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Analysis of Acidic Radicals (Anions): Group Reagents, Tests & JEE MCQs
1. Introduction to Acidic Radicals
In qualitative inorganic salt analysis, the identification of the acidic radical (anion) is typically performed before identifying the basic radical (cation). Anions are derived from acids (e.g., \(Cl^-\) from \(HCl\), \(SO_4^{2-}\) from \(H_2SO_4\)).
Based on their reactions with specific acidic reagents and the volatility of the resulting products, acidic radicals are systematically classified into three main groups:
- Group I (Dilute Acid Group): Anions that react with dilute \(HCl\) or dilute \(H_2SO_4\) to evolve characteristic gases.
- Group II (Concentrated Acid Group): Anions that do not react with dilute acids but react with concentrated \(H_2SO_4\) upon heating.
- Group III (Special/Independent Group): Anions that do not evolve gases with either dilute or concentrated acids and require specific precipitation reagents (like \(BaCl_2\)).
2. Reference Charts & Class Notes
Review these comprehensive summary charts to quickly grasp the group divisions, reagents, and primary observations for all acidic radicals.
Fig 1. Overview of Acid Radicals
Fig 2. Group I and II Classification
Fig 3. Group III Specifics
Fig 4. Group Reagent Observations (A)
Fig 5. Group Reagent Observations (B)
3. Group I: Dilute Acid Group
Group Reagent: Dilute \(HCl\) or Dilute \(H_2SO_4\).
When the dry salt is treated with dilute acid, these anions undergo decomposition to evolve characteristic gases. The identification is primarily based on the smell, color, and specific chemical tests of the evolved gas.
| Anion | Gas Evolved | Observation / Confirmatory Test |
|---|---|---|
| Carbonate (\(CO_3^{2-}\)) | \(CO_2\) | Colorless, odorless gas with brisk effervescence. Turns lime water milky (\(CaCO_3\)). Milkiness disappears on passing excess gas due to formation of soluble \(Ca(HCO_3)_2\). |
| Sulfide (\(S^{2-}\)) | \(H_2S\) | Colorless gas with a strong smell of rotten eggs. Turns lead acetate paper black (\(PbS\)). Gives a purple color with Sodium Nitroprusside solution. |
| Sulfite (\(SO_3^{2-}\)) | \(SO_2\) | Colorless gas with a suffocating smell of burning sulfur. Turns acidified Potassium Dichromate (\(K_2Cr_2O_7\)) paper from orange to green. |
| Nitrite (\(NO_2^-\)) | \(NO_2\) | Reddish-brown fumes with a pungent smell. Gives a positive Griess-Ilosvay Test (red azo dye) and forms a brown ring with *dilute* acid. |
| Acetate (\(CH_3COO^-\)) | \(CH_3COOH\) | Colorless vapors with a distinct vinegar-like smell. Confirmed by the Esterification test (fruity smell with ethanol + conc. \(H_2SO_4\)) or Ferric Chloride test (blood red color). |
4. Group II: Concentrated Acid Group
Group Reagent: Concentrated \(H_2SO_4\) (with heating).
These anions are derived from stronger acids. They do not decompose with dilute acids but require the strong dehydrating and oxidizing properties of hot, concentrated Sulfuric Acid.
| Anion | Gas Evolved | Observation / Confirmatory Test |
|---|---|---|
| Chloride (\(Cl^-\)) | \(HCl\) | Colorless, pungent gas. Gives dense white fumes (\(NH_4Cl\)) when a glass rod dipped in \(NH_4OH\) is brought near the mouth. Confirmed by the Chromyl Chloride Test (red vapors). |
| Bromide (\(Br^-\)) | \(Br_2\) | Reddish-brown fumes. Turns starch iodide paper blue. Gives a pale yellow precipitate with \(AgNO_3\) (sparingly soluble in \(NH_4OH\)). Confirmed by the Layer Test (Chloroform test) giving an orange/brown organic layer. |
| Iodide (\(I^-\)) | \(I_2\) | Deep violet/purple fumes that condense on the cooler parts of the tube. Gives a yellow precipitate with \(AgNO_3\) (insoluble in \(NH_4OH\)). Layer Test gives a violet organic layer. |
| Nitrate (\(NO_3^-\)) | \(NO_2\) | Reddish-brown fumes, intensity increases upon adding copper turnings. Confirmed definitively by the Brown Ring Test using fresh \(FeSO_4\) and conc. \(H_2SO_4\). |
| Oxalate (\(C_2O_4^{2-}\)) | \(CO + CO_2\) | Colorless gas mixture. \(CO\) burns with a blue flame at the mouth of the tube. Decolorizes warm, acidified \(KMnO_4\) solution. |
5. Group III: Special / Independent Group
Group Reagents: Specific precipitating agents (e.g., \(BaCl_2\) for Sulfate, Ammonium Molybdate for Phosphate).
These anions do not evolve characteristic gases with acids. They are identified by their precipitation reactions in aqueous solutions.
1. Sulfate (\(SO_4^{2-}\))
- Barium Chloride Test: To the aqueous extract, add \(BaCl_2\) solution. A heavy white precipitate of Barium Sulfate (\(BaSO_4\)) forms.
- Confirmation: This white precipitate is insoluble in concentrated \(HCl\) and concentrated \(HNO_3\), distinguishing it from sulfites or carbonates which dissolve.
2. Phosphate (\(PO_4^{3-}\))
- Ammonium Molybdate Test: To the aqueous extract, add concentrated \(HNO_3\) and boil. Then add Ammonium Molybdate solution and heat again.
- Confirmation: A distinctive canary yellow precipitate of Ammonium Phosphomolybdate \((NH_4)_3[PMo_{12}O_{40}]\) is formed.
6. Mega Exhaustive MCQ Bank (JEE Main, Advanced & NEET Level)
Test your mastery of Acidic Radicals. Click "Show Solution & Explanation" to reveal the detailed logic behind each answer.
Q1. A salt on treatment with dilute \(HCl\) gives a pungent fuming gas which turns acidified \(K_2Cr_2O_7\) paper green. The anion present is:
- A) Carbonate (\(CO_3^{2-}\))
- B) Sulfite (\(SO_3^{2-}\))
- C) Sulfide (\(S^{2-}\))
- D) Chloride (\(Cl^-\))
Show Solution & Explanation
Explanation: Sulfites react with dilute acids to evolve \(SO_2\) gas. \(SO_2\) is a reducing agent that reduces the orange dichromate ion (\(Cr_2O_7^{2-}\), Cr=+6) to the green chromic ion (\(Cr^{3+}\)).
Q2. Which of the following anions will give a positive result in the Brown Ring test using dilute \(H_2SO_4\)?
- A) Nitrate (\(NO_3^-\))
- B) Nitrite (\(NO_2^-\))
- C) Both A and B
- D) Neither A nor B
Show Solution & Explanation
Explanation: Nitrite (\(NO_2^-\)) is easily reduced by \(Fe^{2+}\) even in the presence of dilute acids to form NO gas, yielding a brown ring. Nitrate (\(NO_3^-\)) is more stable and strictly requires concentrated \(H_2SO_4\) to undergo the reduction.
Q3. The addition of \(AgNO_3\) solution to an aqueous extract of a salt gives a pale yellow precipitate that is sparingly soluble in \(NH_4OH\). The anion is most likely:
- A) Chloride (\(Cl^-\))
- B) Bromide (\(Br^-\))
- C) Iodide (\(I^-\))
- D) Sulfate (\(SO_4^{2-}\))
Show Solution & Explanation
Explanation: Chloride: White ppt (\(AgCl\)), completely soluble in \(NH_4OH\). Bromide: Pale yellow ppt (\(AgBr\)), sparingly/partially soluble in \(NH_4OH\). Iodide: Yellow ppt (\(AgI\)), completely insoluble in \(NH_4OH\).
Q4. A white precipitate of \(BaSO_4\) is formed when testing for sulfate. To confirm it is sulfate and not carbonate or sulfite, the precipitate is treated with:
- A) Distilled water
- B) Dilute \(NaOH\)
- C) Concentrated \(HCl\) or \(HNO_3\)
- D) Ammonium Hydroxide
Show Solution & Explanation
Explanation: Barium carbonate (\(BaCO_3\)) and Barium sulfite (\(BaSO_3\)) are also white precipitates that form upon adding \(BaCl_2\). However, they dissolve in strong mineral acids like \(HCl\) to evolve \(CO_2\) and \(SO_2\) respectively. Barium sulfate (\(BaSO_4\)) is highly insoluble and remains unaffected even in concentrated mineral acids, distinguishing it from the others.
Q5. Heating an unknown salt with concentrated \(H_2SO_4\) and \(MnO_2\) yields a greenish-yellow fuming gas. The anion is:
- A) Chloride
- B) Bromide
- C) Nitrate
- D) Oxalate
Show Solution & Explanation
Explanation: Concentrated \(H_2SO_4\) reacting with a chloride salt produces \(HCl\) gas. When an oxidizing agent like \(MnO_2\) is added, it oxidizes the \(HCl\) into elemental Chlorine gas (\(Cl_2\)), which is characterized by its distinct greenish-yellow color and suffocating odor. (\(MnO_2 + 4HCl \rightarrow MnCl_2 + 2H_2O + Cl_2 \uparrow\)).
Q6. Which of the following tests is definitively used to detect the presence of Acetate (\(CH_3COO^-\)) ions?
- A) Chromyl Chloride Test
- B) Brown Ring Test
- C) Esterification Test (Fruity Smell)
- D) Griess-Ilosvay Test
Show Solution & Explanation
Explanation: When an acetate salt is heated with concentrated \(H_2SO_4\) and ethanol (\(C_2H_5OH\)), an esterification reaction occurs. The product is Ethyl Acetate (\(CH_3COOC_2H_5\)), which is recognized by its pleasant, sweet, fruity odor.
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