When lithium metal encounters water, a rapid chemical reaction occurs. Lithium (Li) is the lightest of all metals and belongs to the alkali metal family, which makes it highly prone to seeking chemical stability. Because its density is only about half that of water, a piece of lithium will float on the surface.
The Observable Physical Reaction
When a piece of lithium is dropped into water, it floats and begins to move erratically across the surface. This skittering movement is caused by the hydrogen gas bubbles rapidly ejecting from the metal’s underside, propelling the small piece like a miniature jet ski. A vigorous fizzing accompanies this movement.
The reaction is notably exothermic, meaning it releases a considerable amount of heat into the surrounding water. This heat generation is typically not sufficient to melt the lithium metal, which has a relatively high melting point of about 180°C. However, the heat quickly raises the temperature of the immediate area, causing the hydrogen gas produced to potentially ignite, resulting in small flames or sparks above the water. The metal gradually reacts and disappears, leaving behind a colorless solution.
The Underlying Chemical Process
The reaction between lithium and water is fundamentally a reduction-oxidation (redox) process driven by the metal’s strong tendency to donate an electron. Lithium metal has a single valence electron that it readily loses to form a positively charged ion (\(\text{Li}^+\)), a process known as oxidation. This electron is transferred to the water molecule (\(\text{H}_2\text{O}\)), where it acts to reduce a hydrogen atom from water, forming hydrogen gas (\(\text{H}_2\)).
This electron transfer is represented by the general chemical equation: \(2\text{Li}(\text{s}) + 2\text{H}_2\text{O}(\text{l}) \rightarrow 2\text{LiOH}(\text{aq}) + \text{H}_2(\text{g}) + \text{Energy}\). The reaction is highly exothermic because the newly formed lithium ion is strongly attracted to the water molecules, releasing a significant amount of energy as it becomes surrounded by them in solution. The speed of the reaction is governed by the energy required to remove lithium’s valence electron, known as the ionization energy, which is higher for lithium than for the other alkali metals.
The Products of the Reaction
The chemical transformation results in the formation of two distinct products: lithium hydroxide and hydrogen gas. Lithium hydroxide (\(\text{LiOH}\)) is a strong base that dissolves in the water, creating an alkaline solution. This compound is caustic, meaning it is corrosive and can cause chemical burns upon contact.
The other product is hydrogen gas (\(\text{H}_2\)), a highly flammable, colorless, and odorless gas. The rapid evolution of this gas causes the visible fizzing and erratic movement.
Context: Lithium’s Place Among Alkali Metals
Lithium is the first element in the alkali metal group (Group 1) of the periodic table, which includes sodium, potassium, and others. While lithium is highly reactive, it is the least violent of the alkali metals when placed in water. This difference in reactivity is attributed to a combination of physical and chemical properties.
Lithium has the smallest atomic size in the group, which means its single valence electron is held more tightly by the nucleus, resulting in a higher ionization energy. This higher energy requirement makes the initial electron loss step slightly slower compared to its heavier counterparts. Furthermore, lithium’s higher melting point means the heat generated by the reaction is not enough to melt the metal, allowing it to maintain a smaller, solid surface area during the process. In contrast, sodium melts almost instantly, forming a sphere that increases the reactive surface area, leading to a much more vigorous reaction, while potassium reacts even more violently, often igniting immediately.