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A student took four test tubes P, Q, R and S and filled about 8 mL of distilled water in each. After that he dissolved an equal amount of Na2SO4 in P, K2SO4 in Q, CaSO4 in R and MgSO4 in S. - CBSE Class 10 - Science

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Question

A student took four test tubes P, Q, R and S and filled about 8 mL of distilled water in each. After that he dissolved an equal amount of Na2SO4 in P, K2SO4 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

Solution

Soaps do not form lather with hard water (water containing calcium and magnesium ions). Among the given test tubes, test tubes R and S contain calcium and magnesium ions, respectively. So the addition of soap solution in these will not produce lather.

On the other hand, water present in test tubes P and Q are soft water (containing sodium and potassium ions), so the addition of soap solution in these will produce a good amount of lather.

Hence, the correct answer is P and Q

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Solution A student took four test tubes P, Q, R and S and filled about 8 mL of distilled water in each. After that he dissolved an equal amount of Na2SO4 in P, K2SO4 in Q, CaSO4 in R and MgSO4 in S. Concept: Soaps and Detergents.
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