Sunita drops a piece of potassium metal in a beaker of cold water and notes…

Alkali metals (Group 1: lithium, sodium, potassium, rubidium, caesium) react with cold water in a strongly exothermic reaction — heat is released, not absorbed — forming the metal's hydroxide and releasing hydrogen gas. Reactivity, and with it the vigour and heat given out, increases down the group, so potassium reacts far more energetically than sodium or lithium.

For potassium specifically: 2K + 2H2O → 2KOH + H2. The heat given out is large enough to melt the potassium and ignite the hydrogen gas as it escapes the water's surface.

• A violent reaction with cold water matches potassium's very high reactivity among the alkali metals.

• Hydrogen gas being evolved matches the products of the reaction 2K + 2H2O → 2KOH + H2, which also releases heat.

• A pop sound being heard matches the evolved hydrogen catching fire from the reaction's own heat.

• Calling the reaction endothermic contradicts the heat that this reaction actually releases — the metal absorbs no heat from its surroundings; it gives heat out.

Cross-check: every alkali-metal-with-water reaction on record is exothermic, enough to ignite hydrogen for potassium, rubidium, and caesium; none is endothermic. So describing the reaction as endothermic is the observation that does not hold, which is why it is the one being asked for here.