Lyophilic sols (solvent-loving): starch, gelatin, gum — formed by simply warming with solvent; reversible. Lyophobic sols (solvent-fearing): metal sols (Au, Ag), As2S3 — prepared by Bredig's arc method, hydrolysis (FeCl3 + boiling water → Fe(OH)3 sol), or chemical reduction. Lyophobic sols are stabilised by adsorbed ions (Hardy–Schulze rule for coagulation by counter-ion charge).
-- NCERT Chemistry Lab Manual Class 12, Unit 1, p. 1Lyophilic Lyophobic Sols
8 MCQs9-step worked example
Source: NCERT Practical and Analytical ChemistryPYQ coverage: NEET 2021Official key: NTA-verifiedLast updated: 24 Sep 2026
Lyophilic Lyophobic Sols, explained for NEET
The trap first: NEET questions on lyophilic vs lyophobic sols often present a substance and ask about its colloidal behaviour — aspirants confuse which sols are reversible, which need stabilising agents, and which preparation method applies to which type. The distinction is not just terminology; it dictates coagulation behaviour, stability, and practical preparation choices.
Core concept: Colloidal sols are classified based on the affinity between the dispersed phase and the dispersion medium (NCERT Chemistry Lab Manual Class 12, Unit 1, page 1).
-
Lyophilic sols ("solvent-loving"): the dispersed phase has strong affinity for the medium. Examples — starch, gelatin, gum arabic in water. These are reversible — evaporate the solvent, add it back, the sol reforms. They are inherently stable (no stabilising agent needed) because extensive solvation layers prevent aggregation. Preparation is simple: direct mixing/dissolution.
-
Lyophobic sols ("solvent-hating"): the dispersed phase has negligible affinity for the medium. Examples — metal sols (gold, silver), As₂S₃, Fe(OH)₃ in water. These are irreversible — once coagulated, they cannot reform by simply re-adding solvent. They require special preparation methods (chemical: reduction, double decomposition, oxidation; physical: Bredig's arc, peptisation) and need stabilising agents or charge to remain dispersed.
Bridge to NEET: Questions test whether you can identify the correct preparation method for a given sol type, predict stability/coagulation behaviour, and distinguish reversible from irreversible character.
Watch-out: "Lyophilic" does NOT mean "stable against all electrolytes" — it means higher coagulation values (Hardy-Schulze) are needed compared to lyophobic sols. Also, when the medium is specifically water, lyophilic = hydrophilic and lyophobic = hydrophobic.
Can you answer these Lyophilic Lyophobic Sols MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
Which of the following is a lyophilic sol?
Show answer and why every option is right or wrong
Answer: C. Starch in water is a lyophilic (hydrophilic) sol — the dispersed starch molecules have strong affinity for water and the sol forms by simple dissolution. Reference: NCERT Chemistry Lab Manual Class 12, Unit 1, page 1.
Why A is wrong: Gold sol is a classic lyophobic sol — gold particles have no affinity for water and require Bredig's arc method or chemical reduction for preparation.
Why B is wrong: Fe(OH)₃ is a lyophobic sol prepared by hydrolysis of FeCl₃; it requires stabilising charge and is irreversible.
Why D is wrong: As₂S₃ is a lyophobic sol prepared by passing H₂S through arsenious oxide solution — it has no solvent affinity.
MCQ 2Easy RecallPractice
A lyophobic sol, once coagulated, cannot be reconverted into a sol by simply adding the dispersion medium. This property is termed:
Show answer and why every option is right or wrong
Answer: A. Lyophobic sols are irreversible — once the dispersed phase aggregates and settles, re-adding solvent does not regenerate the colloidal state. Reference: NCERT Class 12 Chemistry Chapter 5.
Why B is wrong: Peptisation is actually a method of converting a precipitate INTO a sol (the opposite direction) by adding an electrolyte; it does not describe the inability to reform.
Why C is wrong: Dialysis is a purification technique for removing dissolved ions from a sol using a semipermeable membrane — unrelated to reversibility.
Why D is wrong: Tyndall effect is the scattering of light by colloidal particles — it is a property of all sols regardless of lyophilic/lyophobic nature.
MCQ 3Easy RecallPractice
Which method is used to prepare a gold sol?
Show answer and why every option is right or wrong
Answer: D. Gold sol (a lyophobic sol) is prepared by Bredig's arc method — an electric arc is struck between gold electrodes under water with a trace of KOH as stabiliser. Reference: NCERT Class 12 Chemistry Chapter 5.
Why A is wrong: Simple dissolution works only for lyophilic sols (starch, gelatin). Gold has no affinity for water and will not disperse by mere mixing.
Why B is wrong: Adding gelatin to water produces a lyophilic sol of gelatin itself — it has nothing to do with gold sol preparation.
Why C is wrong: Passing H₂S is used to prepare As₂S₃ sol (double decomposition: As₂O₃ + 3H₂S → As₂S₃ sol + 3H₂O), not gold sol.
MCQ 4Direct ApplicationPractice
Which of the following statements correctly distinguishes lyophilic sols from lyophobic sols?
Show answer and why every option is right or wrong
Answer: C. Lyophilic sols require much larger amounts of electrolyte to coagulate (higher coagulation values) because extensive solvation layers protect the particles. Lyophobic sols coagulate easily with small electrolyte additions. Reference: NCERT Class 12 Chemistry Chapter 5.
Why A is wrong: This is reversed — lyophobic sols need stabilising agents (charge or protective colloids); lyophilic sols are self-stabilised by solvation.
Why B is wrong: This is the opposite — lyophilic sols are reversible (can be reconstituted); lyophobic sols are irreversible.
Why D is wrong: Bredig's arc method is used exclusively for lyophobic metal sols (Au, Pt, Ag), not lyophilic sols.
MCQ 5Direct ApplicationPractice
As₂S₃ sol is prepared by:
Show answer and why every option is right or wrong
Answer: B. As₂S₃ sol is prepared by the double decomposition method — passing H₂S through a dilute solution of As₂O₃: As₂O₃ + 3H₂S → As₂S₃ (sol) + 3H₂O. Reference: NCERT Class 12 Chemistry Chapter 5.
Why A is wrong: Peptisation with HCl would not produce As₂S₃ — peptisation uses a specific ion (preferential adsorption) and HCl would introduce Cl⁻, which is not the preferred ion for this system.
Why C is wrong: As₂S₃ is insoluble in water — it cannot form a sol by simple dissolution. This approach works only for lyophilic substances.
Why D is wrong: Bredig's arc method requires metallic electrodes (Au, Pt, Ag) and cannot be applied to a covalent compound like As₂S₃.
MCQ 6Direct ApplicationPractice
Fe(OH)₃ sol can be prepared by:
Show answer and why every option is right or wrong
Answer: A. Fe(OH)₃ sol is prepared by hydrolysis — adding FeCl₃ dropwise to boiling water: FeCl₃ + 3H₂O → Fe(OH)₃ (sol) + 3HCl. The boiling water promotes hydrolysis and the HCl formed is driven off. Reference: NCERT Class 12 Chemistry Chapter 5.
Why B is wrong: Excess NaOH to FeCl₃ would produce a bulk precipitate of Fe(OH)₃ (not a sol) because excess base causes complete and rapid precipitation rather than controlled colloidal dispersion.
Why C is wrong: Fe(OH)₃ is insoluble in water — it is a lyophobic substance that cannot be dispersed by simple dissolution.
Why D is wrong: Bredig's arc method is for metallic sols (Au, Pt, Ag), not for hydroxide sols. Iron electrodes would produce iron sol, not Fe(OH)₃ sol.
MCQ 7CalculationPractice
A gelatin sol is prepared, evaporated to dryness, and then water is added back. The sol reforms. A gold sol is prepared, coagulated by adding NaCl, and then water is added. The gold sol does NOT reform. Which pair of terms correctly describes these behaviours?
Show answer and why every option is right or wrong
Answer: C. Gelatin has strong affinity for water (lyophilic) and reforms on re-addition of solvent (reversible). Gold has no affinity for water (lyophobic) and once coagulated cannot be re-dispersed by adding water alone (irreversible). Reference: NCERT Chemistry Lab Manual Class 12, Unit 1, page 1.
Why A is wrong: Both classifications are swapped — gelatin is lyophilic (not lyophobic) and gold is lyophobic (not lyophilic).
Why B is wrong: Gelatin is correctly identified as lyophilic but it is reversible (not irreversible); gold is correctly lyophobic but it is irreversible (not reversible).
Why D is wrong: Both classifications are again swapped — identical error to option A.
MCQ 8Concept TrapPractice
Peptisation involves converting a freshly prepared precipitate into a colloidal sol. Which of the following correctly describes the role of the peptising agent?
Show answer and why every option is right or wrong
Answer: B. The peptising agent (e.g., FeCl₃ for Fe(OH)₃ precipitate) provides specific ions (Fe³⁺ or Cl⁻) that adsorb on the precipitate particles' surface, imparting charge. The like-charged particles repel and disperse into colloidal dimensions. Reference: NCERT Class 12 Chemistry Chapter 5.
Why A is wrong: Peptisation does NOT dissolve the precipitate — the particles remain in the colloidal size range (1–1000 nm), not molecular or ionic. True dissolution would produce a solution, not a sol.
Why C is wrong: Temperature is not the mechanism of peptisation — it is charge-based dispersion via preferential ion adsorption. Heating may assist but is not the defining step.
Why D is wrong: Emulsification and surface-tension reduction apply to emulsions (liquid-in-liquid), not sols (solid-in-liquid). Peptising agents work by charge, not surface tension.
Free NEET study resources
Get a structured 30-day study plan and a complete formula booklet — delivered to your inbox instantly.
How do you solve a Lyophilic Lyophobic Sols question? A worked example
- 1
Given
A student needs to prepare a sol of sulphur in water. The following reagents are available: Na₂S₂O₃ solution, dilute HCl, ethanol, and water.
- 2
Required
Identify the correct method to prepare sulphur sol and classify it as lyophilic or lyophobic.
- 3
Concept
Sulphur is insoluble in water — it has no affinity for the aqueous medium. Therefore sulphur sol in water is lyophobic. Lyophobic sols require special preparation methods: chemical (oxidation, reduction, double decomposition) or physical (Bredig's arc, peptisation).
- 4
Formula/Principle
Chemical method — oxidation of thiosulphate:
Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂↑ + S↓ (colloidal)
Alternatively, the reaction can yield colloidal sulphur under controlled conditions:
Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + S (sol) + SO₂
(Under gentle acidification, sulphur remains in colloidal form before aggregating.) - 5
Substitution
Add dilute HCl slowly to Na₂S₂O₃ solution. The sulphur produced initially remains dispersed as a colloidal sol (milky appearance).
- 6
Calculation
No arithmetic calculation needed — this is a qualitative identification problem. The key reasoning step: sulphur ≠ solvent-loving → lyophobic → requires chemical method → Na₂S₂O₃ + HCl is the standard oxidation route.
- 7
Final answer
Sulphur sol in water is a lyophobic sol, prepared here by ACID DECOMPOSITION of thiosulphate (Na₂S₂O₃ + 2HCl → 2NaCl + H₂O + SO₂ + S), which is a disproportionation on acidification and NOT the oxidation method. NCERT’s oxidation route to a sulphur sol is 2H₂S + SO₂ → 3S + 2H₂O. It is irreversible — once coagulated, it cannot reform by re-adding water.
- 8
Common trap
Aspirants sometimes confuse sulphur sol with "sulphur dissolved in CS₂" — that system IS lyophilic (sulphur loves CS₂). The same substance can form lyophilic OR lyophobic sols depending on the dispersion medium. NEET exploits this medium-dependence.
- 9
Similar NEET-style question
"Colloidal solution of sulphur is prepared by passing H₂S gas through a solution of SO₂. This method of preparation is known as: (A) Peptisation (B) Oxidation (C) Reduction (D) Double decomposition." [Answer: B — the reaction 2H₂S + SO₂ → 3S (sol) + 2H₂O is an oxidation of H₂S / reduction of SO₂, classified under oxidation method for sol preparation.]
---
What to remember before solving Lyophilic Lyophobic Sols questions
More in Practical and Analytical Chemistry: 6 exam traps and mistakes · 2 formulas · 1 question pattern from its other lessons.
Lyophilic Lyophobic Sols questions from past NEET papers
1 question from NEET 2021. Answers verified against NTA official keys.
NEET 2021
The right option for the statement "Tyndall effect is exhibited by", is :
1Urea solution
2NaCl solution
3Glucose solution
4Starch solution
NTA Answer: Option 4(final)
All 7 past-paper questions from Practical and Analytical Chemistry →
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
Test yourself on this topic with real past-paper questions:
Practice this topic →