Water is often called the “universal solvent” because it dissolves more substances than any other liquid on Earth. This capability stems from its molecular structure, which allows it to chemically interact with and break apart a vast array of compounds. While not truly universal—as some substances resist it—water’s solvent power is unparalleled in chemistry and biology. Understanding why water can dissolve such a wide range of materials requires looking closely at the fundamental properties of the water molecule. This ability to form aqueous solutions is central to life on our planet.
The Polar Structure of the Water Molecule
The solvent power of water begins with its chemical composition: one oxygen atom bonded to two hydrogen atoms (\(\text{H}_2\text{O}\)). This molecule has a distinct bent or V-shape, as the oxygen atom is significantly more electronegative than the hydrogen atoms. This means oxygen exerts a much stronger pull on the shared electrons in the covalent bonds.
This unequal sharing causes electrons to spend more time near the oxygen atom, giving the oxygen end a partial negative charge (\(\delta^-\)). Conversely, the hydrogen ends acquire partial positive charges (\(\delta^+\)). Because the molecule is bent, these charges do not cancel out, creating a net dipole moment. This makes water a highly polar molecule, acting like a tiny magnet with distinct positive and negative poles.
The polarity allows individual \(\text{H}_2\text{O}\) molecules to form weak attractions called hydrogen bonds. A partially positive hydrogen from one molecule is attracted to the partially negative oxygen of a neighboring molecule. This network of constantly forming and breaking hydrogen bonds provides the mechanism by which water interacts with other substances.
How Water Breaks Down Solutes
Water’s polarity dictates the two primary ways it dissolves other substances: electrostatic attraction for ionic compounds and hydrogen bonding for other polar compounds. When an ionic compound, such as table salt, is introduced, its crystal lattice is held together by strong electrostatic forces. Water molecules swarm the crystal, using their negative oxygen ends to attract positive ions (\(\text{Na}^+\)) and their positive hydrogen ends to attract negative ions (\(\text{Cl}^-\)).
These collective ion-dipole attractions overcome the forces holding the crystal together, pulling the individual ions out of the lattice. Once separated, the ions are surrounded by a shell of water molecules, known as a hydration shell. This shell stabilizes the charged ions, preventing them from re-associating and keeping them dispersed evenly throughout the solution.
For polar covalent compounds, such as sugar or alcohol, dissolution occurs differently because these molecules do not break into ions. Instead, they contain their own partially charged regions, often due to hydroxyl (\(\text{-OH}\)) groups. Water molecules form new hydrogen bonds with these polar regions on the solute molecule. This process allows the water network to incorporate the solute molecules, mixing them evenly into the solution.
Defining the Limits: Non-Polar Substances
Water cannot easily dissolve non-polar substances, such as oils, fats, and waxes. This inability is summarized by the chemical principle “like dissolves like,” meaning polar solvents dissolve polar solutes, and non-polar solvents dissolve non-polar solutes. Non-polar molecules lack the partial charges necessary to form strong hydrogen bonds or electrostatic attractions with water.
When a non-polar molecule is placed in water, the water molecules cannot effectively interact with it. Instead, they are forced to reorient themselves to maximize their own hydrogen bonding with each other. This forced reorientation creates a more ordered, cage-like structure of water molecules around the non-polar substance. This localized increase in order is thermodynamically unfavorable for the entire system.
To minimize this disruption to the water’s hydrogen bond network, non-polar molecules tend to aggregate or clump together. By minimizing their exposed surface area to the water, they reduce the number of ordered water molecules required. This phenomenon is called the hydrophobic effect, and it explains why oil and water separate into distinct layers.
The Critical Role in Biological Systems
Water’s solvent capability is fundamental to life on Earth, as all biological processes occur in an aqueous environment. The ability of water to dissolve charged ions and polar molecules allows for the transport of essential substances throughout an organism. For example, blood plasma, which is largely water, dissolves nutrients like glucose, amino acids, and minerals, making them accessible to cells.
This solvent action also enables the body to process and eliminate waste products. Metabolic waste, such as urea, is dissolved in water and subsequently removed from the body via excretion. Water acts as a medium for biochemical reactions within cells, allowing reactants to dissolve and come into contact efficiently. This ability to dissolve and organize molecules permits the complex, life-sustaining chemistry of metabolism to occur.