Class 11 Chemistry Notes Chapter 5 (Chapter 5) – Lab Manual (English) Book
Detailed Notes with MCQs of Chapter 5 from your Chemistry Lab Manual, which deals with Qualitative Analysis of Inorganic Salts. This is a crucial topic, not just for your practical exams, but also because questions based on these tests frequently appear in various government examinations. The goal here is to identify the ions (cation and anion) present in an unknown inorganic salt through systematic chemical tests.
Qualitative Analysis: Identifying the Unknown
Qualitative analysis is the branch of chemistry that deals with the identification of elements or ions present in a substance. For inorganic salts, we aim to identify the acidic radical (anion) and the basic radical (cation). This is done through a series of systematic tests.
I. Preliminary Tests (Often called Dry Tests)
These tests are performed with the solid salt sample and give preliminary clues about the ions present.
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Physical Examination:
- Colour: Observe the colour of the salt.
- Blue: Hydrated Copper (Cu²⁺) salts.
- Green: Hydrated Nickel (Ni²⁺), Iron(II) (Fe²⁺), or Copper(II) carbonate/halides.
- Pink/Lilac: Hydrated Cobalt (Co²⁺) or Manganese (Mn²⁺) salts.
- Brown: Iron(III) (Fe³⁺) salts, some MnO₂, PbO₂.
- Yellow: Chromates (CrO₄²⁻), some Iodides (AgI, PbI₂), hydrated Iron(III) (Fe³⁺).
- Orange: Dichromates (Cr₂O₇²⁻).
- White: May contain cations like Ca²⁺, Ba²⁺, Sr²⁺, Mg²⁺, Al³⁺, Zn²⁺, Pb²⁺, NH₄⁺, Na⁺, K⁺ and anions like CO₃²⁻, SO₄²⁻, Cl⁻, NO₃⁻ etc. (Note: Anhydrous Cu²⁺ salts are often white/pale).
- Smell: Smell the salt carefully.
- Ammoniacal smell: Ammonium (NH₄⁺) salts.
- Vinegar smell: Acetate (CH₃COO⁻) salts (sometimes requires moistening or heating).
- Deliquescence/Hygroscopy: Observe if the salt absorbs moisture from the air and becomes pasty or dissolves (e.g., some chlorides like CaCl₂, MgCl₂, FeCl₃, nitrates). Efflorescence is the loss of water of crystallisation.
- Colour: Observe the colour of the salt.
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Dry Heating Test: Heat a small amount of salt in a dry test tube. Observe changes:
- Gas Evolved:
- Colourless, odourless gas turning lime water milky: CO₂ (from Carbonates, Oxalates).
- Colourless gas with ammoniacal smell: NH₃ (from Ammonium salts).
- Colourless gas smelling like burning sulphur: SO₂ (from Sulphites).
- Colourless gas with rotten egg smell: H₂S (from Hydrated Sulphides).
- Brown gas: NO₂ (from Nitrites or Nitrates of heavy metals like Pb, Cu).
- Violet vapours: I₂ (from Iodides).
- Reddish-brown vapours: Br₂ (from Bromides).
- Sublimation: White sublimate (NH₄Cl), Black sublimate turning violet on heating (Iodine).
- Colour Change:
- White salt turns yellow when hot, white on cooling: ZnO.
- Blue salt turns white: Loss of water from hydrated CuSO₄.
- Green salt turns dirty white/brownish: Loss of water from hydrated FeSO₄.
- Decrepitation: Crackling sound (e.g., NaCl, Pb(NO₃)₂).
- Gas Evolved:
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Flame Test: Make a paste of the salt with conc. HCl on a clean platinum wire loop. Introduce it to a non-luminous Bunsen flame.
- Principle: Metal chlorides are volatile. When heated in a flame, electrons get excited and emit light of characteristic colour upon returning to the ground state.
- Characteristic Colours:
- Crimson Red: Strontium (Sr²⁺)
- Apple Green: Barium (Ba²⁺)
- Brick Red: Calcium (Ca²⁺)
- Golden Yellow: Sodium (Na⁺) (persistent)
- Lilac/Pale Violet: Potassium (K⁺) (viewed through blue glass to filter out Na⁺ yellow)
- Greenish-Blue/Blue-Green: Copper (Cu²⁺)
- Grassy Green (often with blue centre): Sometimes seen with Borates or Ba salts.
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Borax Bead Test: Heat a loop of platinum wire, dip in borax powder (Na₂B₄O₇·10H₂O), and heat until a colourless transparent bead (Na₂B₄O₇ → 2NaBO₂ + B₂O₃) is formed. Touch the hot bead with a tiny amount of salt and heat again in oxidizing (outer) and reducing (inner) flames.
- Principle: Coloured transition metal ions form characteristic coloured metaborates (e.g., Cu(BO₂)₂).
- Observations (Oxidizing Flame/Reducing Flame):
- Copper (Cu²⁺): Blue (hot & cold) / Colourless or Red opaque (cold)
- Iron (Fe²⁺/Fe³⁺): Yellow/Brownish-yellow (hot), Pale yellow (cold) / Bottle green (cold)
- Cobalt (Co²⁺): Deep blue (hot & cold) / Deep blue (cold)
- Nickel (Ni²⁺): Brown/Violet (hot), Brownish-yellow (cold) / Grey/Opaque (cold)
- Manganese (Mn²⁺): Amethyst/Violet (hot & cold) / Colourless (cold)
- Chromium (Cr³⁺): Yellow (hot), Green (cold) / Green (cold)
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Charcoal Cavity Test: Mix salt with Na₂CO₃ (sodium carbonate). Heat the mixture in a charcoal cavity using a blowpipe (reducing flame).
- Principle: Metal carbonates formed decompose to oxides, which may be reduced to metal by carbon.
- Observations:
- Yellow incrustation (hot), pale yellow (cold): Zinc (Zn²⁺)
- Brown incrustation: Cadmium (Cd²⁺)
- Yellow incrustation & malleable bead: Lead (Pb²⁺)
- White incrustation & brittle bead: Antimony (Sb³⁺)
- Red scales/beads: Copper (Cu²⁺)
- White incrustation (hot & cold), garlic odour: Arsenic (As³⁺)
- Silver-white magnetic particles: Iron (Fe), Nickel (Ni), Cobalt (Co)
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Cobalt Nitrate Test: Used for salts leaving a white residue (oxides) in the charcoal cavity test (especially Mg, Al, Zn). Add a drop of cobalt nitrate solution to the white residue and heat strongly.
- Observations:
- Blue mass: Aluminium (Al³⁺) (Thenard's blue)
- Pink mass: Magnesium (Mg²⁺)
- Green mass: Zinc (Zn²⁺) (Rinmann's green)
- Observations:
II. Wet Tests for Anions (Acidic Radicals)
These tests are performed using the salt solution (usually aqueous, sometimes sodium carbonate extract). Anions are typically identified before cations.
Preparation of Sodium Carbonate Extract (Soda Extract): Used when the salt is insoluble in water or when the cation interferes with anion tests (e.g., coloured cations like Cu²⁺, Fe³⁺ or cations forming precipitates like Pb²⁺, Ba²⁺). Boil salt with excess Na₂CO₃ solution, filter. The filtrate contains sodium salts of the anions.
Systematic Analysis of Anions:
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Group I Anions (React with Dilute H₂SO₄ or Dilute HCl): CO₃²⁻, SO₃²⁻, S²⁻, NO₂⁻, CH₃COO⁻
- Test: Add dil. H₂SO₄ to a small amount of salt. Observe the gas evolved.
- CO₃²⁻ (Carbonate): Brisk effervescence of CO₂ (colourless, odourless). Turns lime water [Ca(OH)₂] milky (CaCO₃ ppt). Milky appearance disappears on passing excess CO₂ (due to Ca(HCO₃)₂ formation).
- SO₃²⁻ (Sulphite): Colourless gas with burning sulphur smell (SO₂). Turns acidified K₂Cr₂O₇ paper green. Confirmatory: Add BaCl₂ to soda extract → white ppt (BaSO₃) soluble in dil. HCl. Decolourises acidified KMnO₄ solution.
- S²⁻ (Sulphide): Colourless gas with rotten egg smell (H₂S). Turns lead acetate paper black (PbS ppt). Confirmatory: Add sodium nitroprusside solution to soda extract → purple/violet colouration.
- NO₂⁻ (Nitrite): Light brown fumes (NO₂). Turns starch-iodide paper blue-black. Confirmatory: Add freshly prepared FeSO₄ solution, then carefully add dil. H₂SO₄ → brown ring at the junction (unstable, forms easily without conc. acid unlike nitrate test).
- CH₃COO⁻ (Acetate): Vinegar smell (CH₃COOH) on warming with dil. H₂SO₄. Confirmatory: (i) Ester Test: Warm salt with ethanol and conc. H₂SO₄ → fruity smell of ethyl acetate. (ii) Neutral FeCl₃ Test: Add neutral FeCl₃ solution to neutral salt solution/soda extract → blood-red colouration, disappears on boiling forming reddish-brown ppt.
- Test: Add dil. H₂SO₄ to a small amount of salt. Observe the gas evolved.
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Group II Anions (React with Concentrated H₂SO₄): Cl⁻, Br⁻, I⁻, NO₃⁻, C₂O₄²⁻ (Oxalate), (CH₃COO⁻ also reacts but detected in Group I)
- Test: Warm a small amount of salt with conc. H₂SO₄. Observe the gas/vapours.
- Cl⁻ (Chloride): Colourless pungent gas (HCl). Gives dense white fumes with a rod dipped in NH₄OH. Confirmatory: (i) AgNO₃ Test: Add dil. HNO₃ then AgNO₃ solution to salt solution/soda extract → white curdy ppt (AgCl), soluble in NH₄OH. (ii) Chromyl Chloride Test: Heat salt with K₂Cr₂O₇(s) and conc. H₂SO₄ → reddish-brown vapours (CrO₂Cl₂). Pass vapours into NaOH solution (turns yellow). Acidify with acetic acid and add lead acetate solution → yellow ppt (PbCrO₄).
- Br⁻ (Bromide): Reddish-brown fumes (Br₂). Confirmatory: (i) AgNO₃ Test: Add dil. HNO₃ then AgNO₃ solution → pale yellow ppt (AgBr), sparingly soluble in NH₄OH. (ii) Layer Test (Cl₂ water test): Acidify soda extract with dil. HCl, add CCl₄/CS₂/CHCl₃ and excess chlorine water. Shake well → organic layer turns orange/brown.
- I⁻ (Iodide): Violet vapours (I₂). May see solid sublimate. Turns starch paper blue. Confirmatory: (i) AgNO₃ Test: Add dil. HNO₃ then AgNO₃ solution → yellow ppt (AgI), insoluble in NH₄OH. (ii) Layer Test (Cl₂ water test): Acidify soda extract with dil. HCl, add CCl₄/CS₂/CHCl₃ and chlorine water dropwise. Shake well → organic layer turns violet/purple.
- NO₃⁻ (Nitrate): Light brown fumes (NO₂), intensify on adding copper turnings or paper pellets. Confirmatory: Brown Ring Test: To the aqueous solution/soda extract, add freshly prepared FeSO₄ solution, then add conc. H₂SO₄ carefully along the sides of the test tube → a brown ring ([Fe(H₂O)₅NO]²⁺) forms at the junction of the two layers.
- C₂O₄²⁻ (Oxalate): Colourless gas mixture (CO + CO₂). CO burns with a blue flame, CO₂ turns lime water milky. Confirmatory: (i) Add CaCl₂ to neutral/weakly acidic solution → white ppt (CaC₂O₄), insoluble in acetic acid but soluble in dil. HNO₃/HCl. (ii) Acidify salt solution with dil. H₂SO₄, warm, and add KMnO₄ solution dropwise → pink colour is discharged.
- Test: Warm a small amount of salt with conc. H₂SO₄. Observe the gas/vapours.
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Group III Anions (Independent Group / Special Tests): SO₄²⁻, PO₄³⁻, (BO₃³⁻, F⁻ - less common in basic syllabus)
- SO₄²⁻ (Sulphate):
- BaCl₂ Test: Acidify salt solution/soda extract with dil. HCl, add BaCl₂ solution → thick white ppt (BaSO₄), insoluble in conc. HCl and conc. HNO₃.
- Lead Acetate Test: Add acetic acid, then lead acetate solution → white ppt (PbSO₄), soluble in ammonium acetate solution.
- PO₄³⁻ (Phosphate):
- Ammonium Molybdate Test: Acidify salt solution/soda extract with conc. HNO₃, add ammonium molybdate solution and warm gently → canary yellow ppt (Ammonium phosphomolybdate).
- SO₄²⁻ (Sulphate):
III. Wet Tests for Cations (Basic Radicals)
These tests are performed systematically using the Original Solution (O.S.) of the salt.
Preparation of Original Solution (O.S.): Try dissolving the salt in solvents in the following order:
- Cold water
- Hot water
- Dilute HCl
- Concentrated HCl (Heat if necessary, then dilute)
- Dilute HNO₃
- Concentrated HNO₃ (Heat if necessary, then dilute)
- Aqua Regia (3 parts conc. HCl + 1 part conc. HNO₃)
Systematic Analysis of Cations (Group Separation): Based on differences in solubility products of chlorides, sulphides, hydroxides, and carbonates.
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Group 0: NH₄⁺ (Ammonium)
- Test: Warm the original salt (or solution) with NaOH solution.
- Observation: Colourless gas with pungent ammoniacal smell (NH₃). Turns moist red litmus paper blue. Gives white fumes with a rod dipped in conc. HCl.
- Confirmatory: Pass the gas through/add Nessler's reagent (K₂[HgI₄] in KOH) → Brown or yellow precipitate/colouration.
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Group I: Pb²⁺, Ag⁺, Hg₂²⁺
- Group Reagent: Dilute HCl
- Test: Add dil. HCl to the O.S.
- Observation: White precipitate (PbCl₂, AgCl, Hg₂Cl₂).
- Identification of Pb²⁺: Ppt (PbCl₂) dissolves in hot water. Cool the filtrate → white needle-like crystals reappear. Add KI solution → yellow ppt (PbI₂). Add K₂CrO₄ solution → yellow ppt (PbCrO₄).
- (Ag⁺ and Hg₂²⁺ tests are usually beyond basic syllabus but involve solubility in NH₄OH and reaction with NH₄OH respectively).
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Group II: Pb²⁺, Cu²⁺, As³⁺/As⁵⁺, Sb³⁺/Sb⁵⁺, Sn²⁺/Sn⁴⁺, Bi³⁺, Cd²⁺, Hg²⁺
- Group Reagent: H₂S gas in presence of dilute HCl (acidic medium).
- Test: Pass H₂S gas through the filtrate from Group I (or O.S. acidified with dil. HCl).
- Observation: Precipitate formed (Sulphides: PbS (black), CuS (black), CdS (yellow), As₂S₃ (yellow), Sb₂S₃ (orange), SnS (brown)/SnS₂ (yellow), Bi₂S₃ (brown), HgS (black)).
- Identification of Cu²⁺: Black ppt (CuS) dissolves in dil. HNO₃. To the solution, add excess NH₄OH → deep blue solution ([Cu(NH₃)₄]²⁺). Acidify with acetic acid and add K₄[Fe(CN)₆] solution → chocolate brown ppt (Cu₂[Fe(CN)₆]).
- Identification of Pb²⁺: If not completely removed in Group I, PbS (black ppt) forms here. Test as in Group I after dissolving in HNO₃.
- (Group II is further divided into IIA - insoluble in yellow ammonium sulphide, and IIB - soluble in yellow ammonium sulphide. Detailed separation is complex).
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Group III: Al³⁺, Fe³⁺, Cr³⁺
- Group Reagent: NH₄Cl(s) + NH₄OH solution (alkaline medium).
- Test: To the filtrate from Group II, add solid NH₄Cl, warm, then add NH₄OH solution in excess.
- Observation: Precipitate formed (Hydroxides: Al(OH)₃ (gelatinous white), Fe(OH)₃ (reddish-brown), Cr(OH)₃ (green)).
- Identification of Fe³⁺: Dissolve ppt in dil. HCl. Add K₄[Fe(CN)₆] solution → Prussian blue colour/ppt (Fe₄[Fe(CN)₆]₃). Add KCNS solution → Blood red colouration ([Fe(SCN)(H₂O)₅]²⁺).
- Identification of Al³⁺: Dissolve ppt in dil. HCl. Add NaOH solution dropwise (white ppt, dissolves in excess NaOH). Boil the original solution with blue litmus and NH₄OH dropwise until just alkaline → Blue colour adsorbs on the ppt (Lake Test). Or dissolve ppt in NaOH, acidify with acetic acid, add aluminon reagent and NH₄OH → Red ppt.
- (Cr³⁺ test involves oxidation to yellow chromate (CrO₄²⁻) using NaOH and H₂O₂/Br₂ water, then confirming with lead acetate).
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Group IV: Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺
- Group Reagent: H₂S gas in presence of NH₄Cl + NH₄OH (alkaline medium).
- Test: Pass H₂S gas through the filtrate from Group III.
- Observation: Precipitate formed (Sulphides: ZnS (white/dirty white), MnS (buff/pinkish), NiS (black), CoS (black)).
- Identification of Zn²⁺: Ppt (ZnS) dissolves in dil. HCl. Add NaOH solution dropwise → white ppt, soluble in excess NaOH. Pass H₂S through this alkaline solution → white ppt (ZnS) reappears. Confirm using Cobalt Nitrate test on charcoal cavity → Green mass.
- Identification of Mn²⁺: Ppt (MnS) dissolves in dil. HCl. Add NaOH and Br₂ water/H₂O₂ → brown ppt (MnO₂). Fuse original salt with Na₂CO₃ and KNO₃ → Green mass (Na₂MnO₄).
- Identification of Ni²⁺: Dissolve ppt in aqua regia. Evaporate, dissolve residue in water. Add NH₄OH (pale blue solution), then add Dimethylglyoxime (DMG) reagent → Bright red precipitate.
- Identification of Co²⁺: Dissolve ppt in aqua regia. Evaporate, dissolve residue in water. Acidify with acetic acid, add KNO₂ solution and warm → Yellow ppt (Potassium cobaltinitrite). Add NH₄CNS solution and amyl alcohol/ether → Blue colour in organic layer.
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Group V: Ba²⁺, Sr²⁺, Ca²⁺
- Group Reagent: (NH₄)₂CO₃ solution in presence of NH₄Cl + NH₄OH.
- Test: To the filtrate from Group IV, add (NH₄)₂CO₃ solution.
- Observation: White precipitate (Carbonates: BaCO₃, SrCO₃, CaCO₃).
- Identification: Dissolve the ppt in dilute acetic acid. Divide the solution into three parts.
- Part 1: Add K₂CrO₄ solution → Yellow ppt (BaCrO₄) confirms Ba²⁺. If Ba²⁺ is absent, proceed.
- Part 2 (if Ba²⁺ absent): Add (NH₄)₂SO₄ solution, boil and cool → White ppt (SrSO₄) confirms Sr²⁺. If Sr²⁺ is absent, proceed.
- Part 3 (if Ba²⁺, Sr²⁺ absent): Add ammonium oxalate ((NH₄)₂C₂O₄) solution → White ppt (CaC₂O₄) confirms Ca²⁺.
- Flame Tests: Perform flame tests on the original salt or the ppt dissolved in conc. HCl: Ba²⁺ (Apple green), Sr²⁺ (Crimson red), Ca²⁺ (Brick red).
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Group VI: Mg²⁺, Na⁺, K⁺
- Group Reagent: None (Tested in the filtrate from Group V).
- Test for Mg²⁺: To a portion of the filtrate from Group V, add NH₄OH and then disodium hydrogen phosphate (Na₂HPO₄) or ammonium phosphate ((NH₄)₃PO₄) solution. Scratch the inner sides of the test tube.
- Observation: White crystalline precipitate (Mg(NH₄)PO₄) confirms Mg²⁺.
- Tests for Na⁺, K⁺: Perform flame tests on the original salt. Na⁺ (Golden yellow), K⁺ (Lilac through blue glass).
Important Note: Interfering radicals like Oxalate, Borate, Phosphate can sometimes cause precipitation of Group IV/V cations in Group III. Special procedures exist for their removal if suspected, but are often skipped in introductory analysis.
Multiple Choice Questions (MCQs)
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A salt gives a crimson red colour in the flame test and its aqueous solution gives a white precipitate with ammonium carbonate solution. The cation present is:
a) Ca²⁺
b) Ba²⁺
c) Sr²⁺
d) Mg²⁺ -
The reagent used to test for the ammonium ion (NH₄⁺) which gives a brown precipitate is:
a) Fehling's solution
b) Tollens' reagent
c) Nessler's reagent
d) Benedict's solution -
When H₂S gas is passed through an acidic solution (dil. HCl) containing Cu²⁺ and Zn²⁺ ions, which ion precipitates?
a) Only Zn²⁺
b) Only Cu²⁺
c) Both Cu²⁺ and Zn²⁺
d) Neither Cu²⁺ nor Zn²⁺ -
The formation of a brown ring at the junction of two layers in the test for nitrate ion (NO₃⁻) is due to the formation of:
a) [Fe(H₂O)₆]²⁺
b) [Fe(H₂O)₅(NO)]²⁺
c) Fe₂(SO₄)₃
d) NO₂ -
Which anion gives a colourless gas that turns lime water milky when treated with dilute H₂SO₄?
a) SO₄²⁻
b) Cl⁻
c) CO₃²⁻
d) NO₃⁻ -
The chromyl chloride test is a specific confirmatory test for which anion?
a) Br⁻
b) I⁻
c) SO₄²⁻
d) Cl⁻ -
In the Borax bead test, a blue coloured bead in both oxidizing and reducing flame indicates the presence of:
a) Cu²⁺
b) Ni²⁺
c) Co²⁺
d) Fe³⁺ -
The group reagent for precipitating Group III cations (Al³⁺, Fe³⁺, Cr³⁺) is:
a) Dilute HCl
b) H₂S in acidic medium
c) NH₄Cl(s) + NH₄OH solution
d) (NH₄)₂CO₃ solution -
A white salt turns yellow when hot and white again on cooling during the dry heating test. This indicates the likely presence of:
a) Pb²⁺
b) Zn²⁺
c) Al³⁺
d) Mg²⁺ -
Dimethylglyoxime (DMG) reagent gives a characteristic bright red precipitate with which cation in an ammoniacal medium?
a) Co²⁺
b) Fe³⁺
c) Ni²⁺
d) Mn²⁺
Answer Key for MCQs:
- c) Sr²⁺
- c) Nessler's reagent
- b) Only Cu²⁺ (Group II cation, precipitates in acidic medium; Zn²⁺ is Group IV)
- b) [Fe(H₂O)₅(NO)]²⁺
- c) CO₃²⁻
- d) Cl⁻
- c) Co²⁺ (Copper is blue in oxidizing, red/colourless in reducing)
- c) NH₄Cl(s) + NH₄OH solution
- b) Zn²⁺ (as ZnO)
- c) Ni²⁺
Study these tests systematically, focusing on the reagents, observations (colours, gases, precipitates), and specific confirmatory tests. Good luck with your preparation!