Question:

An alkaline earth metal 'M' readily forms water soluble sulphate and water insoluble hydroxide. Its oxide MO is very stable to heat and does not have rock-salt structure. M is :

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Only Be has a soluble sulphate with an insoluble hydroxide, and BeO has a wurtzite (not rock-salt) lattice.
Updated On: Nov 5, 2026
  • Ca
  • Mg
  • Sr
  • Be
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The Correct Option is D

Solution and Explanation

Sulphate solubility in group 2 decreases down the group: \(\mathrm{BeSO_4}\) and \(\mathrm{MgSO_4}\) are soluble, \(\mathrm{CaSO_4}\) is sparingly soluble, \(\mathrm{SrSO_4}\) and \(\mathrm{BaSO_4}\) are insoluble. So M is Be or Mg (Ca and Sr fail).
Hydroxide solubility increases down the group: \(\mathrm{Be(OH)_2}\) and \(\mathrm{Mg(OH)_2}\) are insoluble, so both still qualify.
Oxide structure: MgO, CaO and SrO have the rock-salt structure; BeO has a wurtzite (covalent) structure and is very stable to heat.
So M is Be.
Answer: (4) Be
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