Explain the cleansing action of soap with a diagram. Why do detergents clean better in hard water?
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define in one line. Soap is the sodium or potassium salt of a long-chain fatty acid, such as sodium stearate, C₁₇H₃₅COONa.
2 · Describe the soap molecule. A hydrophobic hydrocarbon tail (dissolves in oil) and a hydrophilic ionic head, -COO⁻Na⁺ (dissolves in water).
3 · Explain micelle formation.
Tails dissolve in the grease; heads stay in the water.
Molecules surround the oil drop to form a micelle.
Negatively charged micelles repel each other; an emulsion forms.
Rinsing washes away the micelles with the dirt.
4 · Draw a labelled micelle. [Oil drop at the centre; soap molecules with tails inward and charged heads outward; labels: hydrophobic tail, hydrophilic head, micelle, water]
5 · Hard water. Ca²⁺ and Mg²⁺ ions form insoluble scum with soap: 2C₁₇H₃₅COONa + CaCl₂ → (C₁₇H₃₅COO)₂Ca↓ + 2NaCl. Detergents (-SO₃⁻Na⁺) form soluble salts, so there is no scum.
6 · Real life. Detergents suit hard bore-well water, but non-biodegradable ones cause foam in lakes.
7 · Close in one line. Soap cleans by forming micelles; detergents do the same even in hard water.
Scoring tip: The labelled micelle diagram carries 2 marks or more. Always show the charge on the heads.
