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A student takes about 4 mL of distilled water in four test tubes marked P, Q, R and S. He then dissolves in each test tube an equal amount of one salt in

one test tube, namely sodium sulphate in P, potassium sulphate in Q, calcium sulphate in R and magnesium sulphate in S. After that he adds an equal amount of soap solution in each test tube. On shaking each of these test tubes well, he observes a good amount of lather (foam) in the test tubes marked (A) P and Q (B) Q and R (C) P, Q and S (D) P, R and S


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.
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