NEET Salt Analysis Complete Chapter Notes by PW

Understanding the specific reagent, reaction and observation linked to each ion makes salt analysis easier to revise. Colour changes, gas formation, characteristic precipitates and flame colours provide important clues, while selective precipitation helps in the systematic separation and identification of cations.

To make this process simpler and more systematic, Physics Wallah provides complete Salt Analysis notes for NEET, covering important anion and cation tests, observations and reactions. These notes can help students quickly connect each test with its expected result, strengthen recall and approach Salt Analysis questions with greater confidence. 

Salt Analysis Introduction

Salt analysis is the systematic identification of the acidic and basic radicals present in an inorganic salt. The acidic radical generally corresponds to the anion, while the basic radical corresponds to the cation.

Qualitative analysis involves preliminary observations, dry tests, wet tests and confirmatory reactions. Important observations include gas evolution, precipitate formation, precipitate colour, flame colour and characteristic coloured complexes.

The analysis of anions is generally carried out before the systematic analysis of cations.

Fundamental Classification in Salt Analysis

Salt analysis can be approached through the following sequence:

1. Preliminary tests for the anion

2. Confirmatory tests for the anion

3. Preliminary or dry tests for the cation

4. Confirmatory or wet tests for the cation

Anion Classification

Anions can be classified according to their reactions with dilute or concentrated acids.

Important Subgroup IA anions include sulfide, carbonate, acetate and nitrite.

Important Subgroup IB anions include chloride, bromide, iodide, nitrate and oxalate.

If a salt is insoluble in water, a sodium carbonate extract can be prepared for testing certain anions. The extract is treated appropriately with acids and the resulting gas or reaction is observed.

Important Anion Tests

Carbonate Ion

Carbonate ions react with dilute acids to produce carbon dioxide gas with brisk effervescence.

The gas is colourless and odourless and turns limewater milky because of the formation of calcium carbonate.

CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O

When excess carbon dioxide is passed through limewater, the calcium carbonate dissolves because soluble calcium bicarbonate is formed. The solution therefore becomes clear.

Sulfide Ion

Sulfide ions react with dilute acids to produce hydrogen sulfide (H₂S) gas.

Hydrogen sulfide has a characteristic rotten-egg smell. Important tests for sulfide include:

• Moist lead acetate paper turns black due to the formation of lead sulfide.

• Sodium nitroprusside produces a purple-coloured complex.

• Hydrogen sulfide shows acidic behaviour with moist blue litmus paper.

The blackening of lead acetate paper is due to:

Pb²⁺ + H₂S → PbS↓ + 2H⁺

Acetate Ion

Acetate ions can be identified by the characteristic vinegar-like smell of acetic acid produced when an acetate salt is treated with a suitable acid.

A confirmatory test can be performed using ferric chloride, which produces a characteristic red colour that changes on heating and further treatment.

Nitrite Ion

Nitrite ions react with acids to form nitrous acid, which can decompose to produce nitrogen oxides. Nitrogen dioxide is a brown gas that may be observed under suitable conditions.

A confirmatory test involves diazotisation followed by coupling with an aromatic amine. The reaction produces a red-coloured azo dye.

The main steps involve:

1. Formation of nitrous acid.

2. Diazotisation of an aromatic amine.

3. Formation of a diazonium salt.

4. Coupling reaction.

5. Formation of a coloured azo compound.

Chloride Ion

Chloride ions react with concentrated sulfuric acid to produce hydrogen chloride gas.

A major confirmatory test is the silver nitrate test, in which chloride ions produce a white precipitate of silver chloride.

Ag⁺ + Cl⁻ → AgCl↓

Silver chloride dissolves in dilute ammonia solution because of complex formation.

Chromyl Chloride Test

The chromyl chloride test is an important test for chloride ions.

A chloride-containing salt is heated with potassium dichromate and concentrated sulfuric acid. Red-brown vapours of chromyl chloride, CrO₂Cl₂, are produced.

This reaction is associated with chloride identification in qualitative analysis.

Bromide Ion

Bromide ions can be oxidised to bromine by suitable oxidising agents.

Bromine vapours are reddish-brown in colour.

Silver nitrate also gives a precipitate with bromide ions:

Ag⁺ + Br⁻ → AgBr↓

Silver bromide is a cream-coloured precipitate and has different solubility behaviour in ammonia compared with silver chloride.

Iodide Ion

Iodide ions are oxidised to iodine by suitable oxidising agents.

Iodine produces violet vapours and can be identified through characteristic reactions.

With silver nitrate:

Ag⁺ + I⁻ → AgI↓

Silver iodide forms a yellow precipitate.

Iodide can also be oxidised by manganese dioxide in the presence of concentrated sulfuric acid to release iodine.

Nitrate Ion: Brown-Ring Test

The brown-ring test is an important confirmatory test for nitrate ions.

In this test, freshly prepared ferrous sulfate solution is added to the nitrate-containing solution, followed carefully by concentrated sulfuric acid. A brown ring appears at the interface between the two layers.

The brown ring is associated with the formation of a nitrosyl complex of iron.

The test should be performed carefully because concentrated sulfuric acid is involved.

Oxalate Ion

Oxalate ions undergo characteristic reactions on heating with concentrated sulfuric acid.

The products include carbon dioxide and carbon monoxide. Carbon dioxide can be confirmed because it turns limewater milky.

Oxalate can also be tested using calcium salts, which produce a white precipitate of calcium oxalate.

Sulfate Ion

Sulfate ions react with barium chloride to produce a white precipitate of barium sulfate.

Ba²⁺ + SO₄²⁻ → BaSO₄↓

Barium sulfate is highly insoluble and the formation of the white precipitate is an important confirmatory observation for sulfate.

Phosphate Ion

Phosphate ions can be tested using a molybdate-based reagent system.

In the appropriate acidic medium, phosphate produces a canary-yellow precipitate associated with ammonium phosphomolybdate.

This yellow precipitate is an important observation in the qualitative analysis of phosphate ions.

Dry Tests for Cations

Dry tests are performed without preparing an aqueous solution of the salt. Two important dry tests are:

1. Flame test

2. Borax bead test

Flame Test

Certain metal ions produce characteristic colours when their compounds are heated in a flame.

Important observations include:

Be²⁺ and Mg²⁺ do not produce characteristic flame colours under the usual flame-test conditions.

The flame test is based on the excitation of electrons to higher energy levels. When the excited electrons return to lower energy levels, energy is released in the form of light of characteristic wavelengths.

Borax Bead Test

Borax has the formula:

Na₂B₄O₇·10H₂O

On heating, borax loses water and forms a glassy bead containing sodium metaborate and boric anhydride.

The bead can react with certain metal oxides to produce coloured metaborates. Different metal ions can therefore produce characteristic bead colours.

The borax bead test is particularly useful for identifying certain transition metal ions.

Wet Tests and Group Analysis of Cations

Systematic cation analysis is based on the selective precipitation of ions.

Two important principles involved are:

• Solubility product (Ksp)

• Common-ion effect

Cations are separated into groups by adding specific group reagents under controlled conditions.

Group 0

Ammonium ion, NH₄⁺, is tested separately because it does not belong to the precipitation groups used for the metallic cations.

Nessler's reagent, K₂[HgI₄], in alkaline medium gives a brown colour or brown precipitate with ammonium ions.

Group I

The group reagent is dilute hydrochloric acid.

Group I cations form insoluble chlorides.

Important examples include:

• Ag⁺

• Pb²⁺

• Hg₂²⁺

Silver chloride forms a white precipitate.

Lead can show behaviour in both Group I and Group II analysis because of differences in the solubility of its compounds under different conditions.

Group II

Hydrogen sulfide gas is passed through an acidic solution.

Group II cations form sulfide precipitates that are sufficiently insoluble to precipitate at the low sulfide-ion concentration maintained in an acidic medium.

Important Group II cations include sulfides of ions such as Cu²⁺, Cd²⁺, Bi³⁺, Hg²⁺, As³⁺, Sb³⁺ and Sn⁴⁺, depending on the subgroup classification used.

The acidic medium suppresses the concentration of sulfide ions, allowing selective precipitation based on solubility products.

Group III

The group reagent consists of ammonium chloride and ammonium hydroxide.

Cations form hydroxide precipitates under these conditions.

Important observations include:

• Al(OH)₃: White gelatinous precipitate

• Fe(OH)₃: Reddish-brown precipitate

• Cr(OH)₃: Green precipitate

The controlled hydroxide-ion concentration allows selective precipitation of Group III cations.

Group IV

Hydrogen sulfide is passed through a basic or ammoniacal medium.

Important Group IV cations include:

• Ni²⁺

• Co²⁺

• Mn²⁺

• Zn²⁺

The basic medium increases the concentration of sulfide ions, allowing these sulfides to precipitate.

Group V

Ammonium carbonate is used as the group reagent.

Important Group V cations include:

• Ba²⁺

• Sr²⁺

• Ca²⁺

These ions form carbonate precipitates under the required conditions.

Group VI

The remaining cations are tested individually.

Mg²⁺ can be confirmed using disodium hydrogen phosphate, Na₂HPO₄, in the presence of ammonium salts and ammonia.

A white crystalline precipitate of magnesium ammonium phosphate is formed.

Important Confirmatory Cation Tests

Nickel Ion

Ni²⁺ reacts with dimethylglyoxime (DMG) in an ammoniacal medium to form a characteristic red precipitate.

This is an important confirmatory test for nickel ions.

Ferric Ion

Fe³⁺ reacts with thiocyanate ions to form a blood-red complex.

It also gives a Prussian blue colour or precipitate with potassium ferrocyanide.

Copper Ion

Cu²⁺ reacts with potassium ferrocyanide to produce a characteristic chocolate-brown precipitate.

Zinc Ion

Zn²⁺ forms a bluish-white precipitate with potassium ferrocyanide under appropriate conditions.

Role of Ksp and Common-Ion Effect in Group Analysis

The selective precipitation of cations depends on differences in the solubility products of their compounds.

For a sparingly soluble salt:

AB(s) ⇌ A⁺ + B⁻

Its solubility product is:

Ksp = [A⁺][B⁻]

When the ionic product exceeds the value of Ksp, precipitation occurs.

The common-ion effect decreases the solubility of a weakly soluble electrolyte when one of its ions is already present in the solution.

These principles help control which ions precipitate in each group during systematic cation analysis.

Key Takeaways for NEET 2026

• Salt analysis identifies anions and cations through preliminary, dry, wet and confirmatory tests.

• Carbonate produces CO₂, which turns limewater milky.

• Sulfide produces H₂S and gives a black precipitate with lead acetate.

• Chloride gives a white AgCl precipitate with silver nitrate.

• Bromide forms a cream AgBr precipitate, while iodide forms a yellow AgI precipitate.

• The chromyl chloride test is an important test for chloride.

• The brown-ring test is used for nitrate.

• Sulfate produces a white BaSO₄ precipitate with barium chloride.

• Phosphate gives a canary-yellow precipitate with the appropriate molybdate reagent.

• Flame tests help identify certain cations through characteristic flame colours.

• Ni²⁺ gives a characteristic red precipitate with dimethylglyoxime.

• Fe³⁺ gives a blood-red complex with thiocyanate.

• Cation group analysis depends on Ksp, ionic product and the common-ion effect.

• Group I uses dilute hydrochloric acid, while Group II uses hydrogen sulfide in an acidic medium.

• Group III hydroxides are precipitated using ammonium chloride and ammonium hydroxide.

• Group IV sulfides are precipitated in a basic or ammoniacal medium.

• Group V cations are precipitated as carbonates using ammonium carbonate.

• Magnesium is tested individually in Group VI analysis.

Salt analysis requires careful observation of reactions, colours, precipitates and gas evolution. Revise important anion tests, dry tests for cations, systematic cation group analysis, confirmatory reactions and the role of Ksp and the common-ion effect. For NEET 2026 preparation, PW resources such as PYQs, MCQs, revision material and video lectures can support regular practice and help strengthen qualitative analysis concepts. 

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