Statement I: Hard water does not give lather with soap. Statement II: Calcium and magnesium salts, present in hard Abater form precipitate with soap.

Statement I is correct as hard water does not readily form a lather with soap. Statement II provides the correct scientific explanation for this phenomenon. Soap molecules are sodium or potassium salts of long-chain carboxylic acids [1]. When soap is added to hard water, the calcium (Ca2+) and magnesium (Mg2+) ions present in the water react with the soap molecules. This reaction replaces the soluble sodium ions with divalent calcium or magnesium ions, forming insoluble salts known as 'soap scum' or a curdy precipitate. Because these ions precipitate the soap into an insoluble form, the soap is consumed in forming scum rather than creating a lather for cleaning. Only after all the calcium and magnesium ions have been precipitated can the remaining soap begin to form a lather. Thus, Statement II is the direct cause of the observation in Statement I.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 4: Carbon and its Compounds > Activity 4.10 > p. 75