Question

A student takes about 6 ml of distilled water in each of the four test tubes $P$, $Q$, $R$ and $S$, then dissolves an equal amount of four different salts namely sodium chloride in $P$, potassium chloride in $Q$, calcium chloride in $R$ and magnesium chloride in $S$. He then adds 10 drops of soap solution to each test tube and shakes its contents. The test tubes in which scum (insoluble substance) is formed with soap are:

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

Answer 1

Answer: (C)

$R$ and $S$

Soap forms insoluble precipitates with calcium and magnesium ions forming a scum.

Hence, test tube $R$ and $S$ containing calcium chloride and magnesium chloride respectively will form scum.

Option C is correct.