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 Class 12 Chemistry, Ch. 5, p. 152Lyophilic Lyophobic Sols
8 MCQs9-step worked example
Source: NCERT Unit 20PYQ coverage: NEET 2024, 2025Official key: NTA-verifiedLast reviewed: May 2026
Lesson
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 Class 12 Chemistry Chapter 5, page 152).
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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.
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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.
Practice 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?
MCQ 2Easy RecallPractice
A lyophobic sol, once coagulated, cannot be reconverted into a sol by simply adding the dispersion medium. This property is termed:
MCQ 3Easy RecallPractice
Which method is used to prepare a gold sol?
MCQ 4Direct ApplicationPractice
Which of the following statements correctly distinguishes lyophilic sols from lyophobic sols?
MCQ 5Direct ApplicationPractice
As₂S₃ sol is prepared by:
MCQ 6Direct ApplicationPractice
Fe(OH)₃ sol can be prepared by:
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?
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?
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 by the **oxidation method** (treating Na₂S₂O₃ with dilute HCl). 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.] ---
Before solving, remember these
Formulas
Molarity-stoichiometry titration
Use when normality is awkward (e.g., diprotic acids). Stoichiometric coefficients from balanced equation.
| Symbol | Quantity | SI Unit |
|---|---|---|
| M | molarity | mol/L |
| V | volume | L |
| n | coefficient | - |
Valid when
- Balanced equation known
- Same end-point
Normality equation in titration
Equivalents of acid = equivalents of base at end-point. Or for redox: equivalents of oxidant = equivalents of reductant.
| Symbol | Quantity | SI Unit |
|---|---|---|
| N | normality | eq/L |
| V | volume | mL or L |
Valid when
- Same titration end-point
- Equivalent factors known
Exam Traps & Common Mistakes
These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Inorganic Exception
Cations like Pb²⁺ precipitate in BOTH Group I (with HCl) and Group II (with H2S) — assigning to only one group misses the redundancy.
When it triggers
Cation that appears in two analytical groups, e.g. Pb²⁺ (Group I + Group II) or Hg²⁺ vs Hg2²⁺.
How to avoid
Apply confirmatory tests for each candidate group; do not assume mutual exclusivity.
Category: Overthinking
Continuing to add titrant past the first persistent colour change because the colour seemed to fade after a swirl.
When it triggers
Question describes 'colour faded after swirling' or 'persistent colour' — distinguishes transient vs end-point.
How to avoid
End-point = first PERSISTENT colour change (lasts ≥30 s). Transient fades back to original on swirling.
Category: Similar Terms
Phenolphthalein (pH 8.2–10) and methyl orange (pH 3.1–4.4) only mark equivalence when the eq-pt pH falls within their range; using the wrong indicator gives an end-point that disagrees with the actual equivalence point.
When it triggers
Titration prompt mentions a specific weak/strong combination but asks which indicator is suitable.
How to avoid
Match the indicator's pH-change range to the equivalence-point pH: phenolphthalein for eq-pt > 7, methyl orange for eq-pt < 7.
Root cause: concept gap
Correction
Pb²⁺ appears in both Group I (PbCl2 white ppt, soluble in hot water) and Group II (PbS black ppt). Confirm with hot-water solubility test.
Root cause: concept gap
Correction
Strong-acid + strong-base: any indicator (eq pt = 7). Weak-acid + strong-base: phenolphthalein (eq pt > 7). Strong-acid + weak-base: methyl orange (eq pt < 7).
Root cause: rushed under time pressure
Correction
Slow drop-wise addition near the end-point; first persistent colour change is the end-point. Re-do if overshot.
Past Year Questions
3 questions from NEET 2024, 2025. Answers verified against NTA official keys.
NEET 2025
1A-III, B-II, C-I, D-IV
2A-III, B-IV, C-II, D-I
3A-III, B-IV, C-I, D-II
4A-III, B-II, C-IV, D-I
NTA Answer: Option 3(final)
NEET 2024Revised key
1dilute hydrochloric acid
2concentrated sulphuric acid
3dilute nitric acid
4dilute sulphuric acid
NTA Answer: Option 4(revised_final)
NEET 2024Revised key
1B, A, D, C, E
2B, C, A, D, E
3E, C, D, B, A
4E, A, B, C, D
NTA Answer: Option 1(revised_final)
How NEET usually asks this
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
Practical chemistry — titrimetric end-point selection, indicator–pH matching, qualitative-analysis cation/anion identification, organic functional-group tests.
Direct ApplicationMedium
Common distractors
wrong zero error direction
Sign convention is easily flipped.
Sources
NCERT refs: Class 12 Chemistry Chapter 5, p.152
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