Substances that spontaneously combust or release flammable gas when wet belong to which division?

• A. Division 4.3
• B. Division 4.1
• C. Division 3
• D. Division 2

Answer: (A)

Flammable solids are grouped under Class 4, and the subdivision in question is for substances that, when they come into contact with water, emit a flammable gas. This is the behavior described: a material reacts with moisture to release a gas like acetylene or hydrogen, which can ignite. Calcium carbide, for instance, reacts with water to produce acetylene, while certain alkali metals react with water to release hydrogen. That wet-gas-emission criterion is the defining feature of this subdivision, which is why it’s placed in Division 4.3. In contrast, Division 4.1 covers flammable solids themselves that ignite easily without water, and Division 4.2 covers substances liable to spontaneous combustion in air. The wet-gas-producing category, Division 4.3, is the best fit for substances that become flammable gases when they contact water.