# An alkaline earth metal 'M' readily forms water soluble sulphate and water insoluble hydroxide.

Question:

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

1. Sr

2. Ca

3. Mg

4. Be

Correct Option: , 4

Solution:

Solubility of $\mathrm{BeSO}_{4}$ is highest among the given met sulphates

$\mathrm{BeSO}_{4}>\mathrm{MgSO}_{4}>\mathrm{CaSO}_{4}>\mathrm{SrSO}_{4}>\mathrm{BaSO}_{4}$

Solubility order for hydroxide is

$\mathrm{Be}(\mathrm{OH})_{2}<\mathrm{Mg}(\mathrm{OH})_{2}<\mathrm{Ca}(\mathrm{OH})_{2}<\mathrm{Sr}(\mathrm{OH})_{2}<\mathrm{Ba}(\mathrm{OH})_{2}$

Thus $\mathrm{BeSO}_{4}$ is soluble and $\mathrm{Be}(\mathrm{OH})_{2}$ is insoluble.

$\mathrm{BeO}$ does not form rock salt like structure.

In the solid state, it adopts the hexagonal neurtzite structure form while in the vapour phase, it is present as discrete diatomic covalent molecules.