CBSE Class 12 chemistry practical, Organic / techniques · Organic qualitative analysis
Source: CBSE Class 12 chemistry practical syllabus · use your school laboratory manual (NCERT Lab Manual where your school uses it).
Paper chromatography of leaf and flower pigments (Class 12 chemistry practical)
You extract pigments from a leaf or petal and run ascending paper chromatography.
Solvent composition and development time are from your manual — Rf uses your ruler measurements.
Published practice page. Concentrations, masses, and numerical answers come from your school laboratory manual and your own readings — not invented here. Not yet reviewed by a named chemistry reviewer. Version 1.0.0, updated 2026-10-03.
Safety — read before you start
Safety warning. Practise here. Perform the real experiment only in your school lab, with your teacher present.
- Extraction and developing solvents are often flammable — no flames near the chamber.
- Organic solvent vapours are harmful if inhaled — work in a ventilated school lab.
Wear: Lab coat, Splash goggles, Gloves, as your teacher directs.
Follow your teacher’s disposal instructions. Do not pour leftovers down a household drain.
Loads only after you tap. Use the lab hands to pick up and use equipment (pointer hidden in the scene). Falls back to a flat diagram if WebGL is unavailable.
Aim
To separate pigments from a plant sample by paper chromatography and calculate Rf values for the spots observed.
Methods and quantities can differ between schools. Follow your teacher's instructions.
Apparatus and chemicals
| Item | Note |
|---|---|
| Chromatography paper (Whatman strip) | Handle by edges; pencil line only. |
| Chromatography chamber / jar with lid | Vapour-saturated before run. |
| Capillary tube or fine dropper | Spot the extract. |
| Pestle and mortar (optional) | If manual crushes tissue with solvent. |
| Ruler | Measure from origin to spot and to solvent front. |
| Plant material (leaf / petals) (plant) | Use school-supplied or teacher-approved samples. |
| Extraction solvent (solvent) | Often alcohol–water or petroleum ether mix — follow manual. |
| Developing solvent (developer) | Common: pet ether / acetone mixtures — only as listed. |
Theory and reaction
Open
Components partition between stationary (paper water) and mobile (solvent) phases.
Rf = distance travelled by spot / distance travelled by solvent front (both from origin).
Different pigments (chlorophyll, xanthophyll, etc.) have characteristic Rf in a given solvent — compare relative order, not invented absolute values.
Rf equals spot distance divided by solvent front distance from the pencil origin.
Procedure
- Draw a pencil origin line above the bottom of the paper. Do not use ink — it runs.
- Prepare pigment extract as your manual describes. Spot a small drop on the origin. Dry and re-spot if teacher asks.
- Place paper in chamber with solvent depth below the origin. Close lid until front nears top mark.
- Remove paper, mark solvent front with pencil while wet or as taught. Air-dry.
- Measure distances and calculate Rf for each visible spot. Sketch colours in notebook.
Observation table
Enter the readings from your school lab in the practical file below. This page does not fill them in for you.
Calculation
Rf = distance of spot ÷ distance of solvent front. Show one calculation line per spot using your ruler values.
Result
List Rf values and identify pigments in qualitative terms (e.g. higher Rf yellow band) as your manual’s chart allows.
Precautions
- Solvent fumes are flammable — no flames near the chamber.
- Origin must stay above solvent level.
- Cap the jar to saturate vapour space.
Viva questions
Answer one question at a time. The full answers stay closed until you open them.
Question 1 of 4
Rf is always between…
Pick an answer first.
Show all questions and answers
Rf is always between…
Answer: 0 and 1 for a component on one plate. Reason: Spot cannot move farther than the solvent front, so Rf ≤ 1.
Pencil is used for marking because…
Answer: It does not dissolve in organic solvents. Reason: Ink would move and confuse spots.
More polar pigments often…
Answer: Interact differently with paper and solvent — Rf depends on both. Reason: Partition depends on polarity matching between pigment, water on paper, and mobile phase.
Ascending chromatography means…
Answer: Solvent front moves up the paper. Reason: Mobile phase travels up the stationary paper.
Common mistakes
- Using ball-point pen for origin ( ink dissolves ).
- Measuring from wrong baseline after paper curls.
- Copying Rf from a textbook instead of measuring.
Frequently asked questions
- Can I do this at home?
- No. Use this page to practise the order of steps and viva. Do the real experiment only in your school laboratory, with your teacher present.
Your practical file
Name, school, and roll number are optional and are not sent anywhere.
The practical file opens on this device.
Review
Not yet reviewed by a named chemistry reviewer. Published for practice only — follow your teacher and laboratory manual for quantities and safety.
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