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Science (086) — AI-generated practice question

AI-generated practice question — model-generated for extra practice, not a previous-year CBSE board question.

Q1. [1] medium exam-ready
When soap is added to hard water, a white curdy precipitate is formed. What causes this?
  1. A Soap reacts with dissolved oxygen to form insoluble oxides
  2. B Soap reacts with calcium and magnesium salts in hard water to form insoluble salts
  3. C Soap molecules clump together when cooled by hard water
  4. D Hard water contains chlorine that destroys the soap molecules
Generated by claude-sonnet-4-6 · 2026-06-26 01:06 · grounding rag
Model Answer

Answer: B

Soap reacts with calcium and magnesium salts present in hard water to form insoluble salts, which appear as a white curdy precipitate (scum).

Source: Soaps and Detergents, Chapter 4

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Explanation

The textbook explicitly states: "This is caused by the reaction of soap with the calcium and magnesium salts, which cause the hardness of water" — forming an insoluble precipitate. Options A, C, and D are factually incorrect and not supported by the text. Examiners expect students to identify the correct chemical cause (reaction with Ca/Mg salts) and the result (insoluble/curdy precipitate).

Previous-year CBSE Grade 10 board exam questions, organised by subject and chapter, each with a model answer — free to read and print.