AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.
Soap reacts with calcium and magnesium salts present in hard water to form insoluble precipitates (scum/curdy white solid), leaving little soap available to form lather.
Source: Chapter 4, Section 4.5 – Soaps and Detergents
---
The key point examiners look for is two things: (1) the reaction of soap with Ca²⁺/Mg²⁺ ions in hard water, and (2) the result — insoluble precipitate forms, reducing lather. At 1 mark, just one crisp sentence covering both points is enough. Avoid writing extra detail about micelles or detergents — that wastes time and marks are not awarded for extra content.