Which element will react most vigorously with water, and what is the reaction that occurs?
(a) Lithium; 2 Li(s) + 2 H₂O(l) → 2 Li⁺(aq) + 2 H^–(aq) + H₂O₂(aq)
(b) Potassium; 2 K(s) + 2 H₂O(l) → 2 K⁺(aq) + 2 OH^–(aq) + H₂(g)
(c) Magnesium; Mg(s) + 2 H₂O(l) → Mg²⁺(aq) + 2 OH^–(aq) + H₂(g)
(d) Barium; Ba(s) + 2 H₂O(l) → Ba²⁺(aq) + 2 H^–(aq) + H₂O₂(aq)

Alkali metals, such as lithium, potassium, and barium, are known for their high reactivity, especially with water. This reactivity increases down the group in the periodic table, meaning potassium will react more vigorously than lithium. The reaction typically produces hydrogen gas and a corresponding hydroxide, which is a strong base.

When metals react with water, they can either displace hydrogen or form hydroxides. The general reaction for alkali metals involves the formation of metal hydroxides and hydrogen gas. For example, potassium reacts with water to produce potassium hydroxide and hydrogen, which is a characteristic reaction of alkali metals.

Balancing chemical equations is essential for accurately representing the conservation of mass in a reaction. Each side of the equation must have the same number of atoms for each element. Understanding how to balance equations helps in predicting the products and understanding the stoichiometry of the reaction, which is crucial for determining the most vigorous reaction with water.