A solution is a homogeneous mixture where one substance, the solute, is dispersed evenly throughout another substance, the solvent. Water is often referred to as the “universal solvent” because of its remarkable ability to dissolve a greater number of substances than any other liquid. Understanding which substances dissolve in water and why requires examining the fundamental science of water’s molecular structure and the forces that govern the interaction between the solvent and the solute.
Water’s Unique Molecular Structure
The chemical formula for water, H₂O, represents a molecule with a specific bent shape that is responsible for its powerful solvent properties. An oxygen atom is bonded to two hydrogen atoms, but the sharing of electrons in these bonds is not equal. Oxygen is a highly electronegative atom, meaning it has a stronger pull on the shared electrons than hydrogen does.
This uneven electron sharing creates a polar molecule, where the oxygen end develops a slight negative charge and the hydrogen ends develop slight positive charges. These slight charges are referred to as partial charges. Because of the molecule’s bent geometry, the water molecule has a distinct positive pole and a negative pole.
The polarity allows water molecules to form attractive forces with neighboring molecules, known as 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 molecule. These intermolecular forces make liquid water highly cohesive and are the mechanism through which water interacts with and pulls apart other molecules.
The “Like Dissolves Like” Rule
The primary factor determining if a substance will dissolve in water is its chemical compatibility, following the principle known as “like dissolves like.” Since water is a polar solvent, it effectively dissolves substances that are also polar or that possess an electrical charge. Conversely, water will not dissolve non-polar substances, which lack these separated charges.
Solutes that dissolve readily in water are termed hydrophilic, or “water-loving,” and fall into two main categories: polar covalent compounds and ionic compounds. Polar covalent compounds, such as table sugar (sucrose), dissolve because they contain polar bonds that allow them to form new hydrogen bonds with water molecules. These strong, favorable interactions overcome the cohesive forces holding the sugar molecules together.
Ionic compounds, such as table salt (sodium chloride), are composed of positive and negative ions held together by strong electrostatic attraction. Water’s powerful polarity allows its molecules to surround these ions, effectively weakening the ionic bond and pulling the crystal structure apart. In contrast, non-polar substances, like oils or grease, are hydrophobic (“water-fearing”) because their molecules have an even distribution of charge. Water molecules are much more attracted to each other through hydrogen bonding than they are to the non-polar solute, causing the non-polar substance to be excluded and remain separate.
How Solutes Dissolve
The physical process of dissolving differs slightly depending on whether the solute is ionic or polar covalent, but both rely on the water molecule’s polarity.
Dissociation (Ionic Compounds)
When an ionic compound is placed in water, the process is called dissociation. Water molecules collide with the surface of the ionic crystal, and the partially negative oxygen ends orient themselves toward the positive ions, while the partially positive hydrogen ends orient toward the negative ions. These collective attractions pull the ions away from the crystal lattice and into the solution as separate, charged particles. Once in the solution, each ion is completely surrounded by a layer of water molecules, which is known as a hydration shell. This shell shields the ions from one another, preventing them from re-forming the solid compound.
Solvation (Polar Covalent Compounds)
For polar covalent compounds like sugar, the process is known as solvation. The water molecules use their partial charges to form hydrogen bonds with the polar regions of the solute molecule. Unlike ionic compounds, the sugar molecule itself remains intact, separating from the bulk solid as a whole molecule. The water molecules surround the intact sugar molecule, forming a hydration shell that keeps it dispersed within the solution.
Factors That Change Solubility
While chemical compatibility determines whether a substance can dissolve, external factors influence the maximum amount that can be dissolved, which is defined as solubility. Temperature is a significant factor, although its effect varies between different types of solutes. For most solid solutes, an increase in the temperature of the solvent increases solubility because the added thermal energy helps to break the bonds holding the solid together.
Gases, however, exhibit the opposite behavior; their solubility in water decreases as temperature rises. The increased kinetic energy at higher temperatures allows the dissolved gas molecules to escape the liquid phase more easily. Pressure primarily influences the solubility of gases.
Gas solubility is directly proportional to the pressure of the gas above the liquid, which is evident in carbonated beverages where high pressure forces carbon dioxide gas to dissolve. Physical agitation, such as stirring, and increasing the surface area of the solute do not change the maximum solubility, but they do increase the rate at which the substance dissolves by bringing fresh solvent into contact with the solute more quickly.