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Francium is highly radio active. Also it is an alkali metal. As we go down the group, the reaction of alkali metal with water become more and more exothermic and violent. So if Francium reacts with water, francium hydroxide and hydrogen gas would be produced with large amount of heat.
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Francium is the most  reactive one among the alkali metals. 

Francium on treating with water forms  francium hydroxide (Fr Oh). AND this reaction is highly EXOTHERMIC ,so much that an explosion is likely to happen.
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