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Preparation of lyophilic and lyophobic sols: NEET MCQs with answers

49 questions on this topic of Principles Related to Practical Chemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. Which of the following techniques is classified as a physical dispersion method used in the laboratory to prepare colloidal sols?

  1. Hydrolysis of metal salts
  2. Ultrasonic disintegration method
  3. Excessive cooling method
  4. Double decomposition method
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Answer: (B) Ultrasonic disintegration method

Ultrasonic disintegration employs high-frequency ultrasonic waves to break coarse suspension particles into colloidal dimensions, making it a physical dispersion method. Hydrolysis and double decomposition are chemical condensation methods.

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2. When preparing a stable negative colloidal sol of antimony trisulphide (Sb₂S₃) in the laboratory, which ions form the fixed inner layer of the Helmholtz electrical double layer?

  1. S²⁻ ions
  2. Sb³⁺ ions
  3. H⁺ ions
  4. OH⁻ ions
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Answer: (A) S²⁻ ions

In the preparation of Sb₂S₃ sol by passing H₂S through an aqueous solution of potassium antimonyl tartrate or SbCl₃, S²⁻ (or HS⁻) ions from the excess H₂S are preferentially adsorbed on the Sb₂S₃ particles, forming the fixed negative layer.

hard

3. Colloidal gold sol (Faraday gold sol) is a lyophobic sol commonly prepared in practical chemistry. Which chemical method and reagent combination correctly represents its laboratory preparation?

  1. Hydrolysis of AuCl₃ with boiling distilled water
  2. Reduction of auric chloride (HAuCl₄) solution using formalin or tannic acid
  3. Oxidation of metallic gold using concentrated HNO₃
  4. Double decomposition between AuCl₃ and Na₂S
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Answer: (B) Reduction of auric chloride (HAuCl₄) solution using formalin or tannic acid

Gold sol is prepared by the reduction of an extremely dilute aqueous solution of chloroauric acid (HAuCl₄) or AuCl₃ using reducing agents like formaldehyde (formalin), tannic acid, or hydrazine hydrate. The reduced gold atoms nucleate and aggregate to colloidal dimensions.

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4. When preparing gum sol in the laboratory, gum acacia powder is triturated with a small volume of cold distilled water to form a smooth paste before diluting with warm water. Why is direct addition of gum powder to hot water avoided?

  1. Gum undergoes rapid chemical oxidation in hot water
  2. Gum forms large, gummy lumps that hydrate only on the surface and resist uniform dispersion
  3. Hot water causes irreversible denaturation of polysaccharide chains into insoluble charcoal
  4. Gum powder hydrolyses completely into simple sugars in hot water
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Answer: (B) Gum forms large, gummy lumps that hydrate only on the surface and resist uniform dispersion

If gum powder is added directly to hot water, the outer layers swell and form a gelatinous, impermeable barrier around unhydrated powder cores, creating stubborn lumps. Triturating with cold water first ensures uniform wetting and breakup of aggregates before final dispersion.

easy

5. During the laboratory preparation of a starch sol, what is the primary role of adding a smooth starch paste into boiling water with continuous stirring?

  1. To hydrolyse the starch polymers into glucose units
  2. To oxidise amylopectin into a negatively charged lyophobic colloid
  3. To break down starch granules and disperse amylose into water to form a stable lyophilic sol
  4. To act as a peptizing agent that provides electrical charge to starch
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Answer: (C) To break down starch granules and disperse amylose into water to form a stable lyophilic sol

Starch forms a lyophilic sol when added to boiling distilled water with continuous stirring. The hot water causes the starch granules to rupture, releasing amylose and amylopectin which spontaneously hydrate and disperse to form an intrinsically stable colloidal solution without needing a peptizing agent.

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6. A student accidentally added a concentrated solution of NaCl instead of dilute NaCl while attempting to peptize freshly precipitated Fe(OH)₃. What will be the observed result and the reason behind it?

  1. A clearer and more stable sol forms because high ionic strength enhances the zeta potential
  2. The Fe(OH)₃ precipitate dissolves completely to yield a yellow true solution of FeCl₃
  3. No sol will form; instead, the precipitate remains coagulated because high electrolyte concentration suppresses the double layer
  4. The sol inverts into a lyophilic sol due to protective colloid action of Na⁺ ions
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Answer: (C) No sol will form; instead, the precipitate remains coagulated because high electrolyte concentration suppresses the double layer

Peptization requires only a small (trace) quantity of electrolyte to charge the particle surface. When concentrated NaCl is added, the high concentration of coagulating counter-ions (Cl⁻) compresses the electrical double layer beyond the critical threshold, causing coagulation rather than peptization.

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7. In a laboratory experiment, a student mixes 40 mL of 0.1 M AgNO₃ with 60 mL of 0.1 M KI to prepare a silver iodide sol. What is the nature of the electrical charge on the colloidal particles, and which ion forms the fixed layer?

  1. Positively charged, Ag⁺ ion forms the fixed layer
  2. Negatively charged, I⁻ ion forms the fixed layer
  3. Negatively charged, NO₃⁻ ion forms the fixed layer
  4. Positively charged, K⁺ ion forms the fixed layer
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Answer: (B) Negatively charged, I⁻ ion forms the fixed layer

Number of millimoles of AgNO₃ = 40 × 0.1 = 4 mmol. Number of millimoles of KI = 60 × 0.1 = 6 mmol. Since KI is in excess, I⁻ ions are present in excess in the dispersion medium and are preferentially adsorbed onto the AgI precipitate to form a negatively charged fixed layer [AgI]I⁻.

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8. Why can lyophilic sols be recovered by redissolving the residue in the dispersion medium after evaporation to dryness, whereas lyophobic sols cannot?

  1. Lyophilic sols undergo reversible chemical bonding with water molecules
  2. Lyophobic particles have higher molecular masses which prevents redispersal
  3. Lyophilic particles retain their intrinsic affinity for the medium, whereas lyophobic particles irreversibly aggregate due to strong attractive forces when desolvated
  4. Evaporation of lyophobic sols converts the dispersed phase into volatile gaseous products
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Answer: (C) Lyophilic particles retain their intrinsic affinity for the medium, whereas lyophobic particles irreversibly aggregate due to strong attractive forces when desolvated

Lyophilic sols are reversible colloids because the particles have a thermodynamic affinity for the dispersion medium; upon re-addition of solvent, protective solvation barriers naturally reform. Lyophobic sols are thermodynamically unstable and lack this affinity; once their charge barrier is destroyed by evaporation, particles coalesce into large irreversible aggregates.

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9. Arsenious sulphide sol is prepared by:

  1. Mixing starch with water
  2. Passing H₂S gas through a solution of arsenious oxide (double decomposition)
  3. Warming As₂S₃ with water
  4. Hydrolysis of FeCl₃
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Answer: (B) Passing H₂S gas through a solution of arsenious oxide (double decomposition)

As₂O₃ + 3H₂S → As₂S₃ (sol) + 3H₂O. This lyophobic sol is yellow and negatively charged.

NCERT Class 11

easy

10. Which of the following substances can form a lyophilic colloidal sol in water simply by mixing, without requiring any stabilizing agent or special chemical dispersion method?

  1. Gelatin
  2. Ferric hydroxide
  3. Arsenious sulphide
  4. Colloidal gold
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Answer: (A) Gelatin

Gelatin is a lyophilic substance having strong affinity for water. It forms a colloidal sol directly on warm mixing with water without needing any stabilizing agent, whereas Fe(OH)₃, As₂S₃, and Au form lyophobic sols requiring specific preparation methods and stabilizers.

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11. In the laboratory preparation of an egg albumin sol, which thermal condition must be strictly maintained to ensure a stable colloidal system?

  1. The temperature must be maintained above 60 °C to promote dispersion
  2. The mixture must be boiled continuously at 100 °C
  3. The albumin should be dissolved in boiling water with vigorous stirring
  4. The egg albumin must be added to water kept strictly below 35 °C
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Answer: (D) The egg albumin must be added to water kept strictly below 35 °C

Egg albumin is a globular protein that undergoes irreversible thermal denaturation and coagulation at elevated temperatures (above 60 °C). Hence, its lyophilic sol must be prepared in cold water (usually room temperature or below 30-35 °C).

easy

12. Which of the following ions is preferentially adsorbed on the colloidal particles when silver iodide sol is prepared by adding a very dilute AgNO₃ solution dropwise to an excess of dilute KI solution?

  1. Ag⁺
  2. K⁺
  3. NO₃⁻
  4. I⁻
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Answer: (D) I⁻

When dilute AgNO₃ is added to an excess of KI solution, AgI precipitate forms in a medium containing excess I⁻ and K⁺ ions. According to the rule of preferential adsorption of common ions, AgI particles selectively adsorb I⁻ ions, rendering the sol negatively charged.

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