Question

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, viz. 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 tube marked:

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

Answer 1

Answer: (A)

$P$ and $Q$

Soaps are sodium or potassium salts of long chain fatty acids. With $Ca$ and $Mg$, soap forms insoluble precipitate called scum. However, sodium (P) and potassium (Q) sulphates help in the formation of lather.

Hence, option A is correct.