The specific question of whether sodium chloride (\(\text{NaCl}\)) can dissolve in hexane is determined by the fundamental chemical properties of these two distinct compounds. Solubility describes the capacity of a solute to uniformly disperse and form a homogeneous mixture when combined with a solvent. To understand this interaction, it is necessary to examine the intrinsic nature of both the salt and the hydrocarbon solvent.
Defining the Chemical Components
Sodium chloride (\(\text{NaCl}\)) is an ionic compound formed by the complete transfer of electrons between atoms, resulting in full charge separation. It consists of positively charged sodium ions (\(\text{Na}^+\)) and negatively charged chloride ions (\(\text{Cl}^-\)). These oppositely charged ions are held together by strong electrostatic forces, forming a crystal lattice. The presence of these full positive and negative charges makes sodium chloride a highly polar substance.
In contrast, hexane is a simple hydrocarbon with the chemical formula \(\text{C}_6\text{H}_{14}\). It is composed solely of carbon and hydrogen atoms connected by covalent bonds. The electrons within the carbon-carbon and carbon-hydrogen bonds are shared almost equally because the atoms have very similar electronegativity values. This uniform sharing means the hexane molecule lacks any permanent positive or negative poles, classifying hexane as a nonpolar compound.
The Governing Rule of Solubility
The ability of any solvent to dissolve a solute is governed by a chemical principle often summarized as “Like Dissolves Like.” This rule dictates that substances with similar polarity will readily dissolve one another. Polar and ionic substances, which possess significant charge separation, dissolve effectively in polar solvents. Conversely, nonpolar substances are only soluble in other nonpolar solvents.
Dissolution requires that the attractive forces exerted by the solvent molecules must be strong enough to overcome the internal forces holding the solute particles together. In polar or ionic compounds, these internal forces are strong electrostatic attractions. The type and strength of the intermolecular forces that a substance exhibits determine its solubility in a specific solvent.
Why Sodium Chloride Does Not Dissolve
Applying the “Like Dissolves Like” rule confirms that sodium chloride will not dissolve in hexane due to the fundamental mismatch in polarity. The strong electrostatic forces that bind the \(\text{Na}^+\) and \(\text{Cl}^-\) ions into the crystal lattice require a highly energetic environment to be broken apart. The nonpolar hexane solvent simply cannot provide the necessary energy or attractive forces.
The intermolecular forces present in hexane are limited to weak, transient interactions known as London dispersion forces. These forces are far too weak to overcome the considerable ionic attraction within the \(\text{NaCl}\) crystal structure. When sodium chloride crystals are mixed with hexane, the hexane molecules do not possess the partial charges needed to effectively pull the ions away from the lattice. Therefore, the ionic bonds remain intact, and the \(\text{NaCl}\) crystals remain solid, settling quickly to the bottom of the nonpolar liquid.
The Role of Polar Solvents
To appreciate the solubility mismatch, it is useful to examine the behavior of sodium chloride in a solvent where it is highly soluble, such as water (\(\text{H}_2\text{O}\)). Water is a highly polar solvent, characterized by a bent molecular geometry that results in strong, permanent partial charges, or dipoles. The oxygen atom carries a partial negative charge, while the two hydrogen atoms carry partial positive charges.
When \(\text{NaCl}\) is introduced into water, the polar water molecules are able to effectively interact with the charged ions. The partially negative oxygen end of the water molecule surrounds and attracts the positive sodium ions (\(\text{Na}^+\)), while the partially positive hydrogen ends surround the negative chloride ions (\(\text{Cl}^-\)). This process is known as hydration or solvation. The strong attractive forces between the polar water molecules and the fully charged ions are sufficient to overcome the strong electrostatic forces of the crystal lattice, allowing the salt to completely dissolve.