Ever wondered how chemists crack the secret code hidden inside a white powder?
In simple words, salt analysis is a detective game. You add a few chemicals, watch the colors change, and you can tell which ions are hiding in the sample.
Why Salt Analysis Matters in ICSE Chemistry
ICSE exams love practical skills. Knowing the salt analysis steps shows you can link theory to real lab work. It also scores you marks in the analytical chemistry section.
What Are “Radicals” in Salt Analysis?
Here “radicals” means the positively charged ions (cations) that sit in a salt crystal. Think of them as the “players” in a team – each one has a distinct jersey (color or reaction) that helps you spot them.
Step‑by‑Step Procedure to Identify Cation Radicals
We’ll walk through the classic five‑group method used in ICSE labs. The flowchart below gives a quick visual of the order.
graph TD
A[Start with dry salt] --> B[Make aqueous solution]
B --> C[Preliminary test: flame test]
C --> D[Group I test: Add HCl]
D --> E[If precipitate, confirm with HNO3]
E --> F[Group II test: Add H2SO4]
F --> G[Group III test: Add NH4CN]
G --> H[Group IV test: Add NH4Cl + NH4OH]
H --> I[Group V: No precipitate, identify alkali metals]
I --> J[All cations identified]
Group I – Lead, Mercury(I) and Mercury(II) Salts
- Reagent: Dilute hydrochloric acid (HCl)
- What happens: A white precipitate forms if Pb²⁺, Hg₂²⁺ or HgCl₂ is present.
- Confirmatory test: Add nitric acid (HNO₃). The precipitate dissolves for Pb²⁺ but stays for mercury salts.
Group II – Barium, Calcium and Strontium Salts
- Reagent: Dilute sulfuric acid (H₂SO₄)
- What happens: A white precipitate appears for Ba²⁺, Ca²⁺ or Sr²⁺.
- Confirmatory test: Add ammonium carbonate (NH₄)₂CO₃. BaSO₄ stays insoluble, while CaCO₃ and SrCO₃ dissolve.
Group III – Silver, Lead(II) and Mercury(II) Salts
- Reagent: Ammonium cyanide (NH₄CN)
- What happens: A white (Ag⁺) or yellow (Pb²⁺) precipitate forms.
- Confirmatory test: Add dilute nitric acid. The precipitate dissolves for Ag⁺ but not for Pb²⁺.
Group IV – Ammonium and Some Transition Metals
- Reagent: Ammonium chloride (NH₄Cl) followed by ammonium hydroxide (NH₄OH)
- What happens: A pale blue or green precipitate may appear, indicating NH₄⁺ or metals like Cu²⁺.
Group V – Alkali Metal Salts
- Reagent: None needed – they stay in solution throughout the previous tests.
- Identification: Flame test (Na gives bright yellow, K gives lilac, etc.) confirms the alkali metal.
Quick Comparison Table
| Group | Key Cations | Reagent Used | Typical Observation | Confirmatory Test |
|---|---|---|---|---|
| I | Pb²⁺, Hg₂²⁺, HgCl₂ | HCl | White ppt. | HNO₃ – dissolves Pb²⁺ only |
| II | Ba²⁺, Ca²⁺, Sr²⁺ | H₂SO₄ | White ppt. | (NH₄)₂CO₃ – BaSO₄ insoluble |
| III | Ag⁺, Pb²⁺, Hg²⁺ | NH₄CN | White/yellow ppt. | HNO₃ – dissolves Ag⁺ only |
| IV | NH₄⁺, Cu²⁺, Fe³⁺ | NH₄Cl + NH₄OH | Pale blue/green ppt. | Complex‑ion formation |
| V | Na⁺, K⁺, Li⁺ | – (no precipitate) | Flame colours | Flame test |
Worked Example: Identify the Cations in a Sample Salt
Suppose you have an unknown white salt. Follow the five‑group scheme.
- Make a clear solution. No colour, so no immediate clue.
- Add HCl. A white precipitate appears. This puts us in Group I.
- Treat the precipitate with HNO₃. It dissolves, so Pb²⁺ is present (mercury salts would stay).
- Continue with the filtrate (the liquid left after removing the Pb precipitate). Add H₂SO₄ – no new ppt., so Ba²⁺, Ca²⁺, Sr²⁺ are absent.
- Add NH₄CN to the same filtrate. No precipitate, so Ag⁺, Pb²⁺, Hg²⁺ are also absent.
- Finally, perform a flame test on the residue. The flame glows bright yellow – indicating Na⁺.
Result: The unknown salt contains Pb²⁺ and Na⁺.
📝 Likely Exam Questions
- Question: Write the complete procedure to identify Group II cations.
Answer: Dissolve the sample, add dilute H₂SO₄. A white precipitate indicates Ba²⁺, Ca²⁺ or Sr²⁺. Add (NH₄)₂CO₃; if the ppt. remains, it is BaSO₄ (Ba²⁺). If it dissolves, the ion is Ca²⁺ or Sr²⁺, which can be distinguished by heating with HCl – CaCO₃ gives CO₂, SrCO₃ gives a white solid that does not decompose easily. - Question: How does the flame test help in identifying Group V cations?
Answer: Alkali metals do not precipitate with the reagents used for Groups I‑IV, so they stay in solution. Their characteristic flame colours – Na (yellow), K (lilac), Li (crimson) – reveal the ion. - Question: An unknown salt gives a white ppt. with HCl that dissolves on adding HNO₃. Which cation is present and why?
Answer: Lead ion (Pb²⁺) is present. PbCl₂ precipitates with HCl and dissolves in nitric acid, unlike mercury salts which stay insoluble. - Question: Why is ammonium cyanide used for Group III analysis?
Answer: NH₄CN forms insoluble cyanide salts with Ag⁺, Pb²⁺ and Hg²⁺, giving distinct precipitates that can be differentiated by further acid tests. - Question: List the five groups of cations used in ICSE salt analysis and one reagent for each.
Answer: Group I – HCl; Group II – H₂SO₄; Group III – NH₄CN; Group IV – NH₄Cl + NH₄OH; Group V – no precipitating reagent, identified by flame test.
#ICSE#Class 10#Chemistry#Analytical Chemistry#Salt Analysis
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