Pick out the incorrect statement.

Options

A. Except nitrogen , all the elements of group
15 exist in allotropic modification
B. Only at high temperature , greater than
1,070 K, phosphorus dissociates into `P_2` molecules
C. Red phosphorus is obtained by heating white
phosphorus at 540 - 570 K in the absence of
air for several hours
D. White phosphorus is more reactive , but less
soluble in `CS_2` (and other organic solvents )
than red phosphorus

Answer

To solve the question of identifying the incorrect statement regarding the elements of Group 15, we will analyze each statement step by step.

### Step-by-Step Solution:

1. **Understanding Group 15 Elements**:
- The elements in Group 15 of the periodic table are nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi).
- Allotropes are different forms of the same element in the same physical state.

**Hint**: Recall the elements in Group 15 and their common allotropes.

2. **Analyzing the First Statement**:
- "Except nitrogen, all the elements of Group 15 exist in allotropic modification."
- Nitrogen does not have allotropes, while phosphorus, arsenic, antimony, and bismuth do have allotropes.

**Hint**: Consider the allotropes of each element in Group 15.

3. **Analyzing the Second Statement**:
- "Only at high temperature greater than 1070 K phosphorus dissociates into P2 molecules."
- Phosphorus typically exists as P4 at room temperature and dissociates into P2 at high temperatures (around 1070 K).

**Hint**: Think about the molecular forms of phosphorus at different temperatures.

4. **Analyzing the Third Statement**:
- "Red phosphorus is obtained by heating white phosphorus at 540 to 570 K in the absence of air for several hours."
- This is a correct statement as heating white phosphorus under these conditions does yield red phosphorus.

**Hint**: Recall the transformation process between white and red phosphorus.

5. **Analyzing the Fourth Statement**:
- "White phosphorus is more reactive but less soluble in carbon disulfide than red phosphorus."
- This statement is incorrect because white phosphorus is actually more soluble in carbon disulfide compared to red phosphorus, which is insoluble.

**Hint**: Compare the solubility of white phosphorus and red phosphorus in carbon disulfide.

### Conclusion:
The incorrect statement is: "White phosphorus is more reactive but less soluble in carbon disulfide than red phosphorus." This statement is false because white phosphorus is more soluble in carbon disulfide than red phosphorus.

### Final Answer:
The incorrect statement is the fourth one.

---