Why can hydrogen ignite during potassium’s reaction with water?

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A periodic table element box for potassium shows atomic number 19, symbol K, atomic weight 39.0983, electron configuration [Ar]4s1, and name potassium. Icons indicate it is an alkali metal, solid at room temperature, body-centered cubic crystal structure, and has strongly basic higher-valence oxides. A key explains the symbols for alkali metals, solid state, body-centered cubic structure, and strong basicity. — potassium Properties of potassium.

When potassium reacts with water, it does so very vigorously, liberating hydrogen gas and forming a solution of potassium hydroxide.1

This reaction is highly exothermic, generating approximately 47 kilocalories of heat per mole of potassium and water. This significant release of energy provides the heat necessary to ignite the hydrogen gas produced during the reaction. Because hydrogen can ignite when heated to temperatures between 550 °C and 600 °C, or when exposed to a spark or flame, the intense heat from the potassium-water reaction is sufficient to cause the hydrogen to ignite and burn.213