Q1. [1] medium thorough-understanding
Which of the following statements correctly explains why soap fails to clean effectively in hard water but a detergent does not?
(A) Soap molecules are too large to form micelles in hard water, whereas detergent molecules are smaller.
(B) Soap reacts with calcium and magnesium ions in hard water to form insoluble salts, while the charged ends of detergent molecules do not form such precipitates.
(C) Soap is hydrophobic in hard water, so it cannot interact with water, whereas detergent remains hydrophilic.
(D) Hard water destroys the hydrocarbon tail of soap molecules but not that of detergent molecules.
- A Soap molecules are too large to form micelles in hard water, whereas detergent molecules are smaller.
- B Soap reacts with calcium and magnesium ions in hard water to form insoluble salts, while the charged ends of detergent molecules do not form such precipitates.
- C Soap is hydrophobic in hard water, so it cannot interact with water, whereas detergent remains hydrophilic.
- D Hard water destroys the hydrocarbon tail of soap molecules but not that of detergent molecules.
Generated by claude-sonnet-4-6 · 2026-06-26 01:12 · grounding rag
Model Answer
(B) Soap reacts with calcium and magnesium ions in hard water to form insoluble salts (scum), while the charged ends of detergent molecules do not form such precipitates, so detergents remain effective in hard water.
Explanation
The textbook explicitly states: "The charged ends of these (detergent) compounds do not form insoluble precipitates with the calcium and magnesium ions in hard water." Soap forms a curdy scum with Ca²⁺/Mg²⁺ ions, wasting it and reducing cleaning. Options A, C, and D are not supported by the textbook and describe incorrect mechanisms.
Source: Chapter 4, Section 4.5 Soaps and Detergents