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DOT Hazard Class 4.png

A water-reactive substance is one that spontaneously undergoes a chemical reaction with water. Notable examples include alkali metals sodium through caesium.

Water-reactive substances are classified as R2 under the UN classification system and as Hazard 4.3 by the United States Department of Transportation.

Some water-reactive substances are also pyrophoric.

Some of the metals that are reactive with water are lithium, sodium, potassium, rubidium, and caesium. These metals are usually stored in containers with oil so that, if any water is brought into the container, it and the oil will separate, protecting the metal.

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