Water is often called the “universal solvent” because it can dissolve a greater variety of substances than any other liquid. Dissolution occurs when a substance (the solute) breaks down completely into individual molecules or ions and disperses evenly throughout the water, creating a homogeneous solution. For example, when salt dissolves, water molecules pull apart the sodium and chloride ions, surrounding them and preventing them from rejoining. Despite this dissolving power, many common materials remain separate when submerged, refusing to mix at the molecular level. Understanding what water cannot dissolve explains fundamental concepts in chemistry.
The Rule of Polarity
The factor determining solubility is polarity, which is the uneven distribution of electrical charge within a molecule. A water molecule, composed of two hydrogen atoms and one oxygen atom, has a bent shape. Oxygen is highly electronegative, pulling shared electrons closer to itself. This gives the oxygen side a slight negative charge and the hydrogen sides a slight positive charge.
This uneven charge distribution makes water a highly polar molecule, acting like a tiny magnet. This polarity establishes the principle of solubility: “like dissolves like.” Water interacts strongly with other polar molecules and ionic compounds that possess electrical charges. When a polar substance, like sugar, is added to water, the water molecules’ positive ends attract the sugar’s negative regions, and vice versa. This attraction breaks the bonds holding the sugar together, allowing the water to surround each molecule.
The Hydrophobic Rejection of Oils and Fats
The rejection of liquids and soft solids by water is a direct consequence of the polarity rule, involving nonpolar substances. Nonpolar molecules, such as oils, fats, and waxes, have an even distribution of electrical charge and lack the charged ends water requires for attraction. These substances are called “hydrophobic,” meaning “water-fearing.”
When a nonpolar substance is introduced, water molecules prefer to bond strongly to each other rather than interact with the uncharged molecules. The water molecules form an ordered, cage-like structure around the nonpolar substance to maximize their own attractions, forcing the nonpolar molecules to clump together. This clumping minimizes the surface area of contact. Materials like cooking oils, gasoline, and paraffin wax do not dissolve; instead, they separate, often forming a distinct layer or small beads on the surface. Water molecules are not repelling the nonpolar substance, but their strong self-interaction forces the non-interacting material out of the way.
Structural Barriers to Dissolving
Some substances resist dissolving because their internal structure is too robust or too large for water molecules to break apart. This insolubility occurs when the attractive forces between solute particles are stronger than the attraction water can exert.
High Lattice Energy
In some ionic compounds, such as metal oxides and minerals like silica (sand), ions are held together by exceptionally strong electrostatic forces. The energy required to break these internal bonds, known as the lattice energy, is too high for water molecules to overcome. Even though these compounds are ionic, the water’s attraction is insufficient to pull the ions away and overcome the compound’s structural integrity.
Immense Molecular Size
Large, complex molecules like cellulose (the primary component of wood and plant fibers) and most plastics resist dissolving due to their immense size. These polymers are long chains of repeating subunits, often containing thousands of atoms. The sheer volume and complexity of the structure prevent water molecules from completely surrounding the entire chain. The large size means there are too many internal bonds for the water to break all at once, leaving the material intact and insoluble.
Substances That React Instead of Dissolving
A distinct category of materials undergoes an irreversible chemical transformation when they contact water, rather than simply failing to dissolve. These substances react with the water molecule itself to form entirely new chemical compounds. This process differs fundamentally from dissolution, which is a physical change where the original substance retains its chemical identity.
A well-known example involves highly reactive alkali metals, such as sodium or potassium. They react violently with water to produce hydrogen gas and a metal hydroxide. This reaction releases heat, often igniting the hydrogen gas, and leaves behind a new, water-soluble compound. No dissolution occurs because the original metal is consumed and replaced by new substances.
Other materials, such as calcium oxide (quicklime), also undergo a chemical reaction. When quicklime is added to water, it forms calcium hydroxide, a process called slaking, which releases heat. The initial material is chemically transformed by the water, demonstrating that some materials are too chemically unstable to experience the physical process of dissolution.