A solvent is a substance capable of dissolving other molecules, known as solutes, to form a uniform mixture called a solution. Water is often called the “universal solvent” because it dissolves a greater variety of substances than any other liquid.
This property is fundamental to life, enabling biological processes like transporting nutrients and waste within organisms. Water’s dissolving power also shapes geology by eroding and carrying dissolved minerals across landscapes. This capacity is rooted entirely in the unique architecture of the water molecule itself.
The Foundation: Molecular Polarity
Water’s solvent power begins with its molecular structure, which consists of one oxygen atom bonded to two hydrogen atoms. While the atoms share electrons in a covalent bond, the oxygen atom is significantly more electronegative than the hydrogen atoms. Electronegativity describes an atom’s ability to attract a shared pair of electrons toward itself.
The oxygen atom pulls the shared electrons closer to its nucleus, creating an unequal distribution of charge across the molecule. This pull gives the oxygen end a partial negative charge (delta minus). Conversely, the hydrogen atoms develop a partial positive charge (delta plus).
The water molecule is not linear but has a bent shape due to the two unshared pairs of electrons on the oxygen atom. This geometry, combined with the unequal sharing of electrons, results in a permanent molecular dipole moment. The molecule acts like a tiny magnet with distinct positive and negative poles, making it a highly polar compound.
How Water Dissolves Ionic Substances
The polarity of the water molecule is what allows it to effectively dissolve ionic compounds, such as table salt (sodium chloride). Ionic compounds are held together by a strong electrostatic attraction between positive cations and negative anions, forming a crystal lattice.
When salt is introduced to water, the polar water molecules surround the ions in the crystal. The partially negative oxygen atoms are strongly attracted to the cations. Simultaneously, the partially positive hydrogen atoms are drawn toward the anions.
These powerful ion-dipole attractions overcome the forces holding the ions together in the crystal lattice. As the water molecules pull the ions away, dissociation occurs, separating the compound into individual ions. Each ion becomes encased by a shell of water molecules, which stabilizes the dissolved ions and prevents them from re-forming the crystal.
How Water Dissolves Polar Covalent Substances
Water also readily dissolves many substances that possess polarity but do not form ions, such as sugars and alcohols. These polar covalent molecules have regions of partial charge, often due to functional groups like hydroxyl (-OH) groups. The dissolution mechanism relies on the formation of new hydrogen bonds.
When a polar covalent solute is introduced, water molecules break some of their existing hydrogen bonds and form new ones with the polar regions of the solute molecule. This interaction is strong enough to disrupt the intermolecular forces holding the solute molecules together. Water integrates the solute molecules into its existing network, allowing the neutral but polar molecules to disperse evenly throughout the solvent.
The Limits of the Universal Solvent
Despite its name, water is not truly universal and cannot dissolve every substance, which is best summarized by the principle “like dissolves like.” This limitation arises when water encounters non-polar substances, such as oils, fats, and waxes. These molecules lack the partial or full charges necessary to form strong electrostatic or hydrogen bond interactions with the polar water molecules.
Water molecules surrounding a non-polar solute cannot form favorable attractions. Instead, the water molecules are forced to orient themselves in highly ordered, cage-like structures around the non-polar substance. This structural organization significantly reduces the entropy, or disorder, of the water, making the process thermodynamically unfavorable.
This phenomenon is known as the hydrophobic effect. To minimize this unfavorable entropy cost, the non-polar molecules aggregate together, reducing their total surface area exposed to the water. By clustering, they limit the amount of surrounding water held in an ordered state, allowing the bulk water to achieve a higher state of disorder.