Which material is commonly classified as spontaneously combustible (pyrophoric or self-heating)?

• A. Phosphorus
• B. Chlorine
• C. Acetone
• D. Diesel

Answer: (A)

Spontaneously combustible materials ignite in air without an external ignition source because they react with oxygen or heat themselves enough to start a fire. Phosphorus, especially certain forms like white phosphorus, readily reacts with oxygen in the air and can ignite at or near room temperature, which is why it’s known as pyrophoric and is typically stored under water to keep it away from air. The other substances don’t ignite on their own under normal handling: chlorine is an oxidizer and is not a fuel, acetone will burn if there’s a flame or spark but won’t self-ignite, and diesel has a relatively high autoignition temperature and requires significant heat or compression to ignite.