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Why does ordinary soap fail to clean effectively in hard water?
AIt forms an insoluble scum with calcium and magnesium ions
BIt dissolves completely and is washed away unused
CIt reacts with chlorine to form a gas
DIt loses its hydrocarbon tail in hard water
Answer & Solution
Correct answer: A. It forms an insoluble scum with calcium and magnesium ions
1. Hard water contains calcium (Ca²⁺) and magnesium (Mg²⁺) ions.
2. Soap combines with these ions to form an insoluble precipitate called scum.
3. The scum, not over-dissolving or a chlorine reaction, wastes the soap and deposits on cloth; the tail is not lost.
4. Hence soap fails because it forms a scum with Ca²⁺ and Mg²⁺ ions.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 11 "Carbon and its Compounds", p.174_
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