Within the United Nations/Department of Transportation System, a container labeled with a Hazard Class 4 contains a:

• A. Flammable liquid
• B. Flammable solid
• C. Flammable gas
• D. Flammable aerosol

Answer: (B)

In the UN/DOT labeling system, Hazard Class 4 refers to flammable solids. A container labeled with this class indicates the material is a solid that can ignite readily or burn vigorously under the right conditions, rather than a liquid or a gas. That’s why the correct description is a flammable solid. A flammable liquid would fall under a different class designation for liquids, and a flammable gas is categorized separately because gases behave and are stored differently. Flammable aerosols also follow their own labeling guidelines, typically associated with aerosol packaging or a different class, not plain flammable solids. Think of solid materials like certain powders or matches, which can ignite from friction or heat—those are the ones this class covers.