Question

A student took four test tubes P, Q, R and S and lled about 8 mL of distilled water in each. After that he dissolved an equal amount of Na₂ SO₄ in P, K₂ SO₄ in Q, CaSO₄ in R and MgSO₄ in S. On adding an equal amount of soap solution and shaking each test tube well, a good amount of lather will be obtained in the test tubes: (a) P and Q(b) P and R(c) P, Q and S(d) Q, R and S

Answer 1

The correct answer is

(A) P and Q

Answer 2

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 sulphate and magnesium sulphate. Thus, on shaking each of the test tubes well, the student observes a good amount of lather in the test tubes marked P and Q. Hence, the correct option is A.