Which metal in the list reacts explosively with water?

• A. Potassium
• B. Lithium
• C. Sodium
• D. Rubidium

Answer: (D)

Reactivity with water increases as you move down the group of alkali metals. The atoms get larger and lose their outer electron more easily, so the reaction with water releases more energy. That extra heat can ignite the hydrogen gas produced, leading to an explosion for the heavier members. Among the options, rubidium is the heaviest alkali metal listed, so it reacts with water most vigorously and can explode. The reaction can be shown as 2Rb + 2H2O → 2RbOH + H2, with the hydrogen gas potentially igniting due to the exothermic process. Lithium and sodium react more slowly, and potassium also reacts vigorously but typically less violently than rubidium. Hence, rubidium is the best answer.