Water is frequently called the “universal solvent” because of its remarkable ability to dissolve more substances than almost any other liquid. This dissolving power is not limitless, however, as only certain types of molecules can effectively interact with water to form a uniform solution. The fundamental rule governing this process is solubility, which is the maximum amount of a substance (the solute) that can be completely dispersed in a given amount of liquid (the solvent). Understanding what water can dissolve requires exploring the unique molecular structure that gives water its exceptional properties.
The Core Principle: Water’s Polar Nature
The tremendous solvent capability of water stems entirely from the specific arrangement of its atoms. A water molecule (H₂O) consists of one oxygen atom bonded to two hydrogen atoms. The molecule adopts a non-linear, or “bent,” V-shape geometry.
Oxygen is significantly more electronegative than hydrogen, meaning it has a much stronger pull on the shared electrons in the chemical bonds. This unequal sharing causes the electrons to spend more time near the oxygen atom, giving it a slight negative electrical charge. Conversely, the hydrogen atoms acquire a slight positive charge.
This separation of charge creates a permanent dipole moment, making water a polar molecule. The molecule acts like a tiny magnet with a distinct positive end and a negative end. This polarity is the reason for the common chemistry principle that “like dissolves like,” meaning a polar solvent like water primarily dissolves other polar or charged substances.
The bent shape is necessary for this property, as a linear molecule would have the charges cancel each other out. The presence of these permanent partial positive and negative regions allows water molecules to form strong attractive forces, specifically hydrogen bonds, with each other and with other molecules. This cohesive force must be overcome for any substance to successfully dissolve.
Dissolving Power: Ionic and Polar Molecules
Water is highly effective at dissolving two main categories of substances: ionic compounds and other polar molecules. These are collectively known as hydrophilic, or “water-loving,” substances. Ionic compounds, such as table salt (sodium chloride, NaCl), are held together by strong electrostatic forces between oppositely charged ions. When salt is introduced to water, the polar water molecules begin to interact with the charged ions on the crystal surface.
The negatively charged oxygen end of the water molecule is attracted to and surrounds the positive sodium ions (cations). At the same time, the positively charged hydrogen ends of the water molecules are attracted to and surround the negative chloride ions (anions). This attraction, known as an ion-dipole interaction, is strong enough to pull the ions away from the solid crystal structure.
Once separated, the water molecules completely encase the individual ions, forming a hydration shell. This shell stabilizes the ion’s charge and prevents the positive and negative ions from rejoining to form the solid crystal, keeping them suspended and dissolved. This process is how water transports essential electrolytes throughout biological systems.
Water dissolves other polar molecules, such as simple sugars like glucose or alcohols like ethanol, through the formation of hydrogen bonds. These molecules contain groups with partial charges, like hydroxyl (-OH) groups, that can form strong hydrogen bonds with water molecules. The positive poles of the water molecules are attracted to the negative poles on the solute, and vice versa.
These new attractions between the water and the polar solute molecules overcome the forces holding the solute together. The water molecules then pull the solute molecules apart and incorporate them into the vast network of water. This ability to form strong new bonds allows these substances to mix uniformly and dissolve easily.
The Contrast: Molecules That Do Not Dissolve
The principle of “like dissolves like” also explains why water cannot dissolve nonpolar substances, such as oils, fats, and hydrocarbons. These molecules are characterized by a balanced distribution of electrons, meaning they lack the distinct partial positive and negative charges that water possesses. Because they have no significant charge, nonpolar molecules are known as hydrophobic, or “water-fearing.”
Nonpolar substances cannot form the necessary strong attractive forces, like hydrogen bonds or ion-dipole interactions, with water. For a nonpolar substance to dissolve, the water molecules would have to break their existing, energetically favorable hydrogen bonds with one another. Since the nonpolar molecules cannot form new, equally strong bonds, this process is energetically unfavorable.
Instead of dissolving the nonpolar substance, water molecules preferentially interact with each other, maximizing their hydrogen bonding network. This preference for self-association effectively pushes the nonpolar substances away, forcing them to aggregate to minimize their contact surface area with the water. This phenomenon is known as the hydrophobic effect.
This lack of strong interaction is why oil and water separate into distinct layers; the water molecules stick together, and the oil molecules stick together. Ultimately, water’s inability to dissolve nonpolar molecules is not due to repulsion, but rather the strong cohesive forces of water itself that exclude any molecule unable to participate in its extensive polar network.