Which of the following describes white phosphorus?

White phosphorus is known for its highly reactive nature, which causes it to ignite spontaneously upon contact with air. This characteristic makes it particularly hazardous as it can catch fire without an external ignition source, leading to severe burns and fire hazards. The combustion of white phosphorus emits a bright flame and thick white smoke, which is a result of its oxidation process.

The description emphasizing its igniting ability upon exposure to air is critical to understanding its handling and transportation requirements, especially in the context of hazardous materials. Understanding this property helps ensure compliance with safety regulations, as transporters must take great care to prevent accidental ignition and ensure proper storage conditions.

The other options describe different harmful substances or behaviors that do not accurately reflect the properties of white phosphorus. For example, it is not a blood agent in liquid form, nor does it produce choking gas simply by contact with air. Additionally, while certain chemical agents may be used for crowd control, white phosphorus is not typically categorized as such and is primarily recognized for its incendiary and toxic properties.