During a reaction with water, which alkali metal exhibits a small explosion at the end?

The alkali metal that exhibits a small explosion at the end of its reaction with water is potassium. When potassium reacts with water, it generates hydrogen gas and heat, which causes the released hydrogen gas to ignite and create a small explosion. This is due to the highly exothermic nature of the reaction, leading to the rapid formation of hydrogen gas that ignites, especially as potassium burns with a lilac flame.

Lithium and sodium also react with water, but their reactions are less vigorous compared to potassium. Lithium typically reacts quietly, and while sodium does produce some noticeable bubbling and may create a small amount of fire, it does not culminate in an explosion. Rubidium is much more reactive than potassium, but it is less commonly discussed in typical educational settings due to its tendency to react violently with water.

In summary, potassium is characterized by a vigorous reaction that ends with a small explosion due to combustion of the hydrogen gas produced during the reaction.