Experiment: preparation of lyophilic and lyophobic sols

How are lyophilic and lyophobic sols prepared?

Where this experiment comes from. The JEE Main 2027 and NEET (UG) 2026 syllabuses list the preparation of lyophilic and lyophobic sols among the experiments of Unit 20. The CBSE 2026–27 Class 12 practical list also includes the preparation of one lyophilic and one lyophobic sol. The 2026–27 NCERT Class 12 textbooks no longer have a chapter on colloids, so the theory and the method below both follow the NCERT Laboratory Manual (Chemistry, Class 12), Unit 1, Experiment 1.1 (pages 1–3).

In plain words: some solids spread through water by themselves, because water clings to their large molecules. Others have to be made into tiny particles inside the water, where an electric charge keeps them apart.

Sols and true solutions

• In a true solution the solute particles mix evenly with the solvent molecules and form a single phase (Lab Manual, page 1); see types of solutions.
• A colloidal solution is heterogeneous. Very fine particles of one substance, the dispersed phase, are spread through another, the dispersion medium. The particles are either very large molecules or clusters of small molecules, between 10⁻⁹ and 10⁻⁶ m across: bigger than simple molecules, but small enough to stay suspended (Lab Manual, page 1).
• Starch, gum and proteins are very large molecules that form colloids; colloidal sulphur is an example of clusters of small molecules (Lab Manual, page 1).
• A solid dispersed in a liquid is a sol. With water as the medium, lyophilic and lyophobic become hydrophilic and hydrophobic (Lab Manual, page 1).
• Stability: lyophilic sols are the more stable. They are stabilised mainly by solvation of the particles; lyophobic sols are stabilised by the charge on the particles, which keeps them from coagulating (Lab Manual, pages 1–2).

Lyophilic sols of starch, gum and egg albumin

• Egg albumin sol (Lab Manual, page 2):
  1. Make 100 mL of a 5% (w/v) solution of sodium chloride in water in a 250 mL beaker.
  2. Break an egg into a porcelain dish, pipette out the albumin and pour it into the salt solution.
  3. Stir well.
• Starch or gum sol (Lab Manual, page 2):
  1. Boil 100 mL of distilled water in a 250 mL beaker.
  2. Make a paste of 500 mg of starch or gum in hot water and pour it into the boiling water with constant stirring.
  3. Keep the water boiling and stirring for 10 minutes. Comparing the sol with the original paste shows how well it has formed.
• What “directly” means. The Lab Manual says lyophilic sols form directly by mixing and shaking (page 2), yet starch and gum still need boiling water and egg albumin a salt solution. No special method or chemical change is needed, only the right liquid (reasoned).

Lyophobic sols of iron(III) hydroxide and aluminium hydroxide

• Procedure (Lab Manual, pages 2–3):
  1. Boil 100 mL of distilled water in a 250 mL beaker and add 2 g of iron(III) chloride (or aluminium chloride), stirring well.
  2. Boil another 100 mL of distilled water in a second 250 mL beaker.
  3. Pour 10 mL of the chloride solution from step 1, drop by drop and with constant stirring, into the boiling water.
  4. Keep it boiling until a brown sol (iron) or a white sol (aluminium) forms.
• The iron(III) chloride is hydrolysed in the hot water (reasoned), and hydrated iron(III) oxide forms as colloidal particles. This sol is positively charged. Iron(III) chloride added to NaOH solution instead gives a negatively charged sol (Lab Manual, page 2).
• The Lab Manual also prepares an arsenious sulphide sol, a negatively charged lyophobic sol, by passing hydrogen sulphide through a solution of arsenious oxide. It warns that arsenious oxide is poisonous, so hands must be washed every time after handling it (pages 2–3).

The equation is ours, the usual way to write the hydrolysis (reasoned); the Lab Manual gives the method but no equation.

Precautions

• Pour the paste or solution into the boiling water gradually, with constant stirring. Adding too much at once can cause precipitation (Lab Manual, page 3).
• Do not eat, drink or smoke during the experiment. The Lab Manual marks aluminium chloride as corrosive and arsenic compounds as toxic (page 2).

Solved examples

Common mistakes to avoid

Quick revision summary

• Sol: a solid dispersed in a liquid, with particles 10⁻⁹ to 10⁻⁶ m.
• Lyophilic (egg albumin, starch, gum): made by mixing and shaking; stabilised by solvation; more stable.
• Lyophobic (Fe(OH)₃, Al(OH)₃, As₂S₃): made by special methods; stabilised by charge. Fe(OH)₃ from FeCl₃ in boiling water is positive.
• Method: 2 g of chloride in 100 mL of boiling water; 10 mL of it, drop by drop, into another 100 mL of boiling water.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. Why is a lyophilic sol more stable than a lyophobic sol?

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   A lyophilic sol is stabilised mainly by solvation: the particles attract the liquid, which surrounds them. A lyophobic sol depends only on the charge on its particles, which can be neutralised (Lab Manual, pages 1–2).

2. Q2. How does iron(III) chloride give a positively charged sol in one method and a negatively charged sol in another?

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   Added to an excess of hot water, it gives a positively charged hydrated iron(III) oxide sol; added to NaOH solution, it gives a negatively charged one (Lab Manual, page 2). The sign follows the ion in excess, which the particles take up on their surface: Fe³⁺ from the chloride in the first case, OH⁻ from the alkali in the second. In the same way, arsenious sulphide particles, formed while H₂S is in excess (page 3), take up sulphide ions and are negative (reasoned; compare Lab Manual Discussion Question (iii), page 3).