When one substance dissolves into another, the substance doing the dissolving is called the solvent, and the substance being dissolved is the solute. Water is often called the “universal solvent” because it can dissolve more substances than any other liquid on Earth. While it cannot dissolve everything, this title highlights water’s unique and powerful solvent capabilities. These capabilities are fundamental to all biological processes, allowing water to carry nutrients and facilitate chemical reactions. Water possesses this extraordinary dissolving power due to the specific structure of its individual molecules.
The Foundation: Water’s Polar Structure
A single water molecule is constructed from one oxygen atom bonded to two hydrogen atoms, but the electrons in these bonds are not shared equally. Oxygen is a highly “electronegative” element, meaning it has a much stronger pull on the shared electrons compared to hydrogen. This unequal sharing of electrons creates what is known as a polar covalent bond, where the electrons spend more time orbiting the oxygen atom.
This uneven distribution of charge results in the oxygen side of the molecule acquiring a slight negative charge, represented as \(\delta-\). Conversely, the hydrogen atoms are left with a slight positive charge, denoted as \(\delta+\). The water molecule also has a distinctive bent, non-linear geometry. The two pairs of unshared electrons on the oxygen atom push the hydrogen atoms closer together, resulting in a bond angle of about \(104.5^\circ\).
The combination of the unequal electron sharing and the bent shape ensures that the positive and negative charge regions do not cancel each other out. This charge separation is referred to as polarity, and it is the most important property that enables water to be such an effective solvent. This powerful polarity allows water to form attractive forces with other charged or partially charged substances.
Dissolving Ionic Compounds
Water’s polarity is especially effective when interacting with ionic compounds, such as table salt, which is composed of sodium chloride (\(\text{Na}\text{Cl}\)). Ionic compounds are held together by strong electrostatic forces between positively charged cations (like \(\text{Na}^{+}\)) and negatively charged anions (like \(\text{Cl}^{-}\)). To dissolve, these strong ionic bonds must be overcome.
When salt is introduced to water, the charged ends of the water molecules are powerfully attracted to the ions in the salt crystal. The slightly negative oxygen ends of the water molecules swarm around the positive sodium ions (\(\text{Na}^{+}\)). At the same time, the slightly positive hydrogen ends of the water molecules cluster around the negative chloride ions (\(\text{Cl}^{-}\)).
These numerous individual attractions between the water molecules and the ions are collectively strong enough to pull the ions away from the crystal lattice structure. The process of the ionic compound breaking apart into its constituent ions is called dissociation. Once separated, each ion is completely surrounded by a protective cage of water molecules called a hydration shell. This shell insulates the ion, preventing the positive and negative ions from reattaching and keeping them dissolved.
Dissolving Polar Molecules
Water’s solvent ability extends beyond ionic salts to include many neutral molecules that also possess polarity. Substances like sugars and alcohols are not composed of ions, but they contain polar functional groups, most commonly hydroxyl (\(\text{OH}\)) groups.
These polar solutes dissolve not by dissociation, but through the formation of hydrogen bonds with the surrounding water. The partial charges on the solute’s polar groups attract the opposite partial charges on the water molecules. For example, the partial negative oxygen in a sugar molecule can form a hydrogen bond with the partial positive hydrogen of a water molecule.
This extensive network of hydrogen bonding allows the solute to be pulled into the solution and uniformly dispersed. Molecules that can form these hydrogen bonds with water are termed hydrophilic, meaning “water-loving.”
The Practical Importance of Water’s Solvency
The unique solvent property of water is the physical basis for life on Earth. Inside every living organism, water acts as the medium for nearly all biological activity. In the human body, blood plasma is over 90% water and serves as a circulating aqueous solution.
This plasma transports dissolved solutes, including essential nutrients like glucose and amino acids, as well as mineral ions, to every cell in the body. Within the cells themselves, water facilitates countless chemical reactions by allowing reactant molecules to move freely and collide with each other.
Water’s solvent power also plays a crucial role in waste management. The body converts metabolic waste products, such as urea, into water-soluble forms so they can be dissolved and safely flushed out of the body through the kidneys. In plants, the movement of mineral nutrients from the soil up into the leaves is also dependent on water dissolving these substances.