A student takes about 4 ml of distilled water in four test tubes marked P, Q, R, and S. He then dissolves an equal amount of various salts in each test tube, i.e., 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

Q and R

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P , R and S

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P and Q

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P , Q and S

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Solution

The correct option is C P and Q
Hard water is the water that contains salts of calcium and magnesium principally as bicarbonates, chlorides, and sulphates.
Soaps are sodium or potassium salts of long chain fatty acids. With Ca and Mg salts, soap forms insoluble precipitate called scum. However, sodium (P) and potassium (Q) sulphates do not cause hardness,

So option A is the correct answer.

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