A “solvent” is a substance that dissolves another, called the “solute.” The resulting homogeneous mixture is a “solution.” Water plays an indispensable role in nature and daily life. It is often called the “universal solvent” due to its remarkable ability to dissolve a vast array of substances. This capability stems from its unique molecular properties.
Water’s Polarity and Hydrogen Bonds
Water’s solvent properties originate from its molecular structure. A water molecule (H2O) consists of one oxygen atom bonded to two hydrogen atoms, adopting a bent shape. Its bent geometry is important because the oxygen atom is more electronegative than the hydrogen atoms. Electronegativity describes an atom’s ability to attract shared electrons in a chemical bond. Because oxygen has a stronger pull on the shared electrons, they spend more time closer to the oxygen atom. This unequal sharing creates a slight negative charge (δ-) on the oxygen and slight positive charges (δ+) on each hydrogen. A molecule with this uneven distribution of charge is called polar. This inherent polarity allows water molecules to form electrostatic attractions with other charged or polar substances.
The partial charges on water molecules enable them to attract each other through “hydrogen bonds.” A hydrogen bond forms when the partially positive hydrogen atom of one water molecule is attracted to the partially negative oxygen atom of an adjacent water molecule. These hydrogen bonds are weaker than the covalent bonds within a water molecule, but their collective strength provides water with many of its unique characteristics. The continuous formation and breaking of these bonds contribute to water’s fluid nature and its capacity to interact with diverse molecules.
Dissolving Power in Action
Water’s polarity and its ability to form hydrogen bonds allow it to effectively dissolve various types of compounds. When ionic compounds, such as table salt (sodium chloride, NaCl), are introduced to water, the polar water molecules interact with the charged ions. The negatively charged oxygen end of water molecules is attracted to the positively charged sodium ions (Na+), while the positively charged hydrogen ends are drawn to the negatively charged chloride ions (Cl-). These attractions are strong enough to overcome the electrostatic forces holding the ions together in the solid crystal lattice.
Once separated, each ion becomes surrounded by a sphere of water molecules, forming what is known as a “hydration shell.” This shell effectively isolates the ions from each other, preventing them from rejoining and keeping them dispersed in the solution. This process, called hydration or solvation, is why many ionic compounds readily dissolve in water.
Water also effectively dissolves many polar covalent compounds, such as sugar (sucrose). Sugar molecules contain numerous hydroxyl (-OH) groups, which are polar, similar to water’s own structure. Water molecules can form hydrogen bonds with these hydroxyl groups on the sugar molecules. The partial positive hydrogen atoms of water are attracted to the partial negative oxygen atoms in the sugar’s hydroxyl groups, and vice-versa. These extensive hydrogen bonding interactions pull the sugar molecules away from each other and allow them to mix uniformly with the water, leading to dissolution. This is summarized by the principle “like dissolves like,” meaning polar solvents tend to dissolve polar solutes.
Why This Matters
The solvent properties of water are fundamental to biological systems, enabling life to function. In living organisms, water acts as the primary medium for transporting substances. For instance, blood, which is largely water, carries essential nutrients like glucose and oxygen to cells throughout the body. It also transports waste products, such as urea and carbon dioxide, away from cells for elimination.
Many biochemical reactions, which are the basis of metabolism, occur within the aqueous environment inside cells. Water’s solvent capability facilitates the interaction of reactants and molecule movement within and across cellular membranes. Digestion relies on water to break down complex food molecules into simpler, soluble forms that can be absorbed by the body.
Beyond biology, water’s solvent capabilities are widely utilized in everyday life and various industries. In the kitchen, water dissolves sugars, salts, and flavor compounds, distributing them evenly in food and beverages. It is a main component in cleaning products, helping to dissolve dirt, grease, and stains from surfaces and fabrics. Industrial processes, including the manufacturing of pharmaceuticals, foods, and paper, depend on water as a solvent for chemical reactions and purification. Water also plays a significant role in environmental cycles, dissolving minerals from rocks and transporting them, influencing geological formations and supporting diverse ecosystems.
When Water Can’t Dissolve
Despite its reputation as the “universal solvent,” water has limitations. It struggles to dissolve nonpolar substances, such as oils, fats, and waxes. These substances are primarily composed of carbon and hydrogen atoms that share electrons relatively evenly, lacking significant partial positive or negative charges.
Water molecules are more strongly attracted to each other through their extensive hydrogen bond network than they are to nonpolar molecules. Since nonpolar molecules cannot form hydrogen bonds or strong electrostatic interactions with water, water molecules tend to exclude them. Instead of dissolving, nonpolar substances often aggregate together, minimizing their contact with the water. This explains why oil and water separate into distinct layers. The “like dissolves like” principle indicates that nonpolar substances typically dissolve best in other nonpolar solvents, not in polar water.