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Overview

🌳 Concept tree

Qualitative Analysis of Acidic Radicals (Anions)
├── Revision of concepts and laws needed in this chapter

├── Importance

├── Kinds of analysis

├── Qualitative analysis

├── A. Identification of acid radicals (anions)

│ ├── (1) Dilute hydrochloric acid group

│ ├── (2) Concentrated sulphuric acid group

│ └── (3) Barium chloride group

├── Practical differentiation between pairs of salts (typical exam type)

└── Reasoning questions

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 1 / 13

Revision of concepts and laws needed in this chapter

Comparison aspect	Main idea	Keywords
Mole	the amount of substance containing Avogadro’s number (6.02 × 10²³) of particles (molecules atoms ions formula units or electrons). Example: Al³⁺ + 3e⁻ → Al — 3 moles of electrons are needed to reduce 1 mole of Al³⁺ to 1 mole of Al atoms	amountsubstancecontainingAvogadro
Molar mass (g/mol)	= sum of the atomic masses of all atoms in the formula	atomicmassesatomsformula
Number of moles	n = mass (g) / molar mass (g/mol)	massmolar
Number of particles	= n × 6.02 × 10²³	10²³
Volume of a gas at STP	(L) = n × 22.4 L/mol
Gas density at STP	(g/L) = molar mass / 22.4	molarmass
Molarity	M = n (mol) / V (L). Also n = M × V(mL) / 1000	1000
Mass percentage of an element in a compound	= (mass of the element in one mole of compound / molar mass of the compound) × 100	molemolar
Percentage of a compound in an impure sample	= (mass of the compound / mass of the sample) × 100	mass
Dilution	when water is added the number of moles of solute does not change: M₁V₁ = M₂V₂	wateraddednumbermoles

Worked example 1.1.Percentage of sulphur in sodium sulphate Na₂SO₄ (Na = 23, S = 32, O = 16): M = 142 g/mol; %S = 32/142 × 100 = 22.5 %.

Worked example 1.2.Mass of NaOH needed for 250 mL of 0.1 M solution: n = 0.1 × 250/1000 = 0.025 mol; mass = 0.025 × 40 = 1.0 g.

Worked example 1.3.50 mL of 1 M HCl are diluted with water to 250 mL: M₂ = 1 × 50/250 = 0.2 M.

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Importance

Chemical analysis is an important branch of chemistry that plays an important role in the progress of science and in many fields:

Comparison aspect	Main idea	Keywords
Medicine	diagnosis depends on measuring sugar albumin urea and cholesterol it makes the doctor’s diagnosis and treatment easier and it gives the amount of active ingredient in medicines	diagnosisdependsmeasuringsugar
Agriculture	analysing soil (acidity or basicity kinds and percentages of elements) tells fertilizers to add and how to improve soil and crops	analysingsoilaciditybasicity
Industry	ores and products are analysed to check identity and compliance with standard specifications	oresproductsanalysedcheck
Environment	e.g. measuring nitrogen oxides in air and pollutants in water. There are countless other examples based on studying the chemical composition of substances	measuringnitrogenoxidespollutants

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Kinds of analysis

When a substance is analysed we need to know which elements it contains, the percentage of each, and how they are combined (to write the molecular formula, or the compounds if it is a mixture).
Two kinds:

Comparison aspect	Main idea	Example	Keywords
Qualitative analysis	identifies the constituents of a pure substance (	a simple salt) or of a mixture of several substances	identifiesconstituentspuresubstance
Quantitative analysis	estimates the percentage (amount) of each main component. It is carried out after the qualitative analysis which tells us method to choose	—	estimatespercentageamountmain

Think of qualitative analysis as asking "what is present?" and quantitative analysis as asking "how much of it is present?".

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 4 / 13

Qualitative analysis

A pure substance can be recognized from its physical constants (melting point, boiling point, molar mass).
For a mixture the components must first be separated and then identified.
Qualitative analysis is thus a series of well-chosen chemical reactions, each judged by an observable change (evolution of a gas, change of colour, formation of a precipitate).

Organic compoundsidentify the elements and the functional groups.

Inorganic compoundsidentify the ions — the acid radicals (anions) and basic radicals (cations) of the salt. We study only these ions.

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 5 / 13

A. Identification of acid radicals (anions)

Anions are classified into three groups
each with a reagent:

Dilute hydrochloric acid group
Concentrated sulphuric acid group
Barium chloride group

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(1) Dilute hydrochloric acid group

Comparison aspect	Main idea	Keywords
Anions	carbonate CO₃²⁻ bicarbonate HCO₃⁻ sulphite SO₃²⁻ sulphide S²⁻ thiosulphate S₂O₃²⁻ nitrite NO₂⁻	carbonateCO₃²bicarbonateHCO₃
Principle	HCl is a more stable acid than the acids these anions come from	stableacidacidsthese
•	Those acids (H₂CO₃ H₂SO₃ H₂S H₂S₂O₃ HNO₂) are unstable they volatilize or decompose to gases that can be recognized with a suitable reagent	ThoseacidsH₂CO₃H₂SO₃
•	Gentle heating helps to drive off the gas	Gentleheatinghelpsdrive
Main experiment	solid salt + dilute HCl	solidsaltdilute

Anion	Observation and gas	Detection of the gas	Confirmatory test
Carbonate	Effervescence CO₂: Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂	Turns lime water milky: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O. (Excess CO₂ clears it: CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ — so pass the gas only briefly.)	Salt solution + MgSO₄ solution → white ppt. in the cold: Na₂CO₃ + MgSO₄ → Na₂SO₄ + MgCO₃↓; the ppt. dissolves in HCl: MgCO₃ + 2HCl → MgCl₂ + H₂O + CO₂
Bicarbonate	Effervescence CO₂: NaHCO₃ + HCl → NaCl + H₂O + CO₂	Lime water turns milky	Salt solution + MgSO₄ solution → no ppt. in the cold; a white ppt. appears on heating: 2NaHCO₃ + MgSO₄ → Na₂SO₄ + Mg(HCO₃)₂; Mg(HCO₃)₂ → MgCO₃↓ + CO₂ + H₂O
Sulphite	Colourless gas SO₂ with a suffocating smell: Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂	Paper wet with acidified potassium dichromate turns from orange to green: K₂Cr₂O₇ + 3SO₂ + H₂SO₄ → K₂SO₄ + Cr₂(SO₄)₃ + H₂O	Salt solution + AgNO₃ → white ppt. Ag₂SO₃ that turns black on heating: Na₂SO₃ + 2AgNO₃ → Ag₂SO₃↓ + 2NaNO₃
Sulphide	H₂S gas with a rotten-egg smell: Na₂S + 2HCl → 2NaCl + H₂S	Paper wet with lead(II) acetate turns black: (CH₃COO)₂Pb + H₂S → PbS↓ + 2CH₃COOH	Salt solution + AgNO₃ → black Ag₂S: Na₂S + 2AgNO₃ → Ag₂S↓ + 2NaNO₃
Thiosulphate	SO₂ gas and a pale-yellow turbidity of sulphur: Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂ + S↓	SO₂ detected as above	Decolourizes brown iodine solution: 2Na₂S₂O₃ + I₂ → Na₂S₄O₆ (sodium tetrathionate) + 2NaI
Nitrite	Brown fumes in the tube: NaNO₂ + HCl → NaCl + HNO₂; 3HNO₂ → HNO₃ + H₂O + 2NO; colourless NO turns reddish brown at the mouth of the tube: 2NO + O₂ → 2NO₂	—	Salt solution + KMnO₄ acidified with H₂SO₄ → the violet colour is removed: 5NaNO₂ + 2KMnO₄ + 3H₂SO₄ → 5NaNO₃ + K₂SO₄ + 2MnSO₄ + 3H₂O

Notes:

All metal carbonates are insoluble in water except those of sodium, potassium and ammonium; all bicarbonates are soluble.
Dilute HCl cannot distinguish carbonate from bicarbonate (both give CO₂ that clouds lime water), so the MgSO₄ test is used: carbonate gives MgCO₃ at once, because MgCO₃ is insoluble; bicarbonate gives soluble Mg(HCO₃)₂, which decomposes on heating to precipitate MgCO₃.
The acidified dichromate test works because Cr₂O₇²⁻ (orange) is reduced by SO₂ to Cr³⁺ (green). Acidified permanganate (violet) is likewise reduced to colourless Mn²⁺ by sulphite, sulphide or nitrite, so it is not specific for nitrite and is used as a confirmatory test after the HCl test.
Lead(II) acetate paper is used for H₂S because black PbS forms on it. Silver nitrate gives a black Ag₂S precipitate with sulphide.

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 7 / 13

(2) Concentrated sulphuric acid group

Comparison aspect	Main idea	Keywords
Anions	chloride Cl⁻ bromide Br⁻ iodide I⁻ nitrate NO₃⁻	chloridebromideiodidenitrate
Principle	concentrated (hot) H₂SO₄ is more stable (less volatile) than the acids of these anions so it displaces them as volatile acids which appear as gases and are detected with a suitable reagent	concentratedH₂SO₄stableless
Main experiment	solid salt + conc	solidsaltconc
•	H₂SO₄ heat if needed	H₂SO₄heatneeded

Anion	Gas and its detection	Confirmatory test
Chloride	Colourless HCl gas: 2NaCl + H₂SO₄ → Na₂SO₄ + 2HCl. Forms white fumes of NH₄Cl with a glass rod wet with NH₃: HCl + NH₃ → NH₄Cl	Salt solution + AgNO₃ → white ppt. AgCl: NaCl + AgNO₃ → AgCl↓ + NaNO₃; turns violet in sunlight; dissolves in conc. ammonia solution: AgCl + 2NH₃ → [Ag(NH₃)₂]Cl
Bromide	Colourless HBr, partly oxidized by conc. H₂SO₄ to orange-red bromine vapour (turns starch paper yellow): 2NaBr + H₂SO₄ → Na₂SO₄ + 2HBr; 2HBr + H₂SO₄ → 2H₂O + SO₂ + Br₂	AgNO₃ → pale-yellow (white-yellow) ppt. AgBr: NaBr + AgNO₃ → AgBr↓ + NaNO₃; darkens in sunlight; dissolves slowly/with difficulty in conc. ammonia
Iodide	Colourless HI, which is oxidized more easily by conc. H₂SO₄ to violet iodine vapour (turns starch paper blue): 2NaI + H₂SO₄ → Na₂SO₄ + 2HI; 2HI + H₂SO₄ → 2H₂O + SO₂ + I₂	AgNO₃ → yellow ppt. AgI, insoluble in ammonia solution: NaI + AgNO₃ → AgI↓ + NaNO₃
Nitrate	Reddish-brown NO₂ vapour from decomposition of the nitric acid formed: 2NaNO₃ + H₂SO₄ → Na₂SO₄ + 2HNO₃; 4HNO₃ → 4NO₂ + 2H₂O + O₂. Adding copper turnings increases the brown fumes: Cu + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O	Brown-ring test: nitrate solution + freshly prepared iron(II) sulphate; conc. H₂SO₄ is poured carefully down the tube wall. A brown ring forms at the boundary between the acid and the solution: 6FeSO₄ + 2NaNO₃ + 4H₂SO₄ → 3Fe₂(SO₄)₃ + Na₂SO₄ + 2NO + 4H₂O; FeSO₄ + NO → [Fe(NO)]SO₄ (brown ring compound). The ring disappears on heating or shaking.

The silver halides differ in colour and in solubility in ammonia
which helps distinguish them: AgCl (white
dissolves readily in dilute ammonia)
AgBr (pale yellow
dissolves with difficulty in concentrated ammonia)
AgI (yellow
does not dissolve).

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 8 / 13

(3) Barium chloride group

Anions phosphate PO₄³⁻ and sulphate SO₄²⁻.
These do not react with dilute HCl or conc.
H₂SO₄ to give gases; they are identified by precipitation reactions.

Anion	BaCl₂ test	Confirmatory test
Phosphate	White ppt. of Ba₃(PO₄)₂, soluble in dil. HCl: 2Na₃PO₄ + 3BaCl₂ → Ba₃(PO₄)₂↓ + 6NaCl	AgNO₃ → yellow ppt. Ag₃PO₄ soluble in both ammonia and nitric acid: Na₃PO₄ + 3AgNO₃ → Ag₃PO₄↓ + 3NaNO₃
Sulphate	White ppt. of BaSO₄, insoluble in dil. HCl: Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl	Lead(II) acetate → white ppt. PbSO₄: Na₂SO₄ + (CH₃COO)₂Pb → PbSO₄↓ + 2CH₃COONa

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 9 / 13

Practical differentiation between pairs of salts (typical exam type)

Comparison aspect	Main idea	Keywords
Sodium sulphite and sodium sulphate	add dilute HCl to each solid. Sulphite gives SO₂ (suffocating odour turns acidified dichromate paper green) sulphate gives no gas. Alternatively with BaCl₂ both give a white precipitate but BaSO₃ dissolves in dilute HCl while BaSO₄ does not	dilutesolidgivessuffocating
Iron(II) chloride and iron(III) chloride	add NaOH to each solution. Fe²⁺ gives a white-green precipitate Fe(OH)₂ (which darkens in air) Fe³⁺ gives a reddish-brown precipitate Fe(OH)₃: FeCl₂ + 2NaOH → Fe(OH)₂↓ + 2NaCl FeCl₃ + 3NaOH → Fe(OH)₃↓ + 3NaCl	NaOHsolutiongiveswhite
Sodium chloride and aluminium chloride	add NaOH. NaCl gives no precipitate AlCl₃ gives a white gelatinous precipitate that dissolves in excess NaOH: AlCl₃ + 3NaOH → Al(OH)₃↓ + 3NaCl Al(OH)₃ + NaOH → NaAlO₂ + 2H₂O. (Silver nitrate cannot distinguish them because both contain chloride and both give white AgCl.)	NaOHNaClgivesprecipitate
Sodium carbonate and sodium bicarbonate	add MgSO₄ solution in the cold: carbonate gives an immediate white precipitate bicarbonate gives none until heated	MgSO₄solutioncoldgives
Sodium chloride and sodium bromide / iodide	add AgNO₃ and observe white / pale yellow / yellow precipitates or add conc. H₂SO₄ and observe HCl fumes / orange-red bromine vapour / violet iodine vapour	AgNO₃observewhitepale

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Reasoning questions

Why does adding NaOH gradually to aluminium chloride solution give a white precipitate which then disappears? Al(OH)₃ is amphoteric: it dissolves in excess alkali forming soluble sodium meta-aluminate NaAlO₂.
Why is dilute HCl not used to distinguish carbonate from bicarbonate? Both evolve CO₂ that turns lime water milky.
Why does acidified KMnO₄ lose its violet colour when nitrite is added? NO₂⁻ is oxidized to NO₃⁻ and MnO₄⁻ is reduced to colourless Mn²⁺.
Why does lead acetate paper turn black in H₂S? Black lead(II) sulphide forms.
Why do violet vapours appear when conc. H₂SO₄ is added to an iodide? HI is oxidized by the acid to iodine, which vaporizes as violet vapour.

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 11 / 13

Part 12

📖

🔍 In-Depth Explanations, Deductions & Multi-Book Insights:

Chemistry Thanaweya Amma (2026/2027)

Lesson 1 – Qualitative Analysis of Acidic Radicals (Anions)
Detailed Academic Lesson Overview:
Qualitative analysis determines the chemical components of a substance.
Anions are classified into three analytical groups based on the volatility and strength of their corresponding parent acids: Dilute HCl Group, Concentrated H2SO4 Group, and Barium Chloride Group.

🌳 Hierarchy Classification & Conceptual Roadmap

Qualitative Analysis of Anions

├── 1. Dilute HCl Group (Less stable acids)

Comparison aspect	Main idea	Keywords
│ ├── Carbonate (CO3²⁻) & Bicarbonate (HCO3⁻)	Evolve CO2 turning lime water turbid	Evolveturninglimewater
│ ├── Sulfite (SO3²⁻)	Evolves SO2 turning K2Cr2O7 paper from orange to green	EvolvesturningK2Cr2O7paper
│ ├── Thiosulfate (S2O3²⁻)	Evolves SO2 with yellow colloidal sulfur suspension	Evolvesyellowcolloidalsulfur
│ ├── Sulfide (S²⁻)	Evolves H2S turning lead acetate paper black (PbS)	Evolvesturningleadacetate
│ └── Nitrite (NO2⁻)	Evolves NO in solution turns reddish-brown NO2 at test tube mouth	Evolvessolutionturnsreddish
•	├── 2. Conc. H₂SO₄ Group (Moderately stable acids)	ConcH2SO4GroupModerately
│ ├── Chloride (Cl⁻)	Evolves colorless HCl gas forming white fumes with ammonia (NH4Cl)	Evolvescolorlessformingwhite
│ ├── Bromide (Br⁻)	Evolves orange-red Br2 vapors turning starch paper yellow	Evolvesorangevaporsturning
│ ├── Iodide (I⁻)	Evolves violet I2 vapors turning starch paper blue	Evolvesvioletvaporsturning
│ └── Nitrate (NO3⁻)	Evolves dense reddish-brown NO2 vapors confirmed by Brown Ring test	Evolvesdensereddishbrown
•	└── 3. Barium Chloride (BaCl₂) Group (Most stable acids)	BariumChlorideBaCl2Group
├── Sulfate (SO4²⁻)	White ppt of BaSO4 insoluble in dilute HCl	WhiteBaSO4insolubledilute
└── Phosphate (PO4³⁻)	White ppt of Ba3(PO4)2 soluble in dilute HCl	Whitesolubledilute
•	📋 Master Teacher's Blackboard Rules & Exam Secrets	MasterTeacherBlackboardRules
RULE 1	The principle of anion detection: A more stable acid displaces a less stable acid from its salts in the form of a gas that can be identified	principleaniondetectionstable
RULE 2	Confirmatory test for Sulfate vs. Phosphate: Both form a white precipitate with BaCl2 but Barium Sulfate is insoluble in dilute HCl whereas Barium Phosphate is completely soluble	ConfirmatorytestSulfatePhosphate
RULE 3	Brown Ring Test for Nitrate: FeSO4 must be freshly prepared because it easily oxidizes to Fe2(SO4)3 in air destroying the complex	BrownRingTestNitrate
📊 Detailed Comparison Matrix	Diagnostic Silver Nitrate (AgNO3) Confirmatory Matrix	DiagnosticSilverNitrateAgNO3

Anion	Precipitate with AgNO₃	Solubility in Aqueous Ammonia (NH₄OH)
Chloride (Cl⁻)	White ppt of AgCl (turns violet in light)	Rapidly soluble in concentrated NH₄OH
Bromide (Br⁻)	Yellowish-white ppt of AgBr	Slowly soluble in concentrated NH₄OH
Iodide (I⁻)	Bright yellow ppt of AgI	Completely insoluble in NH₄OH
Phosphate (PO₄³⁻)	Bright yellow ppt of Ag₃PO₄	Rapidly soluble in NH₄OH & dilute HNO₃
Sulfide (S²⁻)	Black ppt of Ag₂S	Insoluble in ammonia
Sulfite (SO₃²⁻)	White ppt of Ag₂SO₃ (turns black on heating)	Soluble in dilute acids

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 12 / 13

Detailed Comparison Matrix: Diagnostic Silver Nitrate (AgNO3) Confirmatory Matrix

Anion	Precipitate with AgNO3	Solubility in Aqueous Ammonia (NH4OH)
Chloride (Cl⁻)	White ppt of AgCl (turns violet in light)	Rapidly soluble in concentrated NH4OH
Bromide (Br⁻)	Yellowish-white ppt of AgBr	Slowly soluble in concentrated NH4OH
Iodide (I⁻)	Bright yellow ppt of AgI	Completely insoluble in NH4OH
Phosphate (PO4³⁻)	Bright yellow ppt of Ag3PO4	Rapidly soluble in NH4OH & dilute HNO3
Sulfide (S²⁻)	Black ppt of Ag2S	Insoluble in ammonia
Sulfite (SO3²⁻)	White ppt of Ag2SO3 (turns black on heating)	Soluble in dilute acids

Lesson 1: Qualitative Analysis of Acidic Radicals (Anions) · 13 / 13

Curriculum

Chemistry

Chapter 1 - Transition Elements and Iron Metallurgy

Chapter 2 - Chemical Analysis

Chapter 3 - Chemical Equilibrium

Chapter 4 - Electrochemistry

Chapter 5 - Organic Chemistry

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Lesson 1: Qualitative Analysis of Acidic Radicals (Anions)