Which of the following statements about phosphorus is INCORRECT according to its basic chemical properties?

Introduction / Context:
This question checks your understanding of the properties of phosphorus, a non metal element with several important allotropes. Phosphorus is known for its reactivity with air and the need for careful storage. The question asks which statement is incorrect, so you must know how phosphorus actually behaves with air and water and where it is placed in the periodic table.

Given Data / Assumptions:

• Element: phosphorus, symbol P.
• We consider its reactivity and periodic classification.
• Options describe reactions with water and air, non metallic character, group position and allotropy.
• We assume standard behaviour of white and red phosphorus at school level.

Concept / Approach:
Phosphorus is a non metal found in group 15 (group 5A in older notation) of the periodic table. White phosphorus is highly reactive with oxygen in air and can catch fire spontaneously at relatively low temperatures, which is why it is stored under water to prevent contact with air. However, it does not react vigorously with water itself; the water simply acts as a protective medium. Therefore, any statement claiming vigorous reaction with water is incorrect, while statements about non metallic nature, air reactivity and group position are correct.

Step-by-Step Solution:
Step 1: Recall that phosphorus is classified as a non metal and forms covalent compounds such as phosphoric acid and phosphates. This supports option B as a correct statement. Step 2: Phosphorus, especially white phosphorus, is very reactive with oxygen in air and can ignite at temperatures slightly above room temperature. Hence, it is true that it can catch fire on reacting with air, supporting option C as correct. Step 3: In the periodic table, phosphorus is placed in group 15, along with nitrogen, arsenic and others, so option D is also correct. Step 4: Phosphorus is known to exist in several allotropic forms, including white, red and black phosphorus, so option E is correct. Step 5: Examine option A, which claims that phosphorus reacts vigorously with water. Step 6: In reality, white phosphorus is stored under water to prevent contact with air because it reacts with oxygen, not because it reacts violently with water itself. Step 7: Therefore, the statement that phosphorus reacts vigorously with water is incorrect and should be selected.

Verification / Alternative check:
Typical school experiments and safety notes state that white phosphorus must be kept under water to avoid accidental ignition in air. If phosphorus reacted vigorously with water, storing it under water would be dangerous, which contradicts laboratory practice. References on non metals and their properties highlight that phosphorus is air sensitive rather than water reactive, confirming that water serves as a protective barrier, not as a reactant in a vigorous reaction.

Why Other Options Are Wrong:
In this question, wrong options are actually correct statements about phosphorus, so they cannot be selected as incorrect. Phosphorus is indeed a non metal, it does catch fire on reacting with air, and it is correctly placed in group 15 of the periodic table. It also has well known allotropes, including white and red forms. These details align with standard textbook descriptions and are therefore not incorrect statements.

Common Pitfalls:
Students sometimes misunderstand the reason for storing white phosphorus under water and assume that it reacts with water. It is important to distinguish between being protected by water from air and being chemically reactive with water itself. Another pitfall is to confuse properties of phosphorus with those of alkali metals like sodium, which do react vigorously with water. Careful reading of textbook examples for each element helps avoid such mix ups.

Final Answer:
The incorrect statement is that phosphorus reacts vigorously with water.