On this page
In the chemistry practical, you get an unknown organic compound and a rack of reagents. Students who test at random waste time and get confused by false positives. Students who follow a fixed order usually finish early. This post gives that order, the reaction behind each test, two worked unknowns and the traps the NCERT laboratory manual itself points out.
Which groups do you need to know?
The CBSE Chemistry curriculum for 2026-27 lists Class 12 tests for "unsaturation, alcoholic, phenolic, aldehydic, ketonic, carboxylic and amino (primary) groups". The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus ask for the principles behind detecting hydroxyl (alcoholic and phenolic), carbonyl, carboxyl and amino groups, and the JEE (Advanced) 2026 brochure adds the nitro group. Check the current documents for your exam.
Why follow an order at all?
Laboratory work is meant to teach reasoning, not just procedure. A widely cited review by Hofstein and Lunetta (2004) notes that science educators have long expected rich learning benefits from laboratory activities, and examines how students' lab work is assessed and how activities can serve the intended learning. In a functional group practical, the reasoning is the order of tests and what each result rules out.
The tests, in a sensible order
All tests and observations below are from the NCERT Class 12 chemistry laboratory manual, Unit 8.
1. Is it acidic?
Litmus: blue litmus turning red suggests a carboxylic acid or a phenol.
Sodium hydrogencarbonate: brisk effervescence of CO₂ means a carboxylic acid. Most phenols are not acidic enough, though the manual notes that strongly acidic phenols such as 2,4,6-trinitrophenol do dissolve.
Neutral ferric chloride: a violet, blue, green or red colour indicates a phenolic –OH. Phenol itself gives violet.
2. Is there a carbonyl group?
2,4-DNP: a yellow, orange or orange-red precipitate confirms an aldehyde or ketone.
Tollens' reagent: a silver mirror on warming means an aldehyde.
Fehling's solution: a brick-red precipitate of copper(I) oxide means an aldehyde, but the manual notes that aromatic aldehydes do not give this test.
Schiff's reagent: a pink colour means an aldehyde. Ketones do not respond.
3. Is it an alcohol?
Ceric ammonium nitrate: a red colour shows an alcoholic –OH. Use only a few drops, because excess reagent makes the colour disappear.
Lucas test: the alkyl chloride separates as a layer at once for tertiary alcohols, within 1 to 5 minutes for secondary and very slowly for primary. It works only for alcohols that dissolve in the reagent.
Iodoform test: yellow crystals of iodoform from ethanol and secondary alcohols with a CH₃–CH(OH)– group. NCERT's aldehydes and ketones chapter adds that aldehydes and ketones with a methyl group on the carbonyl carbon also undergo the haloform reaction.
4. Is it an amine?
Solubility in dilute HCl: amines form water-soluble salts.
Carbylamine test: an offensive smell on heating with chloroform and alcoholic KOH shows a primary amine. The isocyanide formed is highly toxic: destroy it at once with concentrated HCl.
Azo dye test: a scarlet red dye with β-naphthol after diazotisation at 0 to 5 °C confirms an aromatic primary amine.
5. Is it unsaturated?
Bromine water and Baeyer's test (alkaline KMnO₄). Both should decolourise.
Worked unknown 1: a neutral liquid
Observations: blue litmus does not change; no effervescence with NaHCO₃; no colour with FeCl₃; orange precipitate with 2,4-DNP; silver mirror with Tollens'; brick-red precipitate with Fehling's; no iodoform.
Reasoning: not an acid or phenol. 2,4-DNP shows a carbonyl group. Tollens' and Fehling's show it is an aldehyde, and since Fehling's is positive it is aliphatic, not aromatic. No iodoform means no CH₃CO– group, so it is not ethanal. Propanal fits. Your report should say "aliphatic aldehyde", with the evidence for each step.
Worked unknown 2: a liquid that dissolves in dilute HCl
Observations: does not dissolve readily in water but dissolves in dilute HCl; an offensive smell in the carbylamine test; a scarlet red dye in the azo dye test.
Reasoning: solubility in acid suggests an amine. The carbylamine test shows a primary amine. The azo dye test shows it is aromatic. Aniline fits. Our post on amines and diazonium salts explains the diazotisation step.
Which mistakes cost marks?
Using one test for unsaturation and missing a phenol.
Calling a compound an aldehyde from 2,4-DNP alone.
Forgetting that formic acid gives a positive Tollens' test, which the manual raises as a discussion question.
Adding too much ceric ammonium nitrate and losing the red colour.
Heating Tollens' reagent directly on a flame instead of in a water bath.
Recording "positive" without the observation. Examiners want "silver mirror formed", not a tick.
A practice routine
Write the five-step order on a card and recite it until you can do it in one minute.
For each test, write the reagent, the observation and the group it shows, in one line.
Make up three unknowns with sets of results for a friend and swap.
Revise alongside our guide to salt analysis in the chemistry practical, since both use the same "test, observe, rule out" habit.
If you keep confusing which reaction lies behind a test, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, so you can revise the chemistry of oxidation or diazotisation before the practical.
Before the practical, write the observation you expect for every test above, then check it against the lab manual.
