Which material is classified as pyrophoric?

Phosphorus is classified as a pyrophoric material because it can ignite spontaneously in air at room temperature. This property makes it particularly dangerous when handling or storing phosphorus, as it can pose significant fire hazards without the need for an external ignition source.

In contrast, nitric acid, formaldehyde, and sulfur do not possess the same spontaneous ignition characteristics. Nitric acid is a strong oxidizer and can enhance combustion when it comes into contact with flammable materials, but it does not ignite on its own under normal conditions. Formaldehyde is a volatile organic compound and can be flammable, but it requires an ignition source to ignite. Sulfur can burn and produce sulfur dioxide gas but also does not spontaneously ignite in air at room temperature. Therefore, phosphorus is unique among these materials as a pyrophoric substance.