Question

A student takes four test tubes marked P, Q, R, and S of 25 mL capacity and fills 10 mL of distilled water in each. He dissolves one spoon full of four different salts in each as - $\mathrm{KCl}\mathrm{in}\mathrm{P},\mathrm{NaCl}\mathrm{in}\mathrm{Q},{\mathrm{CaCl}}_2\mathrm{in}\mathrm{R},\mathrm{and}{\mathrm{MgCl}}_2\mathrm{in}\mathrm{S}$. He then adds about 2mL of a soap solution to each of the above test tubes. On shaking the contents of each of the test tubes, he is likely to observe a good amount of lather(foam) in the test tube marked.

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

Answer 1

The correct option is A

P and Q

The explanation for the correct option:

A: P and Q

• Soaps are composed of Sodium and Potassium salts of long-chain fatty Carboxylic acids.
• The soap reacts with Calcium and Magnesium ions to produce insoluble precipitates.
• These precipitates are known as scum.
• This implies that Sulphates, Bicarbonates, and Chlorides of Calcium and Magnesium make the water hard.
• The hard water does not make any lather when such salts are mixed in it.
• On the other hand when soap reacts with Sodium and Potassium salts, they dissolve easily.
• Thus, on dissolution the neutral solution is formed and due to this lather formation occurs.
• The test tube which contains the salts of Sodium and Potassium will easily make the lather while Calcium and Magnesium salts do not.

Hence we concluded that the test tube which contains the salts of Sodium and Potassium will form a good amount of lather(foam).

Therefore, option A is correct. P and Q.