Revision: Class 12 >> Principles Related to Practical Chemistry NEET (UG) Principles Related to Practical Chemistry

Any reaction that involves both oxidation and reduction occurring simultaneously is called an oxidation-reduction reaction or simply a redox reaction.

or

The chemical reaction in which both oxidation and reduction occur simultaneously is called a redox reaction.

The functional group is defined as an atom or group of atoms joined in a specific manner which is responsible for the characteristic chemical properties of the organic compounds.

• Used for detection of: Nitrogen (N), Sulfur (S), Chlorine (Cl), Bromine (Br), Iodine (I)
• Cannot detect Fluorine (AgF is soluble, so no precipitate forms)
• Principle: Small amount of organic compound + sodium metal → fused → plunged in distilled water → boiled → filtered → Sodium Fusion Extract (SFE) obtained

• SFE is boiled with freshly prepared FeSO₄ solution, then acidified with conc. HCl, then 2–3 drops of FeCl₃ added
• Prussian blue colour confirms nitrogen

Reactions:

• FeSO₄ + 2NaOH → Fe(OH)₂ + Na₂SO₄
• 6NaCN + Fe(OH)₂ → Na₄[Fe(CN)₆] + 2NaOH
• 3Na₄[Fe(CN)₆] + 4FeCl₃ → Fe₄[Fe(CN)₆]₃ (Prussian blue) + 12NaCl

Exceptions: Test NOT given by compounds lacking C atoms that contain N (e.g., NH₂NH₂, NH₂OH) because CN⁻ ion won't form. Also NOT given by diazonium compounds.

When BOTH N and S are present:

• If Na is in insufficient amount → NaSCN (thiocyanate) forms instead of NaCN
• SCN⁻ + Fe³⁺ → [Fe(SCN)]²⁺ → Blood red colour (instead of Prussian blue)

Test 1 — Sodium nitroprusside test:

• SFE + sodium nitroprusside → Deep violet colour confirms sulfur
• S²⁻ + [Fe(CN)₅NO]²⁻ → [Fe(CN)₅NOS]⁴⁻ (violet)

Test 2 — Lead acetate test:

• SFE + acetic acid → acidify + Pb(CH₃COO)₂ → Black precipitate (PbS) confirms sulfur
• Pb(CH₃COO)₂ + S²⁻ → PbS↓ + 2CH₃COO⁻

SFE is first acidified with HNO₃ (to remove CN⁻ and S²⁻ by boiling with conc. HNO₃/glacial acetic acid), then treated with AgNO₃:

• NaCN + HNO₃ → NaNO₃ + HCN (removed)
• Na₂S + 2HNO₃ → 2NaNO₃ + H₂S↑ (removed)

Fluorine cannot be detected by Lassaigne's test since AgF is soluble.
If N or S is present, they must be removed first to prevent interference (AgCN or Ag₂S may form).

Compound is heated with oxidising agent (sodium peroxide) → phosphorus oxidised to phosphate → solution boiled with HNO₃ → then treated with ammonium molybdate [(NH₄)₂MoO₄] + HNO₃ and boiled → canary yellow precipitate of ammonium phosphomolybdate [(NH₄)₃PO₄·12MoO₃] confirms phosphorus.

Na₃PO₄ + 3HNO₃ → H₃PO₄ + 3NaNO₃

\[ \begin{aligned} \mathrm{H}_3\mathrm{PO}_4 + \underset{\substack{\text{Ammonium} \\ \text{molybdate}}}{12(\mathrm{NH}_4)_2\mathrm{MoO}_4} + 21\mathrm{HNO}_3 \longrightarrow{} & \underset{\substack{\text{Ammonium} \\ \text{Phosphomolybdate}}}{(\mathrm{NH}_4)_3\mathrm{PO}_4 \cdot 12\mathrm{MoO}_3} \\ & + 21\mathrm{NH}_4\mathrm{NO}_3 + 12\mathrm{H}_2\mathrm{O} \end{aligned} \]

Mohr's Salt:

• Formula: FeSO₄(NH₄)₂SO₄·6H₂O (ferrous ammonium sulphate)
• Preparation: Dissolve equimolar quantities of hydrated ferrous sulphate and ammonium sulphate in water containing a small amount of dil. H₂SO₄ (to prevent hydrolysis of Fe²⁺ ions) → concentrate → cool → light blue or green crystals
• FeSO₄·7H₂O + (NH₄)₂SO₄ → FeSO₄(NH₄)₂SO₄·6H₂O + H₂O
• It is a double salt and a primary standard in volumetric analysis
• Does NOT lose water of crystallisation on exposure to air; not oxidised in the solid state

Potash Alum:

• Formula: K₂SO₄·Al₂(SO₄)₃·24H₂O → also written as KAl(SO₄)₂·12H₂O
• Preparation: Dissolve equimolar quantities of hydrated aluminium sulphate and potassium sulphate in a minimum amount of water + dil. H₂SO₄ → concentrate → cool → colourless octahedral crystals
• K₂SO₄ + Al₂(SO₄)₃·18H₂O + 6H₂O → K₂SO₄·Al₂(SO₄)₃·24H₂O (Potash Alum)
• Alums = double sulphates (general formula X₂SO₄·M₂(SO₄)₃·24H₂O, where X = monovalent cation, M = trivalent cation)
• Alums are isomorphous, crystalline, soluble in water; they undergo hydrolysis (aqueous solution is acidic)

• Prepared by nitration of acetanilide with HNO₃/H₂SO₄ (below 20°C)
• The acetamido (–NHCOCH₃) group is bulky → causes steric hindrance at ortho → para product is major
• Products: p-Nitroacetanilide (major) + o-Nitroacetanilide (minor)
• On recrystallisation from ethyl alcohol: p-nitroacetanilide separates as crystals; o-isomer remains in solution
• Uses: poultry medicines, anti-oxidants, gasoline, pesticides, rubber chemicals

• Also called para-aminoazobenzene
• First azo dye obtained by coupling reaction; belongs to azo dye family (–N=N– linkage)
• Preparation: 
  
• Product: Ph–N=N–Ph–NH₂ (para-aminoazobenzene)
• Uses: pyrotechnics (yellow smoke), yellow pigments and inks (inkjet printers), insecticides, lacquers, varnishes, waxes, oil stains, styrene resins

• Titration = a definite volume of standard solution of acid (or base) is allowed to neutralise the same volume of base (or acid) solution of the same normality
• Heat of neutralisation = amount of heat evolved when 1 gram equivalent of acid completely neutralises 1 gram equivalent of base in a dilute solution
• For strong acid + strong base: H⁺(aq) + OH⁻(aq) → H₂O + 57.36 kJ

Redox Reactions:

• A substance that oxidises another substance (and is itself reduced) is called an oxidising agent.
• A substance that reduces another substance (and is itself oxidised) is called a reducing agent.

What is Oxidation and Reduction?

• Active H atoms = those attached to electronegative atoms like O, N, S, X, C(sp), and active methylene
• Reaction: A–H + Na → ½H₂↑ + A⁻Na⁺
• Examples that give H₂ gas: 
  \[ \begin{aligned} &\text{1. } \mathrm{R}-\mathrm{NH}_2 \xrightarrow{\mathrm{Na}} \mathrm{R}-\mathrm{NH}^-\mathrm{Na}^+ + \frac{1}{2}\mathrm{H}_2 \uparrow \\[1ex] &\text{2. } \mathrm{R}-\mathrm{COOH} \xrightarrow{\mathrm{Na}} \mathrm{R}-\mathrm{COO}^-\mathrm{Na}^+ + \frac{1}{2}\mathrm{H}_2 \uparrow \\[1ex] &\text{3. } \mathrm{R}-\mathrm{OH} + \mathrm{Na} \longrightarrow \mathrm{R}-\mathrm{O}^-\mathrm{Na}^+ + \frac{1}{2}\mathrm{H}_2 \uparrow \\[1ex] &\text{4. } \mathrm{R}-\mathrm{C} \equiv \mathrm{CH} + \mathrm{Na} \longrightarrow \mathrm{R}-\mathrm{C} \equiv \mathrm{C}^-\mathrm{Na}^+ + \frac{1}{2}\mathrm{H}_2 \uparrow \\[1ex] &\text{5. } \mathrm{R}-\mathrm{CH}_2-\mathrm{O}-\mathrm{CH}_3 + \mathrm{Na} \longrightarrow \text{No reaction.} \end{aligned} \]
• No reaction: R–CH₂–O–CH₃ (simple ethers — no acidic H)

• Terminal alkynes (R–C≡CH): have acidic hydrogen
• With AgNO₃ in ammonia: 
  \[ \mathrm{R} - \mathrm{C} \equiv \mathrm{CH} \xrightarrow{\mathrm{AgNO}_3 + \mathrm{NH}_4\mathrm{OH}} \mathrm{R} - \mathrm{C} \equiv \mathrm{CAg} \downarrow + \mathrm{NH}_4\mathrm{NO}_3 \]
• With ammonical Cu₂Cl₂ (Cu⁺): R – C ≡ CH → R – C ≡ C – Cu↓ → Red precipitate
• Internal alkynes do NOT react with either reagent (no acidic H)
• Acetylene (ethyne): gives both white ppt. with AgNO₃/NH₃ AND red ppt. with Cu₂Cl₂/NH₃

Cerric Ammonium Nitrate Test (Group Reagent for all alcohols):

Alcohols + (NH₄)₂[Ce(NO₃)₆] (cerric ammonium nitrate / CAN) → Red coloured complex

Lucas Test (distinguish 1°, 2°, 3°):

1° alcohol: 
\[ \mathrm{R}_2\mathrm{CH} - \mathrm{OH} \xrightarrow{\text{Conc. HCl} + \text{anhyd. ZnCl}_3} \] No reaction (no turbidity at room temperature)

2° alcohol:
\[ \mathrm{R}_2\mathrm{CH} - \mathrm{OH} \xrightarrow{\text{Conc. HCl} + \text{anhyd. ZnCl}_2} \mathrm{R}_2\mathrm{CH} - \mathrm{Cl} \]
Turbidity in ~5 minutes

3° alcohol: 
\[ \mathrm{R}_2\mathrm{CH} - \mathrm{OH} \xrightarrow{\text{Conc. Hcl} + \text{anhyd. ZnCl}_2} \mathrm{R}_3\mathrm{CCl} \]
Immediate turbidity

Benzyl and allyl alcohols react as rapidly as tertiary (exceptions)

Victor-Meyer's Test:

• 1° alcohol → Nitric acid derivative → red colour (with NaOH)
  \[ \begin{aligned} & \mathrm{RCH}_2-\mathrm{OH} \xrightarrow{\mathrm{P}+\mathrm{I}_2} \mathrm{RCH}_2-\mathrm{I} \xrightarrow{\mathrm{AgNO}_2} \mathrm{RCH}_2-\mathrm{NO}_2 \\[1ex] & \xrightarrow{\mathrm{HONO}} \underset{\text{Nitrolic acid}}{\mathrm{R} - \underset{\underset{\displaystyle \mathrm{NOH}}{||}}{\mathrm{C}} - \mathrm{NO}_2} \xrightarrow{\mathrm{NaOH}} \underset{\text{(Blood red colouration)}}{\mathrm{R} - \underset{\underset{\displaystyle \mathrm{NO}^- \mathrm{Na}^+}{||}}{\mathrm{C}} - \mathrm{NO}_2} \end{aligned} \]

• 2° alcohol → blue colour
  \[ \begin{aligned} & \mathrm{R}_2\mathrm{CHOH} \xrightarrow{\mathrm{P}+\mathrm{I}_2} \mathrm{R}_2\mathrm{CH} - \mathrm{I} \xrightarrow{\mathrm{AgNO}_2} \mathrm{R}_2\underset{\underset{\displaystyle \mathrm{N=O}}{|}}{\mathrm{CH}} - \mathrm{NO}_2 \\[1ex] & \xrightarrow{\mathrm{HONO}} \mathrm{R}_2\mathrm{C} - \mathrm{NO}_2 \xrightarrow{\mathrm{NaOH}} \text{Blue colouration} \end{aligned} \]

• 3° alcohol → no colour
  \[ \begin{aligned} & \mathrm{R}_3\mathrm{C} - \mathrm{OH} \xrightarrow{\mathrm{P} + \mathrm{I}_2} \mathrm{R}_3\mathrm{C} - \mathrm{I} \xrightarrow{\mathrm{AgNO}_2} \mathrm{R}_3\mathrm{H} - \mathrm{NO}_2 \\[1ex] & \xrightarrow{\mathrm{HONO}} \text{No reaction} \xrightarrow{\mathrm{NaOH}} \text{Colourless} \end{aligned} \]

Iodoform Test:

• Organic Compounds + 4I₂ + 6NaOH → yellow ppt. 
  The above reaction is known as the Iodoform test.
• Positive for: CH₃CHO (ethanal), CH₃COCH₃ (acetone), CH₃CHOH–R (2° alcohols with CH₃CHOH–), ethanol only among primary alcohols (CH₃CH₂OH)
• Diethyl ketone does NOT give iodoform test
• To give iodoform: must contain CH₃CO– or CH₃CHOH– group

FeCl₃ Test:

• Phenol + neutral FeCl₃ solution → violet colour (iron phenoxide complex)
  

• Note: m, p-hydroxy benzoic acid and di/trinitrophenols DO NOT give this test; alcohols do not give this test either

Bromine Water Test:

Phenol + Br₂ water → white precipitate of 2,4,6-tribromophenol

Liebermann's Nitroso Reaction:

• Phenol + NaNO₂ + conc. H₂SO₄ → deep green or blue colour (changes to red on dilution with water; alkaline with NaOH → original green/blue restored)

• Used as a test for phenol.

Azo dye formation / Azo-dye test:

• 1° aromatic amines + NaNO₂ + HCl → diazonium salt → coupled with β-naphthol in alkaline solution → orange-red azo dye
• Also confirms phenols (phenol is the coupling partner)

Tests for Acids (Carboxylic acids):

Distinction: Alcohol vs Phenol vs Carboxylic Acid:

• Alcohol: reacts with Na → H₂ gas; does NOT react with NaOH or NaHCO₃
• Phenol: reacts with Na + NaOH → H₂; does NOT react with NaHCO₃
  
• Carboxylic acid: reacts with Na, NaOH, and NaHCO₃ (gives CO₂)

Tests for Esters:

• \[ \mathrm{RCOOR}' + \mathrm{NaOH} + \text{Phenolphthalein (Pink)} \xrightarrow{\Delta} \mathrm{RCOOH} + \mathrm{R}'\mathrm{OH} \text{ (Colorless solution)} \]
• Esters have sweet/fruity smell

Test for Acid amide:

Acid amides (1°) give smell of ammonia when with alkali.

\[ \mathrm{R} - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{NH}_2 \xrightarrow[\Delta]{\mathrm{NaOH}} \mathrm{R} - \overset{\displaystyle \mathrm{O}}{\overset{||}{\mathrm{C}}} - \mathrm{ONa} + \mathrm{NH}_3 \]

NaNO₂ + aq. HCl Test:

• 1° Aliphatic amine: 
  \[ \mathrm{R} - \mathrm{CH}_2\mathrm{NH}_2 \xrightarrow{\mathrm{NaNO}_2 + \text{aq, HCl}} \mathrm{R} - \mathrm{CH}_2 - \mathrm{OH} + \mathrm{N}_2 \uparrow \]

• 1° Aromatic amine: 
  \[ \mathrm{Ph} - \mathrm{NH}_2 \xrightarrow[(0-5^\circ\mathrm{C})]{\mathrm{NaNO}_2 + \text{aq. HCl}} \mathrm{Ph} - \overset{+}{\mathrm{N}}_2\overset{\ominus}{\mathrm{Cl}} \text{ (diazonium salt)} \]

• All secondary amine give yellow oily liquid with nitrous acid i.e. NaNO₂ + HCI

  \[ 2^\circ \text{ Amine \%: } \mathrm{R} - \mathrm{NH} - \mathrm{R} \xrightarrow{\mathrm{NaNO}_2 + \text{aq. HCl}} \mathrm{R}_2\mathrm{N} - \mathrm{NO} \]

  yellow oily liquid (N-Nitroso amine)

  \[ 3^\circ \text{ Amine: } \mathrm{R}_3\mathrm{N} \xrightarrow{\mathrm{NaNO}_2 + \text{aq. HCl}} \text{unstable nitrite} \]

Carbylamine Test (Isocyanide Test):

• Only 1° amines give this test

• \[ \mathbf{1^\circ \text{ Amine : }} \mathrm{R} - \mathrm{CH}_2 - \mathrm{NH}_2 \xrightarrow{\mathrm{CHCl}_3 + \mathrm{KOH}} \mathrm{R} - \mathrm{CH}_2 - \mathrm{N} \equiv \mathrm{C} + 3\mathrm{KCl} + 3\mathrm{H}_2\mathrm{O} \text{ (unpleasant smell of isocyanide)} \]

• \[ \mathrm{Ph} - \mathrm{NH}_2 \xrightarrow{\mathrm{CHCl}_3 + \mathrm{KOH}} \mathrm{Ph} - \mathrm{N} \equiv \mathrm{C} \]

• 2° and 3° amines do NOT give this test

Hofmann Mustard Oil Test (CS₂ test):

Test for 1° amine and aniline

2° amine reacts with CS₂

3° amine + CS₂ → no reaction

Hinsberg Reagent (Benzenesulphonyl chloride, C₆H₅SO₂Cl):

• 1° amine: 
  \[ \mathrm{R} - \mathrm{NH}_2 + \mathrm{PhSO}_2\mathrm{Cl} \xrightarrow{\text{Pyridine}} \mathrm{R} - \mathrm{NH} - \mathrm{SO}_2 - \mathrm{Ph} \xrightarrow{\text{Base}} \text{Compound is soluble in base.} \]

• 2° amine: 
  \[ \mathrm{R}_2\mathrm{NH} + \mathrm{PhSO}_2\mathrm{Cl} \xrightarrow{\text{Pyridine}} \mathrm{R}_2\mathrm{N} - \mathrm{SO}_2 - \mathrm{Ph} \xrightarrow{\text{Base}} \text{Compound is insoluble in base.} \]

• 3° amine: 
  \[ \mathrm{R}_3\mathrm{N} + \mathrm{PhSO}_2\mathrm{Cl} \xrightarrow{\text{Pyridine}} \text{No reaction.} \]

Azo-dye test:

1° aromatic amines + NaNO₂ + HCl → diazonium salt → couple with alkaline β-naphthol → orange-red dye

Used to detect the presence of nitro group (–NO₂)

\[ \mathrm{R} - \mathrm{NO}_2 \xrightarrow[\Delta]{\mathrm{Zn} - \mathrm{NH}_4\mathrm{Cl}} \mathrm{R} - \mathrm{NHOH} \xrightarrow[\text{T.R.}]{\mathrm{AgNO}_3 + \mathrm{NH}_4\mathrm{OH}} \underset{\text{black ppt}}{\mathrm{Ag} \downarrow} \]

This is also called Mulliken's test

Molisch test:

Molisch's test is a chemical test that detects the presence of carbohydrates in a sample.

• One or two drops of an alcoholic solution of α-naphthol are added to 2 mL of glucose solution. 
• 1 mL of conc. H₂SO₄ solution is added carefully along the sides of the test tube.

The formation of a violet ring at the junction of two liquids confirms the presence of a carbohydrate or sugar.

Ninhydrin Test:

• Amino acids react with ninhydrin at pH = 4 → purple (violet) colour
• Proline and hydroxyproline → yellow colour (exception)

Xanthoproteic Test:

• \[ \text{Solution} \xrightarrow{\mathrm{HNO}_3} \text{Yellow colour} \xrightarrow{\mathrm{NaOH}} \text{orange - yellow color} \]
• Detects amino acids with phenolic or indole groups: tyrosine, tryptophan, phenylalanine, glutamic acid

Millon's Test:

• Millon's reagent = mixture of Hg(NO₃)₂ and Hg(NO₂)₂

• \[ \text{Solution} \xrightarrow{\text{Millon's agent}} \text{white precipitate} \xrightarrow{\text{boiling}} \text{brick - red color} \]

• Detects proteins/amino acids with phenolic –OH group (e.g., tyrosine, phenylalanine, β-naphthol)

• Not specific to proteins; positive for any phenolic compound

Biuret Test:

• \[ \text{Protien} + \mathrm{CuSO}_4 \xrightarrow{\mathrm{NaOH}} \text{Copper cation complex (purple to violet)} \]

• Detects peptide bonds (not for amino acids alone)

• Examples: gelatin, casein, albumin

Group A (Dil. H₂SO₄ Test):

Group B (Conc. H₂SO₄ Test):

Group III Anions (SO₄²⁻, PO₄³⁻): No gas with dil. HCl or conc. H₂SO₄ → detected by precipitation.

Test: Salt + NaOH → heat → NH₃ gas evolved (pungent smell; turns moist red litmus blue)

Nessler's Reagent test: NH₄Cl + NaOH →(Δ)→ NH₃ + NaCl + H₂O

• NH₃ + Nessler's reagent (K₂HgI₄ + KOH) → yellow–brown precipitate (iodide of Millon's base)
• [OHg₂NH₂]I (brown ppt.)

Paper soaked in CuSO₄ solution becomes deep blue due to complex with NH₃: CuSO₄ + 4NH₃ → [Cu(NH₃)₄]SO₄

Analysis of Group I Cations (Pb²⁺, Ag⁺, Hg₂²⁺):

Reagent: dil. HCl; precipitate: AgCl, PbCl₂, Hg₂Cl₂ (all white)

PbCl₂ is soluble in hot water (distinguishes Pb²⁺ from Ag⁺ and Hg₂²⁺)

On residue (AgCl + Hg₂Cl₂): add NH₄OH:

• Ag⁺ → Ag(NH₃)₂Cl (soluble, colourless)
• Hg₂²⁺ → Hg + Hg(NH₂)Cl (black precipitate)

Test for Pb²⁺:

• K₂CrO₄ → yellow ppt. of PbCrO₄ (soluble in hot NaOH)
• KI → yellow ppt. of PbI₂ (reversible on cooling)
• dil. H₂SO₄ → white ppt. of PbSO₄
• H₂S → black ppt. of PbS
• NH₃ solution → white ppt. of Pb(OH)₂ (soluble in excess NaOH → [Pb(OH)₄]²⁻, amphoteric)

Group IIA (HgS, PbS, CuS, Bi₂S₃, CdS):

Dissolve in dil. HNO₃ → then dil. H₂SO₄ → Pb²⁺ precipitates as PbSO₄ (white)

Remaining sulphates of Bi³⁺, Cu²⁺, Cd²⁺ → NH₄OH (excess):

• Bi³⁺ → white Bi(OH)₃ precipitate
• Cu²⁺ → deep blue [Cu(NH₃)₄]SO₄ (tetramminecopper II) — confirms Cu²⁺
• Cd²⁺ → colourless solution → add H₂S → yellow CdS precipitate

Test for Cu²⁺ Ion:

• With NH₃ (excess): deep blue solution (tetrammine copper II complex)
• With K₄[Fe(CN)₆]: chocolate brown ppt. of Cu₂[Fe(CN)₆]
• With KCN (sparingly): yellow ppt. of Cu(CN)₂ (quickly decomposes → Cu₂(CN)₂; excess KCN → colourless [CuCN₄]³⁻)
• With NaOH (cold): blue ppt. of Cu(OH)₂ → heated → black CuO
• With KI: white CuI ppt. (intensely brown due to I₃⁻ formation)

Group IIB (As₂S₃, As₂S₅, Sb₂S₃, SnS, SnS₂):

• Soluble in yellow ammonium sulphide [(NH₄)₂Sx]
• Distinction: As₂S₃ insoluble in conc. HCl; Sb₂S₃ and SnS₂ soluble in conc. HCl

Analysis of Group III Cations (Fe³⁺, Al³⁺, Cr³⁺):

• Group reagent: NH₄OH in presence of NH₄Cl
• Precipitates: Al(OH)₃ (white), Fe(OH)₃ (red-brown), Cr(OH)₃ (green, cotton-like)
• Add H₂O + Na₂O₂ to filtrate: Cr³⁺ oxidised → CrO₄²⁻ (yellow filtrate) confirms Cr
• Only Al(OH)₃ and Cr(OH)₃ are soluble in NaOH (amphoteric); Fe(OH)₃ is insoluble

Test for Fe³⁺ Ion:

• KCNS (potassium thiocyanate): Fe³⁺ + SCN⁻ → [Fe(SCN)]²⁺ → blood red colour
• K₄[Fe(CN)₆] (potassium ferrocyanide): 4FeCl₃ + 3K₄[Fe(CN)₆] → Fe₄[Fe(CN)₆]₃ → Prussian blue (insoluble in dil. acids; NaOH converts to Fe(OH)₃ red + [Fe(CN)₆]³⁻)

Test for Al³⁺ Ion:

• Ammonium sulphide: 2Al³⁺ + 3S²⁻ + 6H₂O → 2Al(OH)₃↓ + 3H₂S (white ppt.) → litmus paper turns red (acidic H₂S)
• Lake test: Al(OH)₃ + blue litmus indicator → adsorbs blue dye → blue floating mass in colourless liquid (called "lake")

NH₄NO₃ cannot replace NH₄Cl in Group III because NO₃⁻ ions will oxidise Mn²⁺ → Mn⁴⁺ → MnO₂ precipitate (Mn will then precipitate in Group III instead of IV)

(NH₄)₂SO₄ cannot replace NH₄Cl because SO₄²⁻ will precipitate barium as BaSO₄ in Group V

Analysis of Group IV Cations (Zn²⁺, Mn²⁺, Ni²⁺, Co²⁺):

• Group reagent: H₂S in presence of NH₄OH
• Precipitate colours: ZnS (white), MnS (buff/dirty pink), NiS (black), CoS (black)

Test for Zn²⁺ ion:

• ZnCl₂ + 2NaOH → Zn(OH)₂↓ (white ppt.) → soluble in excess NaOH → Na₂ZnO₂ (confirms Zn²⁺)
• Pass H₂S → white ZnS ppt. forms
• K₄[Fe(CN)₆] after neutralisation with NH₄OH → white or bluish white ppt. of zinc ferrocyanide

Test for Ni²⁺ ion:

• NiS (black) dissolves in aqua regia: NiS + 4HNO₃ + 6HCl → NiCl₂ + NO + ...
• Dimethylglyoxime (DMG) added after making alkaline with NH₄OH → brilliant red precipitate (Ni–DMG complex)

Test for Mn²⁺ ion:

• Mn(OH)₂ (white ppt.) with NaOH
• NaHCO₃ + heat → MnCO₃ → NaOH + Br₂ → boil → black ppt. of Mn₂O₃ (MnO₂)

Analysis of Group V Cations (Ba²⁺, Ca²⁺, Sr²⁺):

• Group reagent: (NH₄)₂CO₃ in presence of NH₄OH
• All precipitate as white carbonates (BaCO₃, CaCO₃, SrCO₃)
• Dissolve in CH₃COOH + add K₂CrO₄ → yellow ppt. of BaCrO₄ (Ba²⁺ confirmed); SrCO₃ and CaCO₃ remain
• Test for Ba²⁺: K₂CrO₄ → yellow BaCrO₄↓; also (NH₄)₂SO₄ → white BaSO₄ (insoluble even in conc. HNO₃)
• Test for Sr²⁺: dissolve ppt. in CH₃COOH + NH₄OH + (NH₄)₂SO₄ → white SrSO₄ precipitate
• Test for Ca²⁺: (NH₄)₂C₂O₄ (ammonium oxalate) → white CaC₂O₄ precipitate; CaC₂O₄ soluble in dil. HCl → solution + K₄[Fe(CN)₆] → white ppt. of calcium potassium ferrocyanide

Flame Test:

• Ba²⁺ → Green flame
• Sr²⁺ → Crimson red flame
• Ca²⁺ → Brick red flame

• A known weight of solid CuSO₄·5H₂O is dissolved in a known volume of water in a calorimeter
• Temperature change is noted
• If Q = heat absorbed when 1 g of solid CuSO₄·5H₂O is dissolved: heat of solution = Q/w × M × 4.184 J/mol
  (where M = molar mass of CuSO₄·5H₂O)
• In this experiment: 1 mole of solute dissolved per 400 moles of water; 7.0 g CuSO₄ dissolved in 200 mL of water.

• Heat of neutralisation = amount of heat evolved when 1 g equivalent of acid completely neutralises 1 g equivalent of base in dilute solution
• H⁺(aq) + OH⁻(aq) → H₂O + 57.36 kJ (for all strong acid–strong base reactions, same value)
• Method: definite volume of standard HCl solution is neutralised with same volume of NaOH of same normality in a thermos flask → temperature rise noted with 1/10th degree thermometer
• Heat evolved = calculated → heat of 1 gram equivalent neutralisation extrapolated

• Crystalloids: Crystalline in nature; dissolved state → easily pass through the vegetable/animal membrane; e.g., NaCl, urea, sugar → form true solutions
• Colloids: Non-crystalline; do NOT diffuse or diffuse slowly through membranes; e.g., starch, ghee, gelatin → form colloidal solutions
• Colloidal particle size: 1–100 nm (can pass through filter paper but NOT parchment/animal membrane)
• True solution particle size: < 1 nm (pass through filter paper AND parchment membrane)
• Suspension particle size: > 100 nm (cannot pass through filter paper)

• Colloidal solution = heterogeneous mixture of dispersed phase (1–100 nm particles) in dispersion medium
• Dispersed phase = component present in small proportion (colloidal dimension particles)
• Dispersion medium = medium in which colloidal particles are dissolved
• Example: sulphur in water → sulphur = dispersed phase; water = dispersion medium
• 8 types of colloids are possible based on the physical state of both phases (gas cannot form a colloid with gas, as a mixture is always homogeneous)

In acidic medium: H₂O₂ + 2I⁻ + 2H⁺ → I₂ + 2H₂O (H₂O₂ oxidises I⁻ to I₂)

The iodine produced is immediately reduced back by sodium thiosulfate:

• I₂ + 2S₂O₃²⁻ → S₄O₆²⁻ + 2I⁻

This continues until all S₂O₃²⁻ is consumed

• After that, any I₂ produced reacts with starch → deep blue complex (I₂ + starch → blue)
• Time for blue colour to appear indicates the rate of I₂ formation → clock reaction
• The rate of reaction changes with a change in the concentration of iodide ions
• As the thiosulphate amount added is fixed and the time for blue colour is measured → rate of I₂ production is determined