Question

Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : Barium carbonate is insoluble in water and is highly stable.
Reason (R) : The thermal stability of the carbonates increases with increasing cationic size.
Choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are true and (R) is the true explanation of (A).
• B. Both (A) and (R) are true but (R) is not the true explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

For Group 2 elements, remember these trends going down the group:
\textbf{Solubility:} Sulphates and Carbonates solubility decreases. Hydroxides solubility increases.
\textbf{Thermal Stability:} Carbonates and Sulphates stability increases.
This is explained by the interplay of lattice energy and hydration energy, and by Fajan's rules regarding polarization.

Answer 1

The Correct Option is A

To determine the correct answer, let's analyze the given statements:

1. Assertion (A): Barium carbonate is insoluble in water and is highly stable.
2. Reason (R): The thermal stability of the carbonates increases with increasing cationic size.

First, let's evaluate the assertion (A). Barium carbonate, indeed, is known to be insoluble in water due to the large size of the barium ion, which does not allow it to form hydrates easily. Moreover, barium carbonate is stable, especially at room temperature and does not decompose easily. Therefore, assertion (A) is true.

Now, let's consider the reason (R). In general, the thermal stability of alkaline earth metal carbonates increases as we move down the group in the periodic table. This is due to the increasing size of the cation, which leads to a decrease in the charge density, reducing the polarizing power of the cation. Less polarization leads to more stable carbonate bonds. Hence, the larger cations like Ba²⁺ result in more stable carbonates. Therefore, reason (R) is also true.

Now, let's assess whether (R) is the correct explanation for (A). The reason provided explains why barium carbonate is more stable than the carbonates of lighter alkaline earth metals. The increased cationic size of Ba²⁺ explains both the insolubility and greater stability of barium carbonate. Thus, (R) is indeed the correct explanation of (A).

Based on this evaluation, the most suitable option is: Both (A) and (R) are true and (R) is the true explanation of (A).