Which metal reacts violently with cold water, producing a metal hydroxide, hydrogen gas, and enough heat for the hydrogen to catch fire?
Potassium
Reactivity with water follows the reactivity series: the most reactive metals (like potassium and sodium) react even with cold water, while less reactive ones react only with steam or not at all.
Potassium reacts violently with cold water: \(2K + 2H_2O \rightarrow 2KOH + H_2\), giving potassium hydroxide and hydrogen. The reaction is so exothermic that the heat ignites the evolved hydrogen, producing a lilac flame.
Calcium reacts far more gently with cold water and does not catch fire; magnesium reacts only slowly with hot water/steam; aluminium is protected by an oxide layer and does not react with cold water.
Hence, the metal is potassium.
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