Question

A student takes a about ${\text{6 ml}}$ 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 ${\text{10}}$ drops of soap solution to each test tube, and shakes its contents. The test tube 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

The atoms and molecules of the hard water interact with the soap molecule to form an insoluble precipitate which is called scum. The soap cations are replaced by the hard water cations in this case.

Complete step by step answer:
Waters that have cations of $+ 2$ charge is called hard waters. Calcium ion $\left( {C{a^{2 + }}} \right)$, and Magnesium ion $\left( {M{g^{2 + }}} \right)$ come in its special category. Soaps are sodium or potassium salt of fatty acid. When soap is dissolved into a hard water, the $C{a^{2 + }}$ ions or the $M{g^{2 + }}$ ions react with the soap molecule, and replace the $N{a^ + }$ or ${K^ + }$ ions to form a precipitate, chemically called Calcium Stearate or Magnesium Stearate. In the first, and second test tubes, P, and Q, the salts present contains $N{a^ + }$ or ${K^ + }$ ions, which are also present in the soap molecules, so, they will not bring any change to the solution after mixed. But, the third, and fourth test tubes, R, and S contain Calcium and Magnesium ions which will react with the soap molecule to form an insoluble scum. So, in test tube R, and S scum (insoluble substance) will form.

So, the correct answer is Option C .

Note:
Soap and soap scum look similar, so, to differentiate between the actual soap and the soap scum, mix equal amounts of soap, and soap scum in equal amounts of water in two different containers. The one with lesser bubbles will be the soap scum as soap scum reduces the ability to form the bubbles.