Collin College Fire Academy Practice Test

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A substance that yields oxygen to support combustion of fuels would be labeled under the United Nations labeling system as which number?

Oxidizing substances provide or release oxygen that supports combustion, which makes them fall into hazard class five in the United Nations labeling system. This class groups materials capable of intensifying fires by supplying oxygen to flames, rather than being the fuel themselves. The other numbers correspond to different hazard classes—for example, flammable liquids, flammable solids, and toxic substances—so they don’t describe substances that yield oxygen to aid burning. Therefore, the correct number is five.