Find the incorrect statement

Options

A. `PH_4^+` ion is tetrahedral like `NH_4^+` ion and is obtained when `PH_3` is bonded to proton
B. `PH_4I` is one of the most stable salt containing the phosphonium ion. It it also more stable than ammonium salts
C. `PH_4I` is decomposed caustic potash to form `PH_3`
D. `PH_3` is used for making Holme's signals

Answer

To find the incorrect statement regarding the properties and stability of phosphonium and ammonium ions, let's analyze each statement step by step.

### Step-by-Step Solution:

1. **Statement Analysis**:
- The first statement claims that the PH4+ ion is tetrahedral and is formed when PH3 bonds to a proton.
- **Evaluation**: This statement is correct. The PH4+ ion has a steric number of 4, indicating sp3 hybridization, which indeed gives it a tetrahedral geometry. Additionally, PH3 can bond with a proton to form PH4+.

2. **Stability Comparison**:
- The second statement asserts that PH4I is one of the most stable salts containing the phosphonium ion and that it is more stable than ammonium salts.
- **Evaluation**: This statement is incorrect. While PH4I is a stable salt, it is less stable than ammonium salts like NH4Cl. The reason is that phosphorus is less basic than nitrogen, making phosphonium ions less stable. Furthermore, PH4I is thermally less stable and decomposes at lower temperatures compared to ammonium salts.

3. **Decomposition Reaction**:
- The third statement mentions that PH4I decomposes with caustic potash (KOH) to form PH3.
- **Evaluation**: This statement is correct. The reaction between PH4I and KOH produces PH3, HI, and water, confirming that PH4I can decompose in the presence of a strong base.

4. **Hallmeyer Signals**:
- The fourth statement states that PH3 is used for Hallmeyer signals.
- **Evaluation**: This statement is also correct. PH3 is indeed involved in the Hallmeyer signal reactions, where it reacts with other gases to form new products.

### Conclusion:
The incorrect statement is the second one, which claims that PH4I is more stable than ammonium salts.