Question

A student takes about 5 mL distilled water in four test tubes marked P, Q, R and S. He dissolves in each test tube an equal amount of ${\mathrm{Na}}_2{\mathrm{SO}}_4$ in P, ${\mathrm{K}}_2{\mathrm{SO}}_4$in Q, ${\mathrm{CaSO}}_4$ in R and ${\mathrm{MgSO}}_4$ in S. After that he adds an equal amount of concentrated soap solution in each test tube. On shaking these test tubes well, he would observe a good amount of lather (foam) in the test tubes marked.

• A. P and Q
• B. Q and R
• C. R and S
• D. P, Q and S

Answer 1

The correct option is A

P and Q

The explanation for the correct option A:

1. Soaps form lather (foam) with soft water and do not form lather (foam) with hard water.
2. Hard water- Hard water is the one that contains chlorides and sulphates of calcium ($\hspace{0.17em}\mathrm{Ca}$) and magnesium ($\hspace{0.17em}\mathrm{Mg}$).
3. Test tube R has Calcium sulphate (${\mathrm{CaSO}}_4$) and test tube S has Magnesium sulphate (${\mathrm{MgSO}}_4$). That means test tube R and S has hard water.
4. Therefore, test tubes R and S do not form lather (foam).
5. Hence, on shaking these test tubes well, the student would observe a good amount of lather (foam) in the test tubes marked P and Q.

The explanation for the incorrect option:

Option B: Test tube R has ${\mathrm{CaSO}}_4$ that makes the water hard and incapable to form lather.

Option C: Test tube R has ${\mathrm{CaSO}}_4$ and test tube S has ${\mathrm{MgSO}}_4$ that make the water hard and incapable to form lather.

Option D: Test tube S has ${\mathrm{MgSO}}_4$ that makes the water hard and incapable to form lather.

The correct option is A., Test tubes P and Q.