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 various salts, 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:

P and Q

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P, R and S

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P, Q and S

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Q and R

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Solution

The correct option is A P and Q

In the test tube P sodium sulphate is added and in test tube Q potassium sulphate is added.

As sodium and potassium sulphate don’t contain multivalent cation, they don’t form hard water and in soft water soap lather is formed.

In test tube R calcium sulphate is added and in test tube S magnesium sulphate is added.

They form hard water and in those test tubes soap doesn’t lather and scum forms.

Test tube P and Q will form lather.

So the correct option is A.

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