Chemistry S & P Block Elements UHS 2022
PMDC Verified Question 48 of 104
Which of the following sulphate compound is insoluble in water?
A
\( \text{BeSO}_4 \)
B
\( \text{MgSO}_4 \)
C
\( \text{BaSO}_4 \)
D
\( \text{CaSO}_4 \)
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: \( \text{BaSO}_4 \)
Concept:

The solubility of an ionic compound in water is decided by the tug-of-war between Lattice Energy (energy holding the crystal together) and Hydration Energy (energy released when ions interact with water).

Solution:

  • For Alkaline Earth Metal (Group II-A) sulphates, solubility decreases drastically as you move down the group.
  • Beryllium (Be) and Magnesium (Mg) have very small ionic radii, giving them extremely high hydration energies. This immense hydration energy easily overcomes their lattice energy, making \( \text{BeSO}_4 \) and \( \text{MgSO}_4 \) highly soluble.
  • Barium (Ba) is at the bottom of the group. Its massive size results in a very low hydration energy.
  • Because the low hydration energy of \( \text{Ba}^{2+} \) cannot compensate for the lattice energy required to break apart the \( \text{BaSO}_4 \) crystal, Barium sulphate remains practically insoluble in water (often forming white precipitates in lab tests).


Why other options are incorrect:

  • Options A and B are highly soluble. Option D (Calcium sulphate) is sparingly soluble, but BaSO4 is definitively the most insoluble.

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