Select the incorrect order .

Answer

To solve the question of selecting the incorrect order among the given options, we will analyze each statement regarding the thermal stability and solubility of nitrates and hydroxides of alkali and alkaline earth metals.

### Step-by-Step Solution:

1. **Understanding Thermal Stability of Nitrates:**
- The thermal stability of nitrates generally increases with the size of the cation.
- For nitrates:
- Lithium nitrate (LiNO₃) < Sodium nitrate (NaNO₃) < Potassium nitrate (KNO₃)
- This order is correct because larger cations lead to more stable nitrates due to weaker lattice energy.

**Hint:** Remember that larger cations stabilize the nitrate structure better.

2. **Analyzing Solubility of Nitrates:**
- The solubility of nitrates in water is influenced by the hydration energy of the cations.
- The order of hydration energy is: Li⁺ > Na⁺ > K⁺.
- Therefore, the solubility order should be:
- Potassium nitrate (KNO₃) > Sodium nitrate (NaNO₃) > Lithium nitrate (LiNO₃)
- The statement that lithium nitrate is least soluble, followed by sodium nitrate, and potassium nitrate being most soluble is incorrect.

**Hint:** Hydration energy is key to determining solubility; higher hydration energy means higher solubility.

3. **Thermal Stability of Hydroxides:**
- Similar to nitrates, the thermal stability of hydroxides increases with cation size.
- For hydroxides:
- Beryllium hydroxide (Be(OH)₂) < Calcium hydroxide (Ca(OH)₂) < Strontium hydroxide (Sr(OH)₂)
- This order is correct as larger cations lead to more stable hydroxides.

**Hint:** Larger cations generally lead to greater thermal stability in hydroxides.

4. **Solubility of Hydroxides:**
- The solubility of hydroxides increases down the group in alkaline earth metals.
- The order of solubility is:
- Be(OH)₂ < Mg(OH)₂ < Ca(OH)₂ < Sr(OH)₂ < Ba(OH)₂
- This order is correct because the basicity and solubility of hydroxides increase down the group.

**Hint:** Basicity and the ability to release hydroxide ions increase down the group.

### Conclusion:
- The incorrect order among the options provided is related to the solubility of nitrates, where the statement about lithium nitrate being the least soluble is wrong. The correct order should be reversed.

**Final Answer:** The incorrect order is **B** (Solubility of nitrates).