)}_2 + 2\text{NaNO}_3 \)
Whereas, on addition of \( \text{NH}_4\text{OH} \) to calcium salts, no precipitation of \( \text{Ca(OH)}_2 \) occurs even with addition of excess of \( \text{NH}_4\text{OH} \) because the concentration of OH⁻ ions from the ionization of \( \text{NH}_4\text{OH} \) is so low that it cannot precipitate the hydroxide of calcium.
So the reagent bottle which gives white precipitate is NaOH and the other is \( \text{NH}_4\text{OH} \).
In simple words: Use calcium salt as a test - NaOH will form a white precipitate but NH₄OH will not because it's too weak to make the precipitate.

📝 Teacher's Note: This is a perfect example of how weak vs strong bases behave differently - use this to reinforce concepts about base strength and ionization.

🎯 Exam Tip: Always state the observation clearly (white precipitate vs no precipitate) and explain the chemical reason for the difference.

Intext Questions

Question 1. What do you understand by the following:
(i) Analysis
(ii) Qualitative analysis
(iii) Reagent
(iv) Precipitation
Answer: (i) Analysis: The determination of chemical components in a given sample is called analysis.
(ii) Qualitative analysis: The analysis which involves the identification of the unknown substances in a given sample is called qualitative analysis.
(iii) Reagent: A reagent is a substance that reacts with another substance.
(iv) Precipitation: It is the process of formation of an insoluble solid when solutions are mixed. The solid thus formed is called precipitate.
In simple words: Analysis means finding out what chemicals are in a sample, qualitative analysis tells us what substances are there, reagents are chemicals we use for testing, and precipitation is when two solutions mix to form a solid.

📝 Teacher's Note: Connect these definitions to everyday examples - like testing water quality (analysis) or soap scum formation (precipitation) to make concepts relatable.

🎯 Exam Tip: Learn these definitions word-for-word as they form the foundation for understanding all analytical chemistry concepts.

Question 2. Write the probable colour of the following salts:
(i) Iron (III) chloride
(ii) Potassium nitrate
(iii) Ferrous sulphate
(iv) Aluminium acetate
(v) Calcium carbonate
Answer: (i) Yellow
(ii) Colourless
(iii) Pale Green
(iv) Colourless
(v) Colourless
In simple words: Iron salts usually have colors (yellow for Fe³⁺, green for Fe²⁺) while most other metal salts are colorless.

📝 Teacher's Note: Show actual samples or pictures of these salts to help students associate colors with specific ions - visual learning is crucial for analytical chemistry.

🎯 Exam Tip: Remember that transition metal salts are usually colored while alkali/alkaline earth metal salts are typically colorless.

Question 3. Name the probable cation present in each of the following solution:
(i) Yellow coloured solution
(ii) blue coloured solution
(iii) Light blue coloured solution
(iv) Pink coloured solution
Answer: (i) \( \text{Fe}^{3+} \)
(ii) \( \text{Cu}^{2+} \)
(iii) \( \text{Cu}^{2+} \)
(iv) \( \text{Mn}^{2+} \)
In simple words: Different metal ions give different colors to solutions - yellow means iron(III), blue means copper, and pink means manganese.

📝 Teacher's Note: Create a color chart with students showing ion-color relationships - this visual aid will be invaluable during practical work.

🎯 Exam Tip: Learn the characteristic colors of common transition metal ions as they're frequently asked in identification questions.

Question 4. Name the metal hydroxides which are:
(i) Sparingly soluble
(ii) Insoluble
(iii) Soluble
In caustic soda solution
Answer: (i) \( \text{Ca(OH)}_2 \)
(ii) \( \text{Fe(OH)}_2 \) and \( \text{Cu(OH)}_2 \)
(iii) \( \text{Zn(OH)}_2 \) and \( \text{Pb(OH)}_2 \)
In simple words: In strong base solution, some hydroxides dissolve completely (Zn, Pb), some dissolve very little (Ca), and some don't dissolve at all (Fe, Cu).

📝 Teacher's Note: Emphasize the concept of amphoteric hydroxides (Zn, Pb) that dissolve in both acids and bases - this explains their solubility in NaOH.

🎯 Exam Tip: Remember the difference between sparingly soluble (very little dissolves) and insoluble (practically nothing dissolves) - examiners often test this distinction.

Question 5. What do you observe when ammonium salt is heated with caustic soda solution? Write the balanced equation.
Answer: When ammonium salt is heated with caustic soda solution, ammonia gas is evolved.
The balance equation is:
\( \text{NH}_4\text{Cl} + \text{NaOH} \xrightarrow{\Delta} \text{NaCl} + \text{H}_2\text{O} + \text{NH}_3 \)
\( (\text{NH}_4)_2\text{SO}_4 + 2\text{NaOH} \xrightarrow{\Delta} \text{Na}_2\text{SO}_4 + 2\text{H}_2\text{O} + 2\text{NH}_3 \)
In simple words: When you heat any ammonium salt with strong base, it releases ammonia gas which you can smell or detect with wet red litmus paper turning blue.

📝 Teacher's Note: This is the confirmatory test for ammonium ions - demonstrate the characteristic smell of ammonia and the litmus test to make it memorable.

🎯 Exam Tip: Always mention both the gas evolution AND a method to detect ammonia (smell or litmus test) for complete marks.

Question 6. How will you distinguish \( \text{NH}_4\text{OH} \) solution from NaOH solution?
Answer: \( \text{NH}_4\text{OH} \) and NaOH can be distinguished by using calcium salts.
For example on adding NaOH to \( \text{Ca(NO}_3\text{)}_2 \), \( \text{Ca(OH)}_2 \) is obtained as white precipitate which is sparingly soluble in excess of NaOH.
\( \text{Ca(NO}_3\text{)}_2 + 2\text{NaOH} \rightarrow \text{Ca(OH)}_2 + 2\text{NaNO}_3 \)
On addition of \( \text{NH}_4\text{OH} \) to calcium salts, no precipitation of \( \text{Ca(OH)}_2 \) occurs even with the addition of excess of \( \text{NH}_4\text{OH} \). This is because the concentration of OH⁻ ions from the ionization of \( \text{NH}_4\text{OH} \) is so low that it cannot precipitate the hydroxide of calcium.
In simple words: Strong base (NaOH) can precipitate calcium hydroxide but weak base (NH₄OH) cannot because it doesn't provide enough OH⁻ ions.

📝 Teacher's Note: This test beautifully demonstrates the concept of strong vs weak bases - use it to reinforce ionization concepts and Kw values.

🎯 Exam Tip: Explain both the observation (precipitate vs no precipitate) and the theoretical reason (strong vs weak base ionization).

Question 7. Name the metal hydroxides which are:
(i) Insoluble (ii) Soluble.
In ammonium hydroxide solution
Answer: (i) \( \text{Fe(OH)}_2 \) and \( \text{Pb(OH)}_2 \)
(ii) \( \text{Cu(OH)}_2 \) and \( \text{Zn(OH)}_2 \)
In simple words: Some metal hydroxides dissolve in weak base (NH₄OH) forming complex ions, while others remain insoluble even in weak base.

📝 Teacher's Note: Explain complex ion formation for Cu and Zn with NH₃ - this helps students understand why these hydroxides dissolve in NH₄OH but not others.

🎯 Exam Tip: Remember that solubility in NH₄OH often involves complex ion formation, not just basic dissolution - mention this for extra marks.