Magnesium chloride (\(\text{MgCl}_2\)), an inorganic salt, is highly soluble in water. This compound naturally occurs in seawater and mineral deposits like bischofite. Upon contact with water, it separates almost immediately into its constituent ions. The high solubility is a defining characteristic that enables its wide range of applications across various industries and health fields.
Why Magnesium Chloride Dissolves So Easily
Magnesium chloride dissolves readily because it is an ionic compound composed of one positively charged magnesium ion (\(\text{Mg}^{2+}\)) and two negatively charged chloride ions (\(\text{Cl}^{-}\)). The forces holding these ions together in a solid crystal lattice, known as lattice energy, must be overcome for dissolution to occur. Water molecules effectively dismantle this strong ionic structure.
Water is a highly polar solvent, meaning each \(\text{H}_2\text{O}\) molecule has a partial negative charge near the oxygen atom and partial positive charges near the hydrogen atoms. When magnesium chloride is introduced, the negative oxygen ends of the water molecules are attracted to the positive magnesium ions. Simultaneously, the positive hydrogen ends surround the negative chloride ions.
This process is called hydration, where water molecules surround and stabilize the separated ions, effectively insulating them from one another. The energy released during this hydration process is greater than the lattice energy required to break the ionic bonds, making the dissolution energetically favorable.
How Solubility is Quantified
Solubility is precisely measured to determine “how much” can dissolve in a specific amount of solvent. The standard measurement is expressed in grams of solute that can dissolve in 100 milliliters (\(\text{mL}\)) of water at a given temperature. At \(20\text{ }^\circ\text{C}\), the solubility of anhydrous magnesium chloride (\(\text{MgCl}_2\)) is approximately \(54.3\) grams per \(100\text{ } \text{mL}\) of water, demonstrating its high capacity for forming concentrated solutions.
Solubility is strongly influenced by temperature. As the water temperature increases, the kinetic energy of both the water molecules and the ions increases, allowing more of the salt to dissolve. At \(100\text{ }^\circ\text{C}\), the solubility significantly increases to about \(72.6\) grams per \(100\text{ } \text{mL}\) of water.
The form of the salt also impacts solubility, as magnesium chloride exists in both an anhydrous (water-free) form and various hydrated forms. The most common hydrated version is magnesium chloride hexahydrate (\(\text{MgCl}_2\cdot 6\text{H}_2\text{O}\)). While the anhydrous form is more concentrated by weight, the hexahydrate is highly soluble, reaching solubility levels of \(468.7\text{ } \text{g}\) per liter at \(20\text{ }^\circ\text{C}\).
Real-World Applications of High Solubility
The high solubility of magnesium chloride drives its utility across a diverse range of practical applications. Its tendency to remain dissolved explains why it is easily sourced from concentrated brines and seawater, where it is one of the most abundant dissolved salts. Extraction techniques rely on this property to separate the compound from other salts through controlled evaporation.
This characteristic is also exploited in health and wellness products, particularly in dietary supplements. Magnesium chloride is used in oral supplements and topical products because its high solubility ensures excellent bioavailability. When consumed, the salt readily dissociates in the digestive tract, allowing the free magnesium ions to be easily absorbed by the body.
In industrial settings, concentrated magnesium chloride solutions (brines) are leveraged for de-icing and dust control. When dissolved, the ions interfere with the formation of ice crystals, lowering the freezing point of the water below \(0\text{ }^\circ\text{C}\). This makes it an effective de-icing agent on roads and highways.
For dust control, the dissolved salt is hygroscopic, meaning the solution actively absorbs and retains moisture from the air. When sprayed onto unpaved roads, this moist brine binds the fine dust particles together, preventing them from becoming airborne.