Salt Analysis NEET: Complete Notes on Anion & Cation Tests, Reactions, Flame Test and MCQs

Salt Analysis NEET is one of the most important topics in Inorganic Chemistry for medical entrance examinations. It helps students understand the identification of cations and anions present in a salt. Questions from this topic are frequently asked in NEET because it combines chemical reactions, observations, and analytical reasoning.

The chapter involves the systematic study of acidic radicals and basic radicals. Students are expected to recognize characteristic tests, colors, precipitates, gases, and reactions associated with different ions. A clear understanding of these concepts helps in solving examination questions quickly and accurately. The topic covers the qualitative identification of various ions through laboratory tests and reaction-based observations.

Salt Analysis NEET PDF

What is Salt Analysis?

Salt analysis is defined as the analysis of cation and anion present in a given mixture. A salt is formed when an acid reacts with a base.

Acid + Base → Salt + H₂O

For example: HCl + NaOH → NaCl + H₂O

In the salt NaCl, Na⁺ is the basic radical (cation) and Cl⁻ is the acidic radical (anion). The goal of qualitative analysis NEET chemistry is to detect these radicals through well-defined tests.

The cations tested in the NEET syllabus are: Pb²⁺, Cu²⁺, Al³⁺, Fe³⁺, Zn²⁺, Ni²⁺, Ca²⁺, Ba²⁺, Mg²⁺, and NH₄⁺.

The anions tested are: CO₃²⁻, S²⁻, SO₄²⁻, NO₃⁻, NO₂⁻, Cl⁻, Br⁻, and I⁻ (insoluble salts are excluded).

Classification of Anions

Anions are divided into two broad classes.

Class A anions give a volatile product with acids. These are further divided into two subgroups:

• Subgroup I: Give a volatile product with dilute acid. Examples include CO₃²⁻, HCO₃⁻, SO₃²⁻, HSO₃⁻, S²⁻, S₂O₃²⁻, CH₃COO⁻, and NO₂⁻.

• Subgroup II: Give a volatile product with concentrated acid. Examples include F⁻, Cl⁻, Br⁻, I⁻, BO₃³⁻, C₂O₄²⁻, and NO₃⁻.

Class B anions do not give a volatile product with acids. These are divided into:

• Subgroup I: Undergo precipitation reactions. Examples include SO₄²⁻, PO₄³⁻, AsO₄³⁻, AsO₃³⁻, and CrO₄²⁻.

• Subgroup II: Undergo redox reactions. Examples include MnO₄⁻, Cr₂O₇²⁻, and CrO₄²⁻.

Preliminary Tests with Dilute Sulphuric Acid

These tests are part of the salt analysis notes for NEET that every student must memorise.

• CO₃²⁻ ion: When carbonate reacts with dilute H₂SO₄, it produces CO₂ gas. CO₂ is colourless, odourless, and tasteless. It produces brisk effervescence. It turns lime water (Ca(OH)₂) milky due to the formation of CaCO₃. With excess CO₂, the solution becomes clear again because Ca(HCO₃)₂ is soluble.

• SO₃²⁻ ion: With dilute H₂SO₄, sulphite produces SO₂ gas. SO₂ is colourless with a suffocating smell of burning sulphur. It turns lime water milky and also turns K₂Cr₂O₇ solution (orange) green.

• S²⁻ ion: With dilute H₂SO₄, sulphide produces H₂S gas. H₂S is colourless with a characteristic rotten egg smell. It turns lead acetate paper black.

• Reaction: H₂S + Pb(CH₃COO)₂ → PbS↓ (black) + CH₃COOH

• NO₂⁻ ion: With dilute H₂SO₄, nitrite produces brown NO₂ gas. NO₂ is paramagnetic due to one unpaired electron. It dimerises to form N₂O₄, which is colourless.

Test with Concentrated Sulphuric Acid

When concentrated H₂SO₄ is used (Subgroup II anions of Class A):

• F⁻ + Conc. H₂SO₄ → HF↑ (white fumes)

• Cl⁻ + Conc. H₂SO₄ → HCl↑ (white fumes)

• Br⁻ + Conc. H₂SO₄ → Br₂↑ (red/reddish brown); turns starch solution yellow

• I⁻ + Conc. H₂SO₄ → I₂↑ (violet); turns starch solution blue

• NO₃⁻ + Conc. H₂SO₄ → NO₂↑

The identification of halogens through these colour reactions is a key area for salt analysis MCQ for NEET. A common question is: which anion gives a brown gas with dilute H₂SO₄? The answer is NO₂⁻ (nitrite), which produces brown NO₂ gas.

Confirmatory Tests of Anions

Before performing confirmatory tests, it is important to remove interfering radicals. The interfering radicals are: F⁻, C₂O₄²⁻, BO₃³⁻, PO₄³⁻, AsO₄³⁻, and SiO₄⁴⁻.

• Test of CO₃²⁻ (Carbonate Ion): Solubility note: All carbonates are insoluble except those of Group IA metals (Li₂CO₃, Na₂CO₃, K₂CO₃) and (NH₄)₂CO₃. Test with dilute acid: CO₃²⁻ + H⁺ → CO₂↑ (brisk effervescence). The CO₂ gas turns lime water milky.

• Test of S²⁻ (Sulphide Ion): With dilute acid: S²⁻ + H⁺ → H₂S↑ (rotten egg smell, turns lead acetate paper black). Reaction: Pb(CH₃COO)₂ + H₂S → PbS↓ (Black) + CH₃COOH

• Test of SO₄²⁻ (Sulphate Ion): Test with BaCl₂ solution: SO₄²⁻ + BaCl₂ → BaSO₄↓ (white precipitate). BaSO₄ is insoluble in concentrated HCl and concentrated HNO₃. This is a distinguishing property.

• Test of Br⁻ (Bromide Ion): On heating with MnO₂ and H₂SO₄: NaBr + MnO₂ + H₂SO₄ → Br₂↑ (Red/Reddish Brown) + MnSO₄ + NaHSO₄

Important Notes on the Chromyl Chloride Test (for Cl⁻)

The chromyl chloride test is an important part of inorganic chemistry salt analysis. Key notes for this test:

1. The test tube must be dry (H₂CrO₄ is involved).

2. NO₂⁻ and NO₃⁻ must be absent.

3. HgCl₂ and Hg₂Cl₂ do not give the chromyl chloride test (high covalent character).

4. SnCl₂ (+2 state) gives the test but SnCl₄ (+4 state) does not.

A common PYQ from salt analysis PYQs NEET: Which ion gives a brown ring test? The answer is NO₂⁻ (nitrite), and it gives the brown ring test with both dilute and concentrated H₂SO₄.

Flame Test

The flame test is used to identify certain cations by the colour they impart to the flame. A platinum wire or loop is used. The salt paste is introduced into the Bunsen flame.

The Bunsen burner flame has several zones:

• Lowest temperature zone (inner dark cone)

• Lower reducing zone

• Middle blue cone (not as hot as outer cone due to insufficient oxygen)

• Lower and upper oxidising zones

• The hottest portion is just above the blue cone

Flame colours to remember:

Wet Test (Original Solution)

The wet test is used for the identification of cations. The salt is dissolved in water. If it does not dissolve in water, dilute HCl is used. If still insoluble, concentrated HCl is tried, then concentrated HNO₃. Aqua Regia is not used.

Analysis of Cations: Group-wise Summary

• Zero Group – NH₄⁺ Ion: NH₄⁺ salt + alkali (NaOH) → NH₃↑ (pungent smell) NH₄Cl + NaOH → NH₃↑ + NaCl NH₃ gas reacts with HCl to form dense white fumes of NH₄Cl. (NH₄)₂SO₄ + Ca(OH)₂ → NH₃↑ + CaSO₄ + H₂O

• Test for Pb²⁺ Ion: Reaction with KI: PbCl₂ + KI → PbI₂ → with excess KI → [PbI₄]²⁻ (soluble)

• Test for Al³⁺ Ion: With NaOH: Al³⁺ + NaOH → Al(OH)₃↓ (white gelatinous precipitate). With excess NaOH, it dissolves to form soluble Na[Al(OH)₄].

Borax Bead Test: The borax bead test is used for transition metal cations. In this test, Na₂B₄O₇ (borax) is fused with the salt on a platinum loop. The bead formed shows a characteristic colour due to the metal metaborate formed.

For example, a chromium(III) salt gives a green colour in the borax bead test. This is due to the formation of Cr(BO₂)₃ (chromium metaborate). This was asked in JEE Main 2019 and is also relevant to salt analysis questions for NEET.

Salt Analysis Tricks for NEET

To score well in Salt Analysis NEET, students should remember the following tricks:

1. Lime water test: CO₂ and SO₂ both turn lime water milky. Differentiate by smell: SO₂ has a suffocating smell, CO₂ is odourless.

2. Lead acetate paper: Only H₂S turns it black. This is specific for S²⁻ ion.

3. Brown gas = NO₂: Any reaction producing a brown gas indicates the presence of either NO₂⁻ or NO₃⁻.

4. BaSO₄ is acid-insoluble: BaSO₄ precipitate is insoluble in conc. HCl and conc. HNO₃. This distinguishes sulphate from sulphite (which is soluble in acid).

5. Carbonates vs Sulphites: Both give CO₂-like effervescence with acid, but sulphite gives SO₂ with a burning sulphur smell. Test with K₂Cr₂O₇ solution (turns green only with SO₂).

6. Solubility rule for sulphides: CdS is yellow and insoluble. Na₂S, BaS, and (NH₄)₂S are soluble. This is an important MCQ point.

7. NH₄⁺ test: Always test with NaOH first. Pungent ammonia smell with formation of dense white fumes with HCl rod confirms NH₄⁺.

8. Flame test mnemonic: "Sodium is Yellow, Potassium is Lilac/Violet, Calcium is Brick Red, Barium is Apple Green, Strontium is Crimson."

Sample MCQ Questions

The following types of questions are common in Salt Analysis NEET examinations:

Q1. Which of the following will give brown gas with dilute H₂SO₄?

(A) NO₂⁻

(B) NO₃⁻

(C) Br⁻

(D) All of these

Answer: (A) NO₂⁻ — Nitrite ion produces NO₂ (brown gas) with dilute H₂SO₄. Nitrate needs concentrated H₂SO₄ to produce NO₂.

Q2. Which of the following gives a suffocating gas when treated with dilute HCl?

(A) Carbonate

(B) Sulphite

(C) Sulphate

(D) Borate

Answer: (B) Sulphite — Sulphite produces SO₂ (suffocating smell of burning sulphur) with dilute acid. Carbonate gives CO₂, which is odourless.

Q3. NO₂⁻ will give a brown ring test with:

(A) Dil. H₂SO₄

(B) Conc. H₂SO₄

(C) Both

(D) None

Answer: (C) Both — NO₂⁻ gives the brown ring test with both dilute and concentrated H₂SO₄.

Q4. Which of the following is insoluble?

(A) Na₂S

(B) BaS

(C) CdS

(D) (NH₄)₂S

Answer: (C) CdS — CdS (cadmium sulphide) is yellow and insoluble. The others are soluble sulphides.

Salt Analysis NEET is a topic that rewards systematic study. Every test has a clear logic behind it, and once the pattern is understood, it becomes easy to solve even tricky multiple-choice questions. Students must focus on the colour of the gas evolved, the odour, the reagent used, and the colour of the precipitate formed. Regular practice of salt analysis, important questions and past year questions helps reinforce memory. This topic is a part of inorganic chemistry salt analysis in Class 12 and carries consistent weightage in NEET each year.

With focused preparation and a clear understanding of the principles of qualitative analysis, students can confidently attempt all salt analysis questions that appear in the examination.