MHT-CET Chemistry · Surface Chemistry
Colloids: Phase and Medium, Kinds of Colloid, Charge on a Sol, and Coagulation
A colloid is one substance dispersed through another in particles bigger than molecules but too small to settle; it is classified by the phases involved and by how the particles form, and a charged sol is coagulated by ions of the opposite charge — the higher their charge, the faster.
Why this matters
21 PYQs — the largest page in the chapter — and none HARD. Five name the phases (milk, fog, cheese, gels, emulsions), five classify multimolecular, macromolecular and associated colloids, four are about lyophilic sols and which sols are positive, five apply the Hardy–Schulze rule, and two are processes. Five cards, all recall.
Concept 1 of 5: Dispersed Phase and Dispersion Medium
Definition
- Aerosol (liquid in gas): fog, mist, clouds. Solid in gas: smoke, dust.
- Emulsion (liquid in liquid): milk, hair cream, cold cream.
- Gel (liquid in solid): cheese, butter, jellies.
- Sol (solid in liquid): paints, gold sol. Foam (gas in liquid): froth, whipped cream. Solid foam (gas in solid): foam rubber, pumice.
Practice this conceptself-check · 2 quick reps
The same idea in a real exam question:
Example 1 · Surface Chemistry · Colloids, Classification, Coagulation and Hardy-Schulze Rule
Concept 2 of 5: Multimolecular, Macromolecular and Associated Colloids
Definition
- Multimolecular: aggregates of small molecules — S₈ sulphur sol, gold sol.
- Macromolecular: single large molecules — starch, cellulose, proteins, nylon, polythene.
- Associated (micelles): soap, detergents — above the critical micelle concentration.
Practice this conceptself-check · 2 quick reps
The same idea in a real exam question:
Example 2 · Surface Chemistry · Colloids, Classification, Coagulation and Hardy-Schulze Rule
Concept 3 of 5: Lyophilic and Lyophobic Sols, and the Charge on a Sol
Definition
- Lyophilic: strong affinity for the medium; reversible; self-stabilised; coagulated only by a large amount of electrolyte (salting out).
- Lyophobic: little affinity; irreversible; easily coagulated by a little electrolyte.
- Positive sols: haemoglobin, Fe(OH)₃, Al(OH)₃, basic dyes (methylene blue).
- Negative sols: As₂S₃, clay, congo red, gold, starch, gum, gelatin, CdS.
Practice this conceptself-check · 1 quick reps
The same idea in a real exam question:
Example 3 · Surface Chemistry · Colloids, Classification, Coagulation and Hardy-Schulze Rule
Assuming every protein sol is negative
Concept 4 of 5: Coagulation and the Hardy–Schulze Rule
Definition
- Only the ion of opposite charge coagulates: anions for a positive sol, cations for a negative sol.
- Hardy–Schulze: coagulating power rises with that ion's charge.
- Positive sol: [Fe(CN)₆]⁴⁻ > PO₄³⁻ > SO₄²⁻ > Cl⁻.
- Negative sol: Al³⁺ > Ba²⁺ > Na⁺.
- Other ways to coagulate: electrophoresis, mixing two oppositely charged sols, boiling. Adding more solvent does not — it only dilutes.
Hardy–Schulze rule
Worked example
Practice this conceptself-check · 2 quick reps
The same idea in a real exam question:
Example 4 · Surface Chemistry · Colloids, Classification, Coagulation and Hardy-Schulze Rule
Choosing the highest charge regardless of sign
Concept 5 of 5: Electrophoresis, Dialysis, Peptization and Emulsification
Definition
- Electrophoresis: charged sol particles move to the oppositely charged electrode under an applied potential (it also coagulates the sol there).
- Dialysis: purification of a colloidal solution — dissolved ions pass a membrane, colloid particles do not.
- Peptization: preparation of a sol from a fresh precipitate by adding a little electrolyte.
- Emulsification: the cleansing action of soap.
Practice this conceptself-check
The same idea in a real exam question:
Example 5 · Surface Chemistry · Colloids, Classification, Coagulation and Hardy-Schulze Rule
| Column I (process) | Column II (application) |
|---|---|
| (I) Dialysis | (A) Cleansing action of soap |
| (II) Peptization | (B) Coagulation |
| (III) Emulsification | (C) Colloidal solution preparation |
| (IV) Electrophoresis | (D) Purification of colloidal solution |
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Coagulation and the Hardy–Schulze Rule
Hardy–Schulze rule
Reference tables (4)
Watch out for (2)
- Assuming every protein sol is negative→ Lyophilic and Lyophobic Sols, and the Charge on a Sol
- Choosing the highest charge regardless of sign→ Coagulation and the Hardy–Schulze Rule
Test yourself on Surface Chemistry
15 past MHT-CET questions from this chapter, timed at 14 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.