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 given ions and their properties, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Analyzing Statement A**:
- The statement claims that \( \text{pH}_4^+ \) ion is a tetrahedral ion like \( \text{NH}_4^+ \) and is formed when \( \text{pH}_3 \) bonds to a proton.
- **Formation**:
- \( \text{NH}_4^+ \) is formed by the reaction of \( \text{NH}_3 \) with a proton (\( \text{H}^+ \)).
- Similarly, \( \text{pH}_4^+ \) is formed from \( \text{pH}_3 \) by adding a proton.
- **Geometry**:
- Both \( \text{NH}_4^+ \) and \( \text{pH}_4^+ \) have a tetrahedral geometry.
- **Conclusion**: This statement is **correct**.

2. **Analyzing Statement B**:
- The statement claims that \( \text{pH}_4^+ \) is one of the most stable salts containing the phosphonium ion and is more stable than ammonium salts.
- **Stability**:
- \( \text{pH}_4^+ \) is known to be unstable and can decompose at elevated temperatures (e.g., 60°C).
- In general, as we move down the group in the periodic table, the stability of hydrides decreases due to increasing bond lengths and weaker bonds.
- **Comparison**:
- \( \text{NH}_4^+ \) is more stable than \( \text{pH}_4^+ \).
- **Conclusion**: This statement is **incorrect**.

3. **Analyzing Statement C**:
- The statement claims that \( \text{pH}_4^+ \) decomposes with caustic potash (potassium hydroxide) to form \( \text{pH}_3 \).
- **Reaction**:
- The reaction is \( \text{pH}_4^+ + \text{KOH} \rightarrow \text{pH}_3 + \text{KI} + \text{H}_2\text{O} \).
- **Conclusion**: This statement is **correct**.

4. **Analyzing Statement D**:
- The statement claims that \( \text{pH}_3 \) is used for making Holmes signal.
- **Holmes Signal**:
- The Holmes signal is produced using phosphine (\( \text{pH}_3 \)), which emits a red light and smoke when burned.
- **Conclusion**: This statement is **correct**.

### Final Conclusion:
The incorrect statement among the options provided is **Statement B**: "pH4 is one of the most stable salts containing the phosphonium ion and is more stable than ammonium salts."