White (yellow) phosphorus is stored and kept immersed under water because of which characteristic property?

Introduction / Context:
This question concerns the physical and chemical properties of white (also called yellow) phosphorus and the safety precautions required when handling it. Some substances are so reactive with air that they ignite or burn spontaneously at or near room temperature. Chemists must therefore store them under special conditions. Here you are asked to identify why phosphorus is kept under water rather than left exposed to air.

Given Data / Assumptions:

• The form of phosphorus considered is white or yellow phosphorus, not red or black phosphorus.
• It is known to be highly reactive and flammable.
• Options mention ignition temperature and critical temperature, as well as solubility.
• Ignition temperature is the minimum temperature at which a substance catches fire spontaneously in air without an external flame.

Concept / Approach:
White phosphorus has a very low ignition temperature, meaning it can catch fire in air at temperatures only slightly above room temperature. It reacts with oxygen to form phosphorus pentoxide, releasing heat and often glowing in the dark due to slow oxidation, a phenomenon called chemiluminescence. To prevent accidental ignition, white phosphorus is stored underwater, where it cannot contact atmospheric oxygen. Water acts as a physical barrier, and because phosphorus is insoluble in water, it remains safe and stable. Critical temperature, by contrast, relates to gas liquefaction and is not the main issue here.

Step-by-Step Solution:
Step 1: Recall that ignition temperature is the lowest temperature at which a substance begins to burn in air spontaneously. Step 2: White phosphorus has a very low ignition temperature, around room temperature or slightly above, making it prone to catching fire if left in air. Step 3: Because of this, to handle white phosphorus safely, it is kept under water to isolate it from oxygen in the air. Step 4: A high ignition temperature would make a substance less likely to burn, which is the opposite of the situation with phosphorus. Step 5: Critical temperature is a property related to the liquefaction of gases and is not directly responsible for spontaneous ignition of a solid like phosphorus. Step 6: Solubility in water is low for white phosphorus, so water is simply used as a protective layer rather than a solvent. Step 7: Therefore, the correct reason is that white phosphorus has a very low ignition temperature.

Verification / Alternative check:
Descriptions of phosphorus in textbooks note that white phosphorus glows faintly in the dark when exposed to air, due to slow oxidation, and can easily ignite, producing dense white fumes of phosphorus pentoxide. Safety data sheets for white phosphorus require that it be stored under water, usually in glass containers, precisely to keep it away from air. Red phosphorus, on the other hand, is more stable and does not require storage under water. This difference emphasises that the key property involved is the low ignition temperature of white phosphorus, not any special behaviour related to critical temperature or solubility.

Why Other Options Are Wrong:
If the ignition temperature were very high, phosphorus would be difficult to ignite and would not need special precautions, which contradicts known behaviour. Critical temperature arguments apply to gases near their liquefaction point and do not explain why a solid element ignites so easily in air. Saying that phosphorus is kept under water because its critical temperature is high or low is therefore incorrect. Phosphorus is also not stored under water because it is highly soluble; in fact, it is only sparingly soluble in water. Water acts as a protective barrier, not as a solvent in this context.

Common Pitfalls:
Students sometimes confuse ignition temperature with melting or boiling point, assuming that a low melting point automatically means a substance catches fire easily. These are different concepts; ignition temperature specifically refers to the temperature needed for combustion in air. Another mistake is to overuse the idea of solubility and think that any substance stored under water must be highly soluble, which is not true for white phosphorus. To avoid such errors, remember key textbook examples: white phosphorus has low ignition temperature and is stored under water, while substances with high ignition temperatures require intense heating or a flame to burn.

Final Answer:
White phosphorus is kept under water because its ignition temperature is very low, so it would otherwise ignite easily in air.