Lithium is the lightest solid element on the periodic table and a member of the alkali metal family. When pure lithium metal meets water, it immediately triggers a rapid and highly exothermic chemical reaction. This inherent chemical volatility makes its interaction with water exceptionally dramatic.
The Visible Phenomenon
Due to its low density, lithium floats on water. The reaction begins immediately, marked by brisk effervescence that causes the metal to move quickly, often described as skittering across the surface. This rapid movement results from hydrogen gas being forcefully ejected from the underside of the floating metal.
The chemical process is exothermic, releasing significant thermal energy. While the heat is not typically intense enough to melt the lithium, the water will warm significantly. If a sufficiently large piece of lithium is used, the heat generated can sometimes ignite the released hydrogen gas. This combustion produces a small, localized flame, often appearing orange or crimson due to excited lithium atoms.
Why Lithium Reacts So Vigorously
Lithium’s intense reactivity is rooted in its atomic structure, specifically its electron configuration. As an element in Group 1, lithium has a single electron in its outermost shell, known as the valence shell. Atoms seek to achieve a stable configuration, which for lithium means losing that lone valence electron.
The metal’s strong tendency to shed this electron makes it a powerful reducing agent, readily giving up its electron. When lithium encounters water (\(\text{H}_2\text{O}\)), the lithium atom rapidly loses its electron to the water molecule. This process, known as oxidation, is the driving force behind the entire reaction.
The water molecules are split by the energy transfer, which reduces a hydrogen atom in the water to form highly reactive hydrogen gas. Because the resulting \(\text{Li}^+\) ion is so small, it attracts water molecules very strongly, releasing a large amount of energy during hydration. This release of energy contributes significantly to the overall heat produced, making the reaction vigorous. The reaction is a displacement reaction that is thermodynamically favored, meaning the products are much more stable than the initial reactants.
The Compounds Created
The chemical transformation between lithium and water produces two distinct compounds: lithium hydroxide and hydrogen gas. The general chemical equation for this process is represented as \(2\text{Li} + 2\text{H}_2\text{O} \rightarrow 2\text{LiOH} + \text{H}_2\).
The lithium hydroxide (\(\text{LiOH}\)) is a strong base that dissolves readily in the remaining water, creating an alkaline solution. This formation of a base is the reason Group 1 elements are collectively known as the alkali metals. The resulting solution can achieve a high \(\text{pH}\) level, often reaching around 13, which is highly corrosive.
The other product, hydrogen gas (\(\text{H}_2\)), is the source of the rapid bubbling and the potential for combustion. This gas is colorless, odorless, and highly flammable. When the heat from the exothermic reaction is sufficient, the hydrogen gas released into the air can ignite, causing the characteristic burst of flame.
Safety Considerations
Due to its extreme reactivity with moisture, pure lithium metal must be handled with great caution and under strictly controlled conditions. It is never stored exposed to the air, which contains water vapor; instead, it is typically submerged in an inert, non-reactive liquid such as mineral oil or kerosene. This liquid barrier prevents any contact with atmospheric moisture.
The reaction with water is not only highly exothermic but also presents a significant fire and explosion hazard. In the event of a lithium fire, common fire extinguishing agents like water, carbon dioxide, or foam must be avoided, as they will only fuel the reaction. Specialized dry powder extinguishers, such as those containing copper powder or an agent called Lith-X, are required to safely smother the blaze.