Common Chemical Tests — Chemistry Mnemonic
Target: mnemonic for common chemical tests qualitative analysis
Why is this hard to memorize?
Chemical tests are the detective tools of chemistry — they identify unknown substances based on characteristic reactions. NEET and JEE test you on: Tollens test (silver mirror for aldehydes), Fehling test (red precipitate for aldehydes), Lucas test (distinguishes 1°/2°/3° alcohols), Iodoform test (methyl ketones and ethanol), Baeyer test (purple KMnO₄ for unsaturation), Lassaigne test (detecting N, S, halogens), and many more. The key is knowing which test detects what, the reagent used, and the positive result observation.
Classic mnemonics you should know
"Tollens = Silver mirror (T for Tin foil shine). Fehling = Red precipitate (F for Fire-red Cu₂O)"
Both Tollens and Fehling test for aldehydes: Tollens (Ag(NH₃)₂⁺) gives a silver mirror on the test tube wall. Fehling (Cu²⁺ in tartrate) gives a red precipitate of Cu₂O. Ketones do NOT give either test (except α-hydroxy ketones).
"3° = Immediate turbidity, 2° = 5 minutes, 1° = No reaction at RT"
Lucas test (ZnCl₂/conc. HCl) distinguishes alcohols by SN1 reactivity: 3° alcohols react instantly (most stable carbocation), 2° react in minutes, 1° don't react at room temperature. "3-2-1 countdown: 3 is fastest."
"CH₃CO- or CH₃CH(OH)- gives CHI₃ yellow precipitate with I₂/NaOH"
The iodoform test is positive for: methyl ketones (CH₃COR), acetaldehyde (CH₃CHO), ethanol (CH₃CH₂OH), and secondary alcohols with CH₃CH(OH)- group. Product is yellow CHI₃ with a characteristic smell.
The complete list
- Tollens test (silver mirror → aldehyde)
- Fehling test (red Cu₂O → aldehyde)
- Lucas test (turbidity timing → 1°/2°/3° alcohol)
- Iodoform test (yellow ppt → CH₃CO- group)
- Baeyer test (KMnO₄ decolourized → unsaturation)
- Lassaigne test (Na fusion → N, S, halogens)
- Biuret test (violet colour → peptide bond)
- Ninhydrin test (purple → amino acids)
- Molisch test (violet ring → carbohydrates)
Frequently asked questions
How do I remember the difference between Tollens and Fehling test?
Both detect aldehydes but with different observations: Tollens(T) → silver mirror (T for Tin/shiny). Fehling(F) → red precipitate of Cu₂O (F for Fire-red). Tollens uses Ag⁺, Fehling uses Cu²⁺. Both involve oxidation of aldehyde to carboxylate.
What compounds give a positive iodoform test?
Any compound with a CH₃CO– group (methyl ketone) or CH₃CH(OH)– group (which gets oxidized to CH₃CO– first). Specific examples: acetone (CH₃COCH₃), acetaldehyde (CH₃CHO), ethanol (CH₃CH₂OH), 2-propanol. A yellow precipitate of CHI₃ (iodoform) confirms the test.
How does the Lucas test work?
Lucas reagent (anhydrous ZnCl₂ + conc. HCl) converts alcohols to alkyl chlorides via SN1 mechanism. Alkyl chlorides are insoluble → solution turns turbid. 3° alcohols form the most stable carbocation → react immediately. 2° alcohols react in ~5 min. 1° alcohols don't react at room temperature (would need SN2 conditions).
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