Question

A student takes 4 mL of distilled water in each of four test tubes I, II, III and IV, and then dissolves an equal amount of four different salts namely NaCl in I, CaCl₂ in II, MgCl₂ in III and KCl in IV. He then adds 8 drops of the given soap solution to each test tube and shakes the contents of the test tube 10 times. In which test tubes will enough lather (foam) be formed ?
(A) I and II
(B) II and III
(C) I and IV
(D) III and IV

Answer 1

Soap molecules are sodium or potassium salts of long-chain carboxylic acids. On reaction with calcium or magnesium ions, the soap solution forms insoluble precipitates. However, the formation of foam by the soap solution is hardly influenced by the presence of sodium and potassium ions.

Thus, after adding soap to each test tube and shaking them well, a good amount of lather will be formed in the test tubes marked I and IV.

Hence, the correct option is C.