CDS Chemistry · Carbon and Its Compounds
Soaps, Micelles and Hydrogenation
How a soap molecule cleans by forming micelles, why micelles stay spread out, and how nickel turns vegetable oil into solid fat.
Why this matters
Three CDS questions, two of them from 2026 (I). One asks for the hydrogenation catalyst; the others are statement checks on micelles and soap structure.
Concept 1 of 2: Soap molecules and micelles
Definition
The facts:
- Soap = sodium or potassium salt of a long-chain carboxylic (fatty) acid.
- Hydrophilic (water-loving) part: the ionic carboxylate head. Hydrophobic part: the hydrocarbon tail. The oil is not part of the soap.
- Micelle: tails point inward around the grease at the centre, heads outward in the water.
- The heads carry like charges, so micelles repel each other and stay dispersed; they do not precipitate.
- Micelles are colloid-sized, so they scatter light; soap solution looks cloudy.
- Detergents (sulphonate or ammonium salts) also work in hard water, where soap forms scum.
| Part or idea | What it is |
|---|---|
| Soap | Na or K salt of a long-chain carboxylic acid |
| Hydrophilic part | Ionic head (–COO⁻ Na⁺) |
| Hydrophobic part | Long hydrocarbon tail |
| Grease in a micelle | Trapped in the centre |
| Why micelles stay dispersed | Like charges on the heads repelQ CDS 2026 (I): 'micelles precipitate out because of ion-ion repulsion' is the false statement. |
Practice this conceptself-check · 3 quick reps
The same idea in a real exam question:
Example 1 · Carbon and Its Compounds · Soaps, Detergents and Hydrogenation of Oils
The hydrophilic part is the ionic head, not the oil
Repulsion keeps micelles apart
Concept 2 of 2: Hydrogenation of oils
Definition
The reaction:
- Unsaturated oil + H₂ → saturated fat, with finely divided nickel as the catalyst (platinum or palladium also work).
- This is how vanaspati ghee is made from vegetable oil.
- Copper, iron and zinc do not catalyse this addition.
- Related: in esterification (acid + alcohol → ester + water), concentrated H₂SO₄ acts as the dehydrating agent.
| Process | Reagent or catalyst | Result |
|---|---|---|
| Hydrogenation of oil | H₂ with nickel | Solid saturated fat (vanaspati)Q |
| Esterification | Concentrated H₂SO₄ (removes water) | Ester + water |
Practice this concept3 quick reps
The same idea in a real exam question:
Example 2 · Carbon and Its Compounds · Soaps, Detergents and Hydrogenation of Oils
The catalyst is nickel
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.
Reference tables (2)
Soap molecules and micelles5 rows
| Part or idea | What it is |
|---|---|
| Soap | Na or K salt of a long-chain carboxylic acid |
| Hydrophilic part | Ionic head (–COO⁻ Na⁺) |
| Hydrophobic part | Long hydrocarbon tail |
| Grease in a micelle | Trapped in the centre |
| Why micelles stay dispersed | Like charges on the heads repelQ CDS 2026 (I): 'micelles precipitate out because of ion-ion repulsion' is the false statement. |
Hydrogenation of oils2 rows
| Process | Reagent or catalyst | Result |
|---|---|---|
| Hydrogenation of oil | H₂ with nickel | Solid saturated fat (vanaspati)Q |
| Esterification | Concentrated H₂SO₄ (removes water) | Ester + water |
Watch out for (3)
- The hydrophilic part is the ionic head, not the oil→ Soap molecules and micelles
- Repulsion keeps micelles apart→ Soap molecules and micelles
- The catalyst is nickel→ Hydrogenation of oils
Test yourself on Carbon and Its Compounds
10 past CDS questions from this chapter, timed at 10 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.