What conditions cause white phosphorus to ignite?

White phosphorus is highly reactive and can ignite spontaneously upon contact with air. This is due to its unique chemical property where it reacts with oxygen in the atmosphere. The resultant exothermic reaction generates enough heat to ignite the phosphorus itself, leading to combustion. This characteristic makes it particularly hazardous, as it can catch fire without requiring an external ignition source.

The other conditions listed do not accurately describe the ignition behavior of white phosphorus. For example, placing it in water actually extinguishes it, as water prevents the phosphorus from coming into contact with the oxygen needed for combustion. Exposure to extreme heat may increase the likelihood of ignition but is not the primary condition that causes it to ignite under normal circumstances. Lastly, low humidity is not a triggering factor for ignition; white phosphorus can ignite regardless of humidity levels as long as there is sufficient oxygen.

Understanding these properties is crucial for safe handling and storage practices in a fire department setting, ensuring personnel are aware of the reactive nature of materials like white phosphorus.