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

Answer 1

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