Which Class 4 subcategory represents materials that ignite spontaneously without an ignition source?

• A. 4.2 Spontaneous combustible
• B. 4.1 Self reactive
• C. 4.3 +water = Flammable gases
• D. 4.4 Water reactive solids

Answer: (A)

Ignition without an external ignition source happens when a material is spontaneously combustible. This means heat is generated by the substance itself through internal oxidation or other exothermic processes, and the heat cannot dissipate fast enough, allowing the temperature to rise to the point of ignition without a flame or spark being applied. Think of materials that can self-warm and catch fire simply from sitting in air or under normal conditions; that’s the hallmark of spontaneous combustion. The other descriptions point to different behaviors: self-reactive substances can decompose or react aggressively under certain conditions rather than reliably lighting off in air on their own; materials that emit flammable gases when in contact with water need water to trigger gas production, not air ignition; and solids that react with water simply have a chemical reaction with moisture, not spontaneous ignition in air. Thus, the category for materials that ignite spontaneously is the spontaneous combustible group.