Water is not soluble in hexane, a fact easily observed when the two liquids are mixed. Water is known as a universal solvent because of its ability to dissolve many different compounds. Hexane is an organic hydrocarbon, a common component in gasoline and various industrial solvents. The key to understanding why these two liquids do not mix lies in their fundamental molecular structures and chemical characteristics.
Defining Molecular Polarity
Solubility is governed by a foundational concept known as molecular polarity, which describes how electrical charge is distributed across a molecule. This distribution originates from the unequal sharing of electrons between atoms within a chemical bond. Atoms possess different abilities to attract electrons, a property called electronegativity. When two atoms have a significant difference in electronegativity, the electrons spend more time near the more attractive atom, creating a polar bond.
This uneven electron distribution results in one end of the bond acquiring a slight negative charge and the other end acquiring a slight positive charge, forming an electric dipole. A molecule’s overall polarity depends not only on the presence of these polar bonds but also on the molecule’s three-dimensional shape. If the individual bond dipoles are positioned symmetrically, they cancel each other out, resulting in a nonpolar molecule. Conversely, an asymmetrical arrangement of polar bonds leads to a net electrical charge separation, making the entire molecule polar.
The Chemical Nature of Water and Hexane
Water (H₂O) is a highly polar molecule. Its structure features one oxygen atom bonded to two hydrogen atoms, possessing a bent geometry. Oxygen has a much higher electronegativity than hydrogen, meaning it strongly pulls the shared electrons toward itself. This gives the oxygen side a partial negative charge and the hydrogen side a partial positive charge, resulting in a net dipole moment of approximately 1.85 Debye.
Hexane (C₆H₁₄) is a straight-chain alkane composed solely of carbon and hydrogen atoms. The electronegativity values of carbon and hydrogen are very similar, making the carbon-hydrogen bonds nonpolar. The overall symmetrical arrangement of the six-carbon chain causes any minor dipoles to cancel each other out. This molecular symmetry classifies hexane as a nonpolar compound, with a negligible dipole moment often cited as 0.00 Debye.
The Principle of Immiscibility
The outcome of mixing water and hexane is explained by the rule of solubility: “like dissolves like.” This principle means that polar substances are soluble in other polar solvents, and nonpolar substances dissolve in nonpolar solvents. For a substance to dissolve, the attractive forces between the solute and solvent molecules must be strong enough to overcome the forces holding the solvent molecules together and the forces holding the solute molecules together.
Water molecules are held together by strong attractive forces called hydrogen bonds, a direct result of their high polarity. For hexane to dissolve, these strong hydrogen bonds must be broken to make room for the nonpolar hexane molecules. The weak attractive forces (London dispersion forces) that would form between the polar water and nonpolar hexane are not sufficient to provide the energy needed to break the water’s hydrogen-bonding network.
When water and hexane are combined, they are described as immiscible, meaning they will not blend to form a single, uniform solution. The two liquids separate into distinct layers. Hexane is less dense (0.655 grams per milliliter) than water (1.000 grams per milliliter), so the nonpolar hexane layer floats on top of the polar water layer.