Qualitative Analysis — Cation Groups Mnemonic
Target: mnemonic for qualitative analysis cation groups chemistry
Why is this hard to memorize?
Qualitative salt analysis uses systematic separation of cations into groups using group reagents. The five groups are: Group 0 (NH₄⁺ — no group reagent needed, identified by smell/litmus), Group I (Pb²⁺, Ag⁺, Hg₂²⁺ — precipitated by dilute HCl), Group II (Cu²⁺, Hg²⁺, Bi³⁺, Cd²⁺, As³⁺, Sb³⁺, Sn²⁺ — precipitated by H₂S in acidic medium), Group III (Fe³⁺, Al³⁺, Cr³⁺ — precipitated by NH₄OH + NH₄Cl), Group IV (Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ — precipitated by H₂S in basic medium), Group V (Ba²⁺, Sr²⁺, Ca²⁺ — precipitated by (NH₄)₂CO₃). Knowing the group reagent and which cations belong to each group is essential for NEET practicals.
Classic mnemonics you should know
"I=dil HCl, II=H₂S(acid), III=NH₄OH, IV=H₂S(base), V=(NH₄)₂CO₃"
Each group has a specific reagent that precipitates its cations: Group I uses dilute HCl (chlorides are insoluble). Group II uses H₂S in acidic solution. Group III uses ammonium hydroxide. Group IV uses H₂S in basic solution. Group V uses ammonium carbonate.
"Pb, Ag, Hg₂ — "Plumber Argues with Mercury" — form insoluble CHLORIDES"
Group I cations form precipitates with dilute HCl: PbCl₂ (white, soluble in hot water), AgCl (white, curdy, turns grey in light), Hg₂Cl₂ (white, turns black with NH₃). These are the three common insoluble chlorides.
"Ba, Sr, Ca — "BarStar Calcium" — form insoluble CARBONATES"
Group V cations (alkaline earth metals) are precipitated by ammonium carbonate: BaCO₃, SrCO₃, CaCO₃. Distinguished by flame test: Ba=green, Sr=crimson, Ca=brick red.
The complete list
- Group 0: NH₄⁺ (detected by NaOH → NH₃ gas)
- Group I: Pb²⁺, Ag⁺, Hg₂²⁺ (dil. HCl → chlorides)
- Group II: Cu²⁺, Hg²⁺, Bi³⁺, Cd²⁺, As³⁺, Sb³⁺, Sn²⁺ (H₂S acidic)
- Group III: Fe³⁺, Al³⁺, Cr³⁺ (NH₄OH → hydroxides)
- Group IV: Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺ (H₂S basic)
- Group V: Ba²⁺, Sr²⁺, Ca²⁺ ((NH₄)₂CO₃ → carbonates)
- Group VI: Mg²⁺, Na⁺, K⁺ (no group reagent)
Frequently asked questions
Why must cation analysis follow a specific order?
Each group reagent only works if previous groups have been removed. For example, H₂S in acidic medium (Group II) would also precipitate Group IV cations if they were present. By removing Group I first (with HCl) and Group II next, you ensure clean separation. The order goes from most insoluble precipitates to least.
How do I confirm a specific cation after group separation?
After separating into groups, individual cations are confirmed by specific tests: Pb²⁺ → yellow PbCrO₄ with K₂CrO₄. Ag⁺ → soluble in NH₃ (forms [Ag(NH₃)₂]⁺). Cu²⁺ → blue colour with excess NH₃. Fe³⁺ → Prussian blue with K₄[Fe(CN)₆]. These confirmatory tests are the "final answer."
What is Group 0 and why is NH₄⁺ tested first?
NH₄⁺ is tested before systematic analysis because: (1) ammonium salts are added in later steps (NH₄Cl, NH₄OH), which would interfere with NH₄⁺ detection. (2) Detection is simple: add NaOH and heat → if NH₃ gas is released (turns wet red litmus blue), NH₄⁺ is present. Always test for NH₄⁺ with the original salt BEFORE adding any reagents.
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