Assertion (A) : Soaps do not form lather with water containing salts of calcium and magnesium. Reason (R) : Calcium and magnesium salts of long-chain fatty acids are insoluble in water.

Assertion (A) is true because hard water contains dissolved salts of calcium and magnesium, which prevent soap from forming a lather [t1][t6]. Instead of producing foam, soap reacts with these divalent cations to form a white, curdy precipitate known as 'soap scum' [t2][t3]. Reason (R) is also true and provides the correct explanation for (A). Soaps are sodium or potassium salts of long-chain fatty acids (like stearic or palmitic acid) [c2][t2]. When added to hard water, the sodium ions are replaced by calcium or magnesium ions, resulting in the formation of calcium or magnesium stearate [t1][t2]. These specific salts of long-chain fatty acids are insoluble in water, causing them to precipitate out [t1][t3]. This precipitation consumes the soap molecules, making them unavailable for lather formation until the ions are fully precipitated [t3][t7].

Sources

1. [1] https://www.griet.ac.in/nodes/EC_UNIT_1.pdf
2. [2] https://en.wikipedia.org/wiki/Hard_water
3. [3] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.10 > p. 75