Lithium hydroxide (LiOH) is an inorganic compound that exists as a white, crystalline solid and is classified as a strong alkali base. Lithium hydroxide is highly soluble in water, readily dissolving to form a strong, corrosive, and transparent solution. This high solubility makes it a highly reactive compound in aqueous environments, which is fundamental to its many specialized applications in technology and industry.
Why Lithium Hydroxide Dissolves
The dissolution of lithium hydroxide is driven by the powerful interaction between its ions and the polar nature of water molecules. Lithium hydroxide is an ionic compound, consisting of positively charged lithium ions (Li+) and negatively charged hydroxide ions (OH-) held together in a rigid crystal lattice structure. For the solid to dissolve, water must supply enough energy to pull these ions apart from their neighbors in the crystal.
This separation relies on a careful energetic balance between two opposing forces: lattice energy and hydration energy. Lattice energy is the energy required to break the strong electrostatic forces holding the solid crystal together. Hydration energy is the energy released when water molecules surround the freed ions, forming protective shells.
Water molecules are highly polar, having partial negative charges near the oxygen atom and partial positive charges near the hydrogen atoms. When the solid is placed in water, the negative oxygen ends are attracted to the positive lithium ions, while the positive hydrogen ends surround the negative hydroxide ions. This process of surrounding the ions is called hydration, and the energy released must be greater than the lattice energy.
Due to the very small size of the lithium ion, its positive charge is highly concentrated, allowing it to attract water molecules very strongly and release a significant amount of hydration energy. This successfully overcomes the lattice energy. Complete dissociation of a single LiOH unit has even been modeled to require at least seven surrounding water molecules to fully separate the ions into a stable, dissolved state.
Measuring Solubility and Temperature Dependence
Chemists quantify solubility by measuring the maximum amount that can dissolve in a specific volume of water at a given temperature. At a standard temperature of 20°C (68°F), the solubility of anhydrous lithium hydroxide is approximately 12.8 grams dissolved per 100 milliliters of water. This figure demonstrates its high solubility compared to many other compounds.
The relationship between temperature and the solubility of lithium hydroxide presents an unusual trend compared to most ionic solids. While most solids increase solubility steadily as the water temperature rises, LiOH exhibits a unique behavior. The solubility initially increases as the temperature rises up to the boiling point, reaching about 17.5 grams per 100 milliliters at 100°C (212°F).
However, when the water temperature is increased further, particularly in high-pressure systems, the solubility begins to decrease after reaching a maximum near 115°C (240°F). This counter-intuitive effect is attributed to the strong hydration shell formed around the tiny lithium ion. At high temperatures, water molecules lose some of their structure and cannot hold the lithium ion as effectively, leading to less solubility at extreme heat.
Practical Uses for Dissolved Lithium Hydroxide
The high solubility and strong basicity of lithium hydroxide make its aqueous solutions indispensable for several high-stakes and industrial applications.
One of its most recognized uses is in carbon dioxide (CO2) scrubbing systems for enclosed environments, such as spacecraft, submarines, and rebreathers. The dissolved LiOH is used to efficiently remove exhaled carbon dioxide, which is a toxic byproduct of respiration in confined spaces.
In this application, the dissolved lithium hydroxide reacts with the gaseous CO2 to produce lithium carbonate (Li2CO3) and water, effectively trapping the harmful gas. This reaction is highly efficient, allowing for the regeneration of breathable air in places where ventilation is impossible.
The compound is also widely used in the manufacturing of materials for the energy sector, particularly in the preparation of electrolytes for specific types of lithium-ion batteries. In this process, the dissolved LiOH serves as a precursor material, providing the lithium source necessary for creating the cathode materials that facilitate the flow of ions during charging and discharging.
The ability to create highly concentrated and pure solutions ensures the quality and performance of the final battery components. Beyond high-tech applications, dissolved lithium hydroxide is used in industrial processes such as saponification, where it acts as a strong base to react with fatty acids, primarily to produce lithium stearate grease.