Science (086) — AI-generated practice question

(i) Cleansing Action of Soap:

(a) A soap molecule is a sodium/potassium salt of a long-chain carboxylic acid. It has two ends: a hydrophilic (ionic) end that attracts water, and a hydrophobic (hydrocarbon tail) end that repels water but interacts with oil/grease.

(b) When soap is added to water containing oily dirt, the hydrophobic tails of soap molecules orient themselves into the oil droplet, while the hydrophilic ionic ends face outward toward water. This cluster is called a micelle. The oily dirt is trapped in the interior of the micelle.

(c) Micelles remain suspended in water as a colloid and do not precipitate because of ion–ion repulsion between the negatively charged ionic ends on the surface. The suspended dirt is then rinsed away easily.

(ii) In hard water, soap reacts with calcium and magnesium salts to form an insoluble white scum (curdy precipitate), reducing its cleaning ability. Detergents are sodium salts of sulphonic acids whose charged ends do not form insoluble precipitates with Ca²⁺/Mg²⁺ ions, so they remain effective in hard water.

(iii) When ethyl ethanoate is treated with NaOH, saponification occurs:

$$\text{CH}_3\text{COOC}_2\text{H}_5 + \text{NaOH} \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{CH}_3\text{COONa}$$

(Ethyl ethanoate) → (Ethanol) + (Sodium ethanoate)

Source: Chapter 4, Section 4.5 Soaps and Detergents; Section 4.4.2 Properties of Ethanoic Acid

---

• For part (i), examiners look for all three terms: hydrophilic, hydrophobic, micelle, and the specific reason micelles don't precipitate (ion–ion repulsion). Missing any loses a mark.
• For part (ii), the key words are scum/insoluble precipitate (for soap's failure) and sulphonic acid salts / no insoluble precipitate (for detergents). Don't just say "detergents work in hard water" — explain why.
• For part (iii), write the balanced equation clearly and name the reaction (saponification) for full credit.