Chemistry S & P Block Elements UHS 2024
PMDC Verified Question 3 of 104
The solubility of sulphates of alkaline metals generally
A
Increases down the group
B
Increase then decrease down the group
C
Decrease down the group
D
Doesn't change down the group
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option C: Decrease down the group
Concept:

Note: The question says "alkaline metals", which in historical exam parlance often refers to alkaline earth metals (Group II), as alkali metal sulphates are universally soluble. The trend for Group II sulphates is asked here.

Solution:

  • For Alkaline Earth Metal Sulphates (e.g., \( \text{BeSO}_4, \text{MgSO}_4, \text{CaSO}_4, \text{SrSO}_4, \text{BaSO}_4 \)), solubility depends on hydration vs. lattice energy.
  • The sulphate ion (\( \text{SO}_4^{2-} \)) is very large. Thus, the lattice energy remains relatively constant down the group because changing the size of the smaller metal cation doesn't disrupt the large-anion lattice much.
  • However, as the metal cation gets larger (moving down the group), its hydration energy drops precipitously.
  • Because hydration energy falls much faster than lattice energy, the compounds become much harder to dissolve.
  • Therefore, the solubility of Group II sulphates heavily decreases down the group (with \( \text{BaSO}_4 \) being notoriously insoluble).


Why other options are incorrect:

  • Hydroxides increase down the group; sulphates and carbonates definitively decrease.

Quality & Fidelity Assurance: Every question on BeambePrep is rigorously curated against the official PMDC syllabus with zero filler, zero out-of-syllabus content, and zero typos. When an authentic past paper originally contained a historical mistake or ambiguity from the examining board (such as UHS or NUMS), BeambePrep faithfully reflects the original paper while detailing the nuance and scientific consensus in the autopsy above.

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