Elementary lithium is not very water soluble; instead, it vigorously reacts with water to form lithium hydroxide and hydrogen gas.
While the question of whether lithium can dissolve in water seems straightforward, it's important to differentiate between a substance dissolving and reacting. Elemental lithium, a soft, silvery-white alkali metal, does not simply dissolve in water in the way sugar dissolves. Instead, it undergoes a significant chemical transformation.
The Reactivity of Elemental Lithium with Water
When elemental lithium (Li) comes into contact with water (H₂O), it engages in a chemical reaction rather than a simple dissolution process. This reaction is energetic, releasing heat and producing two new substances:
- Lithium hydroxide (LiOH): This is a strong base that is highly soluble in water.
- Hydrogen gas (H₂): This gas is released during the reaction and can ignite due to the heat generated, especially when larger pieces of lithium are involved.
The chemical equation for this reaction is:
2Li(s) + 2H₂O(l) → 2LiOH(aq) + H₂(g)
This means that while the original lithium metal doesn't dissolve, its reaction product (lithium hydroxide) does.
Key Characteristics of Lithium's Reaction with Water:
- Not true dissolution: The lithium changes its chemical identity.
- Exothermic: The reaction releases heat.
- Gas production: Hydrogen gas is produced, which is flammable.
- Base formation: Lithium hydroxide, a strong base, is formed in the water.
Solubility of Lithium Compounds
While elemental lithium reacts with water, many of its compounds are indeed water-soluble. These compounds are formed when lithium has already reacted with other elements, creating stable ionic bonds. The solubility of these compounds varies significantly.
Here's a look at the solubility of some common lithium compounds:
| Lithium Compound | General Solubility in Water | Example Solubility |
|---|---|---|
| Lithium Hydroxide (LiOH) | Highly soluble | 129 g/L at 20°C (e.g., Wikipedia: Lithium Hydroxide) |
| Lithium Chloride (LiCl) | Very highly soluble | 63.7 g/100 mL at 20°C (e.g., PubChem: Lithium Chloride) |
| Lithium Carbonate (Li₂CO₃) | Sparingly soluble | 1.3 g/100 mL at 20°C (e.g., Wikipedia: Lithium Carbonate) |
| Lithium Phosphate (Li₃PO₄) | Low solubility | 0.039 g/100 mL at 20°C (e.g., PubChem: Lithium Phosphate) |
| Lithium Fluoride (LiF) | Low solubility | 0.13 g/100 mL at 25°C (e.g., PubChem: Lithium Fluoride) |
As seen in the table, compounds like lithium hydroxide and lithium chloride are readily soluble, while others such as lithium carbonate, lithium phosphate, and lithium fluoride are much less soluble. For instance, lithium hydroxide, which is a product of lithium reacting with water, boasts a considerable solubility of 129 grams per liter at room temperature.
Distinguishing Between Dissolution and Reaction
Understanding the difference between dissolution and a chemical reaction is key when discussing lithium and water:
- Dissolution occurs when a substance disperses uniformly into a solvent, maintaining its original chemical identity. Think of salt dissolving in water – it's still salt, just spread throughout the water.
- Reaction involves a chemical change where the original substance transforms into new substances with different chemical properties. This is what happens when elemental lithium meets water.
Therefore, while elemental lithium does not dissolve in water, it reacts to form soluble compounds.
