Which decomposes on heating-

Options

A. NaOH
B. KOH
C. LiOH
D. RbOH

Answer

To determine which hydroxide decomposes upon heating among the options given (NaOH, KOH, LiOH, RbOH), we can analyze the thermal stability of these compounds based on the polarizing power of their respective cations.

### Step-by-Step Solution:

1. **Understanding Thermal Stability**:
- The thermal stability of a compound is inversely related to the polarizing power of the cation. Higher polarizing power means lower thermal stability.

2. **Identifying the Cations**:
- The cations in the given hydroxides are:
- Na⁺ (Sodium)
- K⁺ (Potassium)
- Li⁺ (Lithium)
- Rb⁺ (Rubidium)

3. **Comparing Cation Sizes**:
- In the alkali metals group (Li, Na, K, Rb), as we move down the group, the size of the cations increases:
- Li⁺ < Na⁺ < K⁺ < Rb⁺
- This means that lithium has the smallest size and rubidium has the largest size.

4. **Determining Polarizing Power**:
- The polarizing power of cations is higher for smaller cations. Therefore, lithium (Li⁺) has the highest polarizing power among the given options.

5. **Decomposition of Hydroxides**:
- Lithium hydroxide (LiOH) is known to decompose upon heating into lithium oxide (Li₂O) and water (H₂O):
\[ 2 \text{LiOH} \rightarrow \text{Li}_2\text{O} + \text{H}_2\text{O} \]
- The other hydroxides (NaOH, KOH, RbOH) do not decompose upon heating under normal conditions.

6. **Conclusion**:
- Based on the analysis, the hydroxide that decomposes upon heating is **LiOH**.

### Final Answer:
The correct answer is **LiOH** (Option 3).