Question

Assertion (A): \( MgO, CaO, SrO, \) and \( BaO \) are insoluble in water.
Reason (R): In aqueous medium, the basic strength of \( MgO, CaO, SrO, \) and \( BaO \) increases with increase in the atomic number of the metal.
The correct option among the following is

• (A) and (R) are correct. (R) is the correct explanation of (A).
• (A) and (R) are correct, but (R) is not the correct explanation of (A).
• (A) is correct but (R) is not correct.
• (A) is not correct but (R) is correct.

Hint:

The solubility of metal oxides generally increases down the group due to decreasing lattice energy.

Answer 1

The Correct Option is D

Assertion (A): MgO, CaO, SrO, and BaO are insoluble in water.
This statement is not entirely correct. While MgO has very low solubility in water, the
solubility of the oxides of alkaline earth metals generally increases as you go down the
group. CaO, SrO, and BaO react with water to form their respective hydroxides, which are
soluble to varying degrees.
Reason (R): In an aqueous medium, the basic strength of MgO, CaO, SrO, and BaO
increases with an increase in the atomic number of the metal.
This statement is correct. As you move down the group in the periodic table, the size of the
metal cation increases. This leads to a weaker attraction between the metal cation and the
oxide ion (O²), making it easier for the oxide to donate electrons and act as a base.
Therefore, the basic strength of the oxides increases down the group.
Correct Option:
Since the assertion is not entirely correct, but the reason is correct, the answer is (4) (A) is
not correct but (R) is correct. Explanation:
The solubility and basic strength of these oxides are related to the size and charge density of
the metal cations. Smaller cations with higher charge density (like Mg²) have a stronger
attraction to the oxide ion, making the oxide less soluble and less basic. Larger cations with
lower charge density (like Ba²) have a weaker attraction to the oxide ion, making the oxide
more soluble and more basic.