Question

A student takes about 4 ml of distilled water in 4 separate test tubes labelled P, Q, R and S each containing an equal amount of different salts:
P – Sodium Sulphate
Q – Potassium Sulphate
R – Calcium Sulphate
S – Magnesium Sulphate
After doing so, he then adds an equal amount of soap solution to each of the aforementioned tubes. Upon shaking well, he observes a good amount of lather 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

Hint: Think about the difference of the action of soap solution in hard water when compared to its action in soft water. What results in a major difference between the two?

Step-by-Step Solution:
To solve this question, we need to first remember what the action of soap solution on hard water is. Remember that soaps are long-chain fatty acids of either Sodium or Potassium and that the hardness of water is caused by the presence of sulphates, carbonates and bicarbonates of Calcium and Magnesium in water.
When the soap solution is subjected to the hard water solution in test tubes R and S (formed as a result of the presence of Ca and Mg Sulphates respectively), the sodium salts present in the soap solution are transformed into their Ca and Mg salts respectively which are precipitated out in the form of scum, which hampers the action of soap solution.

$C{{a}^{2+}}+\text{ }R-COONa\text{ }\to \text{ }R-COOC{{a}_{(scum)}}+\text{ 2N}{{\text{a}}^{+}}$

$M{{g}^{2+}}+\text{ }R-COONa\text{ }\to \text{ }R-COOM{{g}_{(scum)}}+\text{ 2N}{{\text{a}}^{+}}$

Since Magnesium and Calcium salts are present in neither of test tubes P and Q, they are able to produce good lather due to the relative softness of the solution inside.
Therefore, the answer to this question is a) P and Q

Note: The knowledge of the reaction of soap solutions to hard and soft water is essential to solve this question, please try and be thorough with this concept before attempting this question.