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CBSE Class 11 chemistry · purification and crystallisation · Preparation and purification

Source: CBSE Class 11 chemistry practical syllabus · use your school laboratory manual (NCERT Lab Manual where your school uses it).

Purification by crystallisation (Class 11 chemistry practical)

You dissolve the impure solid in minimum hot solvent, filter hot if needed, and cool slowly so pure crystals form.

Which compound, solvent, and masses come from your laboratory manual.

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.

  • Hot solutions scald.
  • Broken glass on filtration.

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 purify an impure sample of potash alum, copper(II) sulphate, or benzoic acid by crystallisation from a suitable solvent, using the quantities in your laboratory manual.

Methods and quantities can differ between schools. Follow your teacher's instructions.

Apparatus and chemicals

Apparatus and chemicals
ItemNote
Beaker or conical flaskFor dissolution and cooling.
Glass rod and spatulaStirring and transfer.
Funnel and filter paperHot filtration if manual requires.
Watch glass or coverSlow cooling; reduce dust.
Ice bath (optional)Only if your manual directs controlled cooling.
Impure solid (alum / CuSO₄·5H₂O / benzoic acid) (impure sample)Use the concentration, mass, volume, and sample names printed in your school laboratory manual. This page does not invent numerical results for you.
Solvent (water or as manual) (solvent)Minimum hot solvent to dissolve — manual gives volume.
Volumetric flask with calibration mark and meniscus
Make up to the mark after the solid has dissolved. Read the meniscus at eye level so the curve sits on the line.

Theory and reaction

Open

Solubility of many solids rises with temperature. Hot filtration removes insoluble grit; on cooling, the solute crystallises while many soluble impurities stay in the mother liquor.

Slow cooling gives larger, purer crystals. Fast cooling gives small crystals with occluded mother liquor.

Alum and copper sulphate crystallise from aqueous solution; benzoic acid often uses hot water or a mixed solvent as your manual states.

Dissolved salt separates as hydrated crystals on cooling — exact form as per sample.

Procedure

  1. Weigh the impure sample as your manual directs. Add minimum solvent and heat until dissolved (not charred).
  2. Filter hot through fluted paper if insoluble impurities are present, collecting filtrate in a clean beaker.
  3. Cover and cool slowly at room temperature. Use an ice bath only if the manual says so.
  4. Collect crystals by filtration, wash with a small volume of cold solvent if directed, and dry as your teacher shows (filter paper / desiccator).
  5. Note crystal colour and form. Compare with starting material qualitatively.

Observation table

Enter the readings from your school lab in the practical file below. This page does not fill them in for you.

Type the readings in your practical file

Calculation

If your manual asks for percentage recovery, use (mass of dry crystals / mass of impure sample) × 100 with your weighed values only.

Result

Record masses you measured and a short note on crystal appearance. Do not invent a recovery percentage.

Precautions

  • Dissolve in minimum hot solvent — excess solvent keeps material in solution on cooling.
  • Avoid bumping when heating — stir and use appropriate vessel.
  • CuSO₄ solutions stain; benzoic acid hot water is scalding.

Viva questions

Answer one question at a time. The full answers stay closed until you open them.

Question 1 of 4

Crystallisation purifies because…

Answer choices

Pick an answer first.

Show all questions and answers

  1. Crystallisation purifies because…

    Answer: The desired solid separates on cooling while impurities stay largely in the mother liquor. Reason: Selective separation on cooling is the basis.

  2. Why use minimum hot solvent?

    Answer: So the solution is saturated on cooling and yields crystals. Reason: Minimum solvent maximises recovery on cooling.

  3. Fluted filter paper during hot filtration helps…

    Answer: Increase filtration rate and clarity. Reason: Larger surface area speeds hot filtration.

  4. Potash alum crystals are often…

    Answer: Colourless / white octahedral crystals. Reason: Alum gives characteristic octahedral crystals.

Common mistakes

  • Cooling too quickly → small impure crystals.
  • Skipping hot filtration when grit is present.
  • Washing with too much cold solvent → loss of product.
  • Copying a classmate’s recovered mass.

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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