Water-reactive

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