Which statement best describes Division 4.2 self-heating substances?

• A. They do not react with oxygen
• B. They are always liquids
• C. They generate heat through oxidation and may self-ignite in bulk
• D. They are explosive on impact

Answer: (C)

Division 4.2 self-heating substances are materials that generate heat through oxidation and may self-ignite in bulk. The heat comes from a slow exothermic reaction with oxygen in the air. When large quantities are stored together, cooling can be insufficient to remove that heat, so temperatures can rise to the point where ignition occurs without an external flame or spark. That’s why these substances require careful handling and storage to prevent heat buildup. They do react with oxygen (so they are not “do not react with oxygen”), they are not limited to liquids (many are solids), and they are not defined by being explosive on impact. Examples include oily rags or other materials that can oxidize gradually and heat up in bulk.