Chemistry Class 10 ICSE Solutions — Selina Chapter 4

Chemistry Class 10 ICSE solutions for Selina Chapter 4 cover Analytical Chemistry, the branch of chemistry that identifies substances by their reactions. The chapter teaches the colours of salts, the action of sodium hydroxide and ammonium hydroxide on salt solutions, and the behaviour of alkalis on metals and metal oxides. Each test forms a precipitate whose colour and solubility in excess reagent identify the metal ion.

Download Analytical Chemistry Class 10 ICSE Chemistry Chapter 4 PDF

Download the Analytical Chemistry Class 10 ICSE Chemistry chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 6 pages, 5 sections, 23 questions.

What is inside Chapter 4

6 pages · 5 sections · 23 exercise questions

• Intext Questions: 7 questions
• EXERCISE: 16 questions

4.1 Colours of the Salts and Their Solutions

The salts of representative elements, which sit in Groups 1, 2 and 13 to 17 of the periodic table, are generally colourless. The salts of transition elements, in Groups 3 to 12, are generally coloured. Colour is the first clue in qualitative analysis, and the same test methods return in later chapters of the Class 10 Chemistry book.

A salt looks colourless only when both its ions are colourless. The common colourless cations are ammonium NH_4^+, sodium Na^+, potassium K^+, calcium Ca^{2+}, magnesium Mg^{2+}, aluminium Al^{3+}, lead Pb^{2+} and zinc Zn^{2+}. The common colourless anions are chloride, sulphate, carbonate, nitrate, hydrogen carbonate, sulphide, bromide and acetate.

The coloured cations do the identifying work. The cupric ion Cu^{2+} is blue, the ferrous ion Fe^{2+} is light green, and the ferric ion Fe^{3+} is yellowish brown. The nickel ion Ni^{2+} and chromium ion Cr^{3+} are green, while the manganous ion Mn^{2+} is pink.

Coloured anions matter too. The permanganate ion MnO_4^- is pink or purple, the dichromate ion Cr_2O_7^{2-} is orange, and the chromate ion CrO_4^{2-} is yellow.

Precipitation is the formation of an insoluble solid when two solutions are mixed. The solid is called the precipitate. Only compounds that are insoluble in water form precipitates.

Worked pattern: predict a salt’s colour from its ions. Ferrous sulphate contains Fe^{2+}, so its solution is light green. Potassium nitrate holds only colourless ions, so it is colourless. Cupric chloride holds Cu^{2+}, so it is blue. Potassium dichromate holds Cr_2O_7^{2-}, so its solution is orange. The exercise asks for several of these colour predictions directly.

4.2 Action of Sodium Hydroxide Solution on Certain Metallic Salt Solutions

Sodium hydroxide solution, also called caustic soda, is added drop by drop to the salt solution. Each drop forms the metal hydroxide, which appears as a precipitate. The precipitate’s colour identifies the metal ion. Some metal hydroxides dissolve in excess sodium hydroxide to form soluble complexes.

The dropwise method matters because a precipitate may redissolve in excess alkali. If the alkali is added too quickly, it is easy to miss a precipitate that forms and then dissolves. The chapter’s Intext Question 7 makes exactly this point.

The table below collects the six metal ions tested with sodium hydroxide solution.

Zinc hydroxide dissolves in excess sodium hydroxide to form sodium zincate: Zn(OH)_2 + 2NaOH \rightarrow Na_2ZnO_2 + 2H_2O. Lead hydroxide dissolves the same way to form sodium plumbite: Pb(OH)_2 + 2NaOH \rightarrow Na_2PbO_2 + 2H_2O. Lead hydroxide is also readily soluble in acetic acid.

Ammonium salts behave differently. Heating an ammonium salt with sodium hydroxide or any water-soluble base evolves ammonia gas: NH_4Cl + NaOH \rightarrow NaCl + H_2O + NH_3. Potassium hydroxide, caustic potash, shows similar behaviour to sodium hydroxide.

Worked pattern: add sodium hydroxide solution to ferric sulphate. The balanced equation is Fe_2(SO_4)_3 + 6NaOH \rightarrow 2Fe(OH)_3 + 3Na_2SO_4. A reddish brown precipitate of ferric hydroxide forms, and it stays insoluble in excess alkali. The metal ion present is ferric, Fe^{3+}.

To identify an unknown metal ion from these tests, read two clues. Note the precipitate colour first, then test its solubility in excess sodium hydroxide. A white precipitate that dissolves in excess alkali points to zinc or lead. Add excess ammonium hydroxide to separate them: zinc hydroxide dissolves in it, but lead hydroxide does not.

Ferrous and ferric precipitates are the most-swapped answers. Ferrous hydroxide is dirty green because ferrous salts are pale green. Ferric hydroxide is reddish brown because ferric salts are yellowish brown. Tie each precipitate to the salt’s colour, and the confusion disappears.

4.3 Action of Ammonium Hydroxide on Certain Salt Solutions

Ammonium hydroxide is a weak base. It ionises feebly, so the concentration of hydroxide ions in solution stays low. That concentration is too low to precipitate calcium hydroxide. So calcium salts give no precipitate with ammonium hydroxide, even in excess.

The table below compares the same metal ions with ammonium hydroxide.

Copper gives the characteristic test for the cupric ion. Ammonium hydroxide first forms a pale blue precipitate of copper hydroxide: CuSO_4 + 2NH_4OH \rightarrow Cu(OH)_2 + (NH_4)_2SO_4. Excess ammonium hydroxide dissolves this precipitate to a deep blue solution of tetrammine copper(II) sulphate. The chapter gives a second form of this complex too, tetrammine copper hydroxide.

Zinc behaves similarly. Its gelatinous white hydroxide dissolves in excess ammonium hydroxide to a colourless solution of tetrammine zinc(II) sulphate. The alternative form, tetrammine zinc hydroxide, also appears in the chapter. Both names are accepted in naming questions.

Worked pattern: run the copper detection flow with copper nitrate instead of copper sulphate. First, Cu(NO_3)_2 + 2NH_4OH \rightarrow Cu(OH)_2 + 2NH_4NO_3. A pale blue precipitate appears. Excess ammonium hydroxide then dissolves it to a deep blue solution of tetrammine copper(II) nitrate. The colour sequence matches the copper sulphate case exactly.

The side-by-side view below is the one to memorise for the distinguish questions.

Three contrasts decide most answers. Calcium precipitates with sodium hydroxide but not with ammonium hydroxide. Copper hydroxide stays insoluble in excess sodium hydroxide, yet dissolves in excess ammonium hydroxide. Lead dissolves in excess sodium hydroxide but not in excess ammonium hydroxide.

4.4 Action of Alkalis on Certain Metals

Zinc, aluminium and lead react with hot concentrated caustic alkalis. With sodium hydroxide or potassium hydroxide, each metal gives a soluble salt and liberates hydrogen gas.

The aluminium equation includes water, and that is the point students miss. The balanced reaction is 2Al + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2. With potassium hydroxide it is 2Al + 2KOH + 2H_2O \rightarrow 2KAlO_2 + 3H_2. Zinc and lead do not need the water term, which is why it is easy to forget for aluminium.

The gas liberated is hydrogen. It burns with a pop sound. The exercise asks you to name a metal that gives this gas when boiled with alkali solution, and zinc, aluminium and lead all fit the description.

Worked pattern: react aluminium with hot concentrated potassium hydroxide. The balanced equation is 2Al + 2KOH + 2H_2O \rightarrow 2KAlO_2 + 3H_2. The salt is potassium meta aluminate, colourless and soluble in water. Two moles of aluminium produce three moles of hydrogen gas.

Three mistakes cost marks in this chapter. The list below covers the ones students repeat.

• Omitting the 2H_2O term from the aluminium equations.
• Writing ferrous hydroxide as reddish brown; it is dirty green.
• Expecting a calcium precipitate with ammonium hydroxide, which never forms.

4.5 Action of Alkalis on Metal Oxides

Most metal oxides are basic in nature. They dissolve in water to form hydroxides, and they neutralise acids. Sodium oxide is the chapter’s example: Na_2O + H_2O \rightarrow 2NaOH, and Na_2O + 2HCl \rightarrow 2NaCl + H_2O. Basic oxides do not react with bases.

A few metallic oxides and hydroxides show dual character. They behave as both bases and acids. The chapter defines them as amphoteric oxides and hydroxides, which react with both acids and alkalis to form salt and water.

Zinc oxide shows both reactions clearly. With hydrochloric acid it gives zinc chloride and water: ZnO + 2HCl \rightarrow ZnCl_2 + H_2O. With sodium hydroxide it gives sodium zincate and water: ZnO + 2NaOH \rightarrow Na_2ZnO_2 + H_2O. Zinc hydroxide reacts the same way with both reagents.

Three metals dominate this section: zinc, aluminium and lead. The mnemonic ZAP keeps the trio in order — Zinc, Aluminium, Lead. Their oxides and hydroxides all react with acids and alkalis.

Worked pattern: prove lead oxide’s dual character in two reactions. With dilute hydrochloric acid, PbO + 2HCl \rightarrow PbCl_2 + H_2O. With sodium hydroxide, PbO + 2NaOH \rightarrow Na_2PbO_2 + H_2O. The same oxide forms salt and water with both an acid and an alkali, exactly as the definition requires.

What to practise from Analytical Chemistry Class 10 ICSE Solutions

Two practice blocks follow the five sections of this ICSE chapter. Work through both on paper, writing every balanced equation fully.

• Intext Questions — 7 questions
• EXERCISE — 16 questions

The exercise builds its questions from the chapter tables. Question 12 asks you to distinguish salt solutions by adding sodium hydroxide and ammonium hydroxide. Question 16 asks for conversion equations, walking zinc sulphate to sodium zincate and copper sulphate to the tetrammine complex. Question 15 asks for the definition of an amphoteric oxide and balanced equations for its reaction with a caustic alkali.

These salt tests sit in the analytical chemistry part of the Class 10 ICSE syllabus. For the official ICSE Chemistry syllabus and examination format, check the CISCE board website. The reactions here also build directly on the acid-base ideas from Chapter 3 on acids, bases and salts.

Analytical Chemistry Class 10 ICSE frequently asked questions

Why does ammonium hydroxide not give a precipitate with a calcium salt solution?

Ammonium hydroxide is a weak base and ionises feebly. Its hydroxide ion concentration is too low to precipitate calcium hydroxide. Sodium hydroxide, a strong alkali, supplies enough hydroxide ions and does give a white precipitate with calcium salts.

How can I distinguish NaOH solution from NH4OH solution?

Add the alkali dropwise to a calcium salt solution. Sodium hydroxide gives a white precipitate of calcium hydroxide. Ammonium hydroxide gives no precipitate at all. The chapter’s exercise uses this contrast directly in its distinguish questions.

Why is alkali added drop by drop to a salt solution?

Some metal hydroxides dissolve in excess alkali. If the alkali is added too quickly, a precipitate can form and redissolve before you see it. Adding it drop by drop lets you observe the precipitate at the moment it forms.

Which hydroxide precipitate dissolves in excess NaOH but not in excess NH4OH?

Lead hydroxide. It dissolves in excess sodium hydroxide to form sodium plumbite, but it stays insoluble in excess ammonium hydroxide. This contrast separates lead salts from zinc salts, because zinc hydroxide dissolves in both alkalis.

Where can I get chemistry class 10 icse solutions for Selina Chapter 4 Analytical Chemistry?

This page explains each section of this ICSE chapter in the order the book presents it. The chapter PDF on this page carries the full exercise for study and practice. Work the Intext Questions and the 16 EXERCISE questions after reading each section.

Reference: ICSE Class 10 Chemistry textbook material, chapter 4: Analytical Chemistry — CISCE-aligned.

More from this book

• Chapter 3: Acids Bases and Salts
• Chapter 6: Electrolysis
• Chapter 12: Organic Chemistry