Lyophilic and Lyophobic Colloids | Peptization for Class 12, IIT-JEE, NEET
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What Are Lyophilic and Lyophobic Colloids?
Surface Chemistry is one of the highest-scoring chapters in Class 12 Chemistry for both JEE Main and NEET, contributing 2–3 questions almost every year according to NTA's official question paper analysis. Within this chapter, the classification of colloids into lyophilic and lyophobic types — and the methods used to prepare them — forms a core concept that students must master precisely.
Lyophilic colloids (from Greek: lyo = solvent, philic = loving) are colloidal solutions where the dispersed phase has considerable affinity for the dispersion medium. Examples include dispersions of gelatin, starch, gum, and proteins in water. These sols form simply by mixing the solid with the liquid medium, often on gentle warming.
Lyophobic colloids (solvent-hating) are sols where the dispersed phase has little or no attraction for the medium. Examples include gold sol, sulphur sol, and silver sol in water. Because there is no natural attraction between the phases, these sols cannot form spontaneously — they require special preparation methods and the addition of stabilising agents to prevent immediate coagulation.
Understanding this distinction precisely is essential because JEE questions frequently test property comparisons, stability mechanisms, and preparation methods as single-choice or multiple-correct MCQs.
Difference Between Lyophilic and Lyophobic Colloids
This comparison is directly tested in JEE Main and NEET. Learn each row — not just the headings.
| Property | Lyophilic Colloids | Lyophobic Colloids |
|---|---|---|
| Affinity for medium | High affinity (solvent-loving) | No/very little affinity (solvent-hating) |
| Ease of preparation | Form easily by mixing with the medium | Require special methods (dispersion/condensation) |
| Stability | Highly stable; not easily coagulated | Unstable; coagulated easily by electrolytes |
| Reversibility | Reversible — dried sol redisperses easily | Irreversible — once coagulated, cannot redisperse |
| Viscosity | Much higher than pure solvent | Nearly equal to pure solvent |
| Surface tension | Lower than pure solvent | Nearly equal to pure solvent |
| Charge on particles | Little or no charge | Particles carry a significant charge |
| Examples | Gelatin, starch, gum, albumin in water | Gold sol, Fe(OH)3 sol, As2S3 sol in water |
Exam Shortcut: If a JEE MCQ asks which colloid is "reversible" or "self-stabilising," the answer is always lyophilic. If it asks which is "easily coagulated by electrolytes," it is always lyophobic.
Methods of Preparation of Lyophobic Sols
Lyophilic sols require no special technique — simply warming the dispersed phase with the medium is enough (e.g., starch with water). Lyophobic sols, however, must be prepared through two broad categories of methods:
- Dispersion methods — large particles are broken down to colloidal size.
- Condensation methods — small ions or molecules aggregate to form colloidal-sized particles.
Electro-Dispersion: Bredig's Arc Method
Bredig's arc method is a dispersion method used to prepare colloidal solutions of metals such as gold, silver, and platinum.
Procedure
- Two electrodes of the metal (e.g., platinum) are placed under cold water containing a small amount of a stabilising agent such as KOH (trace quantity).
- An electric arc is struck between the two electrodes.
- The intense heat of the arc vaporises some of the metal.
- The metal vapour immediately condenses in the cold water, producing colloidal-sized particles.
- The container is kept in an ice bath to ensure rapid condensation and prevent aggregation.
Key Notes for JEE/NEET
- This method involves both dispersion and condensation — the metal first disperses as vapour, then condenses.
- It is not suitable for organic dispersion media because the intense arc causes charring of the organic liquid.
- KOH acts as a stabiliser by supplying OH− ions that adsorb on the metal particles, giving them a negative charge and preventing coagulation.
Chemical Methods (Condensation)
In condensation methods, colloidal particles are built from smaller units — individual ions or molecules — through chemical reactions carried out in a medium where the product is sparingly soluble. Supersaturation is achieved, but actual macroscopic precipitation is prevented.
Double Decomposition
Arsenious sulphide sol (As2S3):
A dilute solution of arsenious oxide (As2O3) in hot water is cooled and filtered. This solution is then gradually added to water saturated with H2S, while a stream of H2S gas is passed through it. A yellow-coloured As2S3 sol is formed. Excess H2S is removed by bubbling hydrogen gas through the sol.
$$\text{As}_2\text{O}_3 + 3\text{H}_2\text{S} \rightarrow \text{As}_2\text{S}_3(\text{sol}) + 3\text{H}_2\text{O}$$
Oxidation
A colloidal sol of sulphur is obtained by passing H2S through a solution of SO2:
$$2\text{H}_2\text{S} + \text{SO}_2 \rightarrow 3\text{S}(\text{sol}) + 2\text{H}_2\text{O}$$
Sulphur sol can also be obtained when H2S is bubbled through an oxidising agent such as bromine water or dilute nitric acid.
Reduction
Colloidal solutions of metals such as gold, silver, and platinum are obtained when their salt solutions are treated with a reducing agent:
$$2\text{AuCl}_3 + 3\text{HCHO} + 3\text{H}_2\text{O} \rightarrow 2\text{Au}(\text{sol}) + 3\text{HCOOH} + 6\text{HCl}$$
What Is Peptization?
Peptization is the process of converting a freshly prepared precipitate into a colloidal solution by the addition of a small amount of a suitable electrolyte. The electrolyte used in this process is called the peptizing agent.
Peptization is strictly a dispersion method — no chemical reaction occurs; instead, the precipitate is broken into colloidal particles by adsorbing ions from the electrolyte.
Examples of Peptization
| Precipitate | Peptizing Agent | Colloidal Sol Formed |
|---|---|---|
| Freshly precipitated Fe(OH)3 | FeCl3 solution | Dark reddish-brown Fe(OH)3 sol |
| Freshly precipitated SnO2 | Dilute HCl | Stable SnO2 colloidal solution |
| Freshly precipitated AgCl | Dilute HCl | AgCl colloidal solution |
| CdS precipitate | H2S | CdS sol |
Note: The peptizing agent always supplies a common ion — for example, FeCl3 supplies Fe3+ ions which are common with Fe(OH)3. This is the key to the selectivity of peptizing agents.
Mechanism of Peptization
How Does Peptization Work?
Peptization works through the selective adsorption of ions from the electrolyte (peptizing agent) on the surface of the precipitate particles. Here is the step-by-step mechanism:
- A freshly prepared precipitate consists of aggregated, loosely bound particles.
- A small amount of electrolyte (peptizing agent) is added.
- The ions supplied by the electrolyte — particularly the common ions — are preferentially adsorbed on the surface of the precipitate particles.
- This adsorption imparts an electric charge to the particles (all particles get the same charge).
- Because all particles now carry like charges, they repel each other and break apart from the aggregate.
- The individual charged particles, now in the colloidal size range (1–1000 nm), remain dispersed in the medium — forming a stable colloidal sol.
Example: Fe(OH)3 Peptization
- FeCl3 dissociates to give Fe3+ and Cl− ions.
- Fe3+ ions (common with Fe(OH)3) are adsorbed on the surface of precipitate particles.
- Particles acquire a positive charge.
- Electrostatic repulsion prevents re-aggregation.
- A stable, positively charged Fe(OH)3 sol is formed.
For a complete look at all NCERT-covered concepts in Class 12 Chemistry, the NCERT Books for Class 12 are your primary reference. Cross-referencing these with your classroom notes helps catch every MCQ-level detail.
Students in eSaral's JEE Dropper batch — taught by IIT Bombay faculty, including AIR-41 rankers — regularly use the 5-layer doubt-solving system to clarify exactly which peptizing agent applies to which precipitate, since wrong pairings are a common MCQ error.
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Frequently Asked Questions
Find answers to common questions.
Why is Bredig's arc method considered both a dispersion and a condensation method?
In Bredig's arc method, the metal first disperses as vapour due to the intense heat of the electric arc, and then condenses into colloidal-sized particles as the vapour meets the cold water. Because both steps occur, the method is classified simultaneously as a dispersion method and a condensation method — a dual nature that is directly tested in JEE MCQs.
What is peptization in chemistry with an example?
Peptization is the conversion of a freshly prepared precipitate into a colloidal solution using a small amount of an electrolyte called the peptizing agent. For example, freshly precipitated Fe(OH)₃ treated with a small amount of FeCl₃ solution forms a stable reddish-brown colloidal sol. The Fe³⁺ ions from FeCl₃ are adsorbed on precipitate particles, giving them a positive charge that prevents re-aggregation.
What is the main difference between lyophilic and lyophobic colloids?
Lyophilic colloids have strong affinity between the dispersed phase and the dispersion medium, making them stable and reversible (e.g., gelatin sol). Lyophobic colloids have no such affinity, are unstable, irreversible, and easily coagulated by electrolytes (e.g., gold sol). Stability in lyophobic sols depends on the charge carried by colloidal particles, not solvent interaction.
Is Bredig's arc method suitable for organic solvents?
No. Bredig's arc method is not suitable when the dispersion medium is an organic liquid. The intense heat of the electric arc causes significant charring (burning) of the organic liquid, destroying it before the metal can condense into a stable sol. This method is only used with water as the dispersion medium.
What is the role of a peptizing agent in peptization?
A peptizing agent supplies common ions that are preferentially adsorbed on the surface of the precipitate particles. This adsorption gives all particles the same electric charge, causing them to repel each other and split away from the aggregate as individual colloidal particles. The common-ion principle means the peptizing agent must share an ion with the precipitate.
Why are lyophobic sols irreversible?
Lyophobic sols are irreversible because the stability of these sols depends on the charge layer around colloidal particles, not on affinity for the medium. Once coagulated by an electrolyte (which neutralises the charge), the particles aggregate permanently. Since there is no solvent affinity to re-disperse them, the coagulated sol cannot be re-formed simply by adding the medium again.