Water is the most abundant substance on Earth’s surface and is often called the “universal solvent.” This description is based on its ability to dissolve a greater variety of substances than any other liquid. This power lies in a foundational physical property: the unique molecular structure of water, which enables the dissolution of many compounds while also defining the limitations of this process.
Understanding Water’s Molecular Structure
Water’s solvent power originates from its molecular structure, defined by polarity. A single water molecule is composed of one oxygen atom covalently bonded to two hydrogen atoms. The oxygen atom has a significantly higher electronegativity than the hydrogen atoms, causing an unequal sharing of bonding electrons. This unequal sharing means electrons spend more time orbiting the oxygen atom, resulting in the oxygen atom developing a partial negative charge (\(\delta^-\)), while each hydrogen atom acquires a partial positive charge (\(\delta^+\)).
The molecule’s geometry is bent, not linear, with the two hydrogen atoms positioned on one side of the oxygen atom. This bent shape ensures the opposing charges do not cancel each other out, creating a net dipole moment. This permanent separation of charge makes water a polar molecule, allowing it to interact strongly with other charged particles through electrostatic attraction.
How Water Breaks Down Ionic Compounds
The polarity of water allows it to effectively dissolve ionic compounds, such as table salt (sodium chloride, NaCl), by overcoming the strong attractive forces holding the crystal lattice together. When an ionic solid is placed in water, the highly charged ions at the surface are immediately exposed to the water molecules.
The oxygen end of the water molecule, which carries the partial negative charge, is strongly attracted to the positively charged ions (cations). Simultaneously, the partial positive hydrogen ends are drawn toward the negatively charged ions (anions). This specific attraction between a charged ion and a polar molecule is known as an ion-dipole interaction.
These ion-dipole forces are often powerful enough to pull the individual ions away from the solid crystal lattice structure. As an ion separates from the solid, water molecules surround it in an organized fashion, forming what is known as a hydration shell or sphere of hydration. Positive ions are encased by water molecules oriented with their oxygen atoms facing inward, while negative ions are surrounded by water molecules with their hydrogen atoms pointing inward.
The formation of these stable hydration shells releases energy, known as the enthalpy of hydration, which helps compensate for the energy required to break the ionic bonds in the solid. Once surrounded by water molecules, the formerly bonded ions are shielded from one another, preventing them from rejoining and reforming the crystal structure. The ions are then free to disperse and move independently throughout the solution.
Why Some Substances Resist Dissolution
Water cannot dissolve every substance, particularly those that lack significant electrical charges. The principle known as “like dissolves like” dictates that polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes. Therefore, nonpolar compounds, such as oils, waxes, and hydrocarbons, do not dissolve in water because they lack the necessary partial or full charges.
Water molecules cannot form the strong attractions required to pull nonpolar molecules apart and surround them. Instead, introducing a nonpolar substance forces the water molecules to reorganize their hydrogen-bonding network around the solute, forming a rigid, cage-like structure called a clathrate cage. The formation of this organized structure significantly decreases the disorder (entropy) of the water molecules, which is energetically unfavorable.
To minimize this unfavorable structuring, the nonpolar molecules aggregate together, reducing the total surface area exposed to the water. This phenomenon is known as the hydrophobic effect, which causes oil to separate from water.