Dissolution is the process where a solute (a solid, liquid, or gas) mixes into a solvent (a liquid) to form a homogeneous solution. Solubility refers to the maximum concentration of a solute that can dissolve in a solvent under specific conditions, such as a given temperature. Water is frequently referred to as the “universal solvent” because it can dissolve more substances than any other common liquid. Understanding what dissolves in water requires examining the unique molecular structure of the water molecule and the principles that govern how substances interact with it.
Why Water is an Exceptional Solvent
The ability of water to dissolve a wide array of materials stems from its unique molecular structure, which consists of one oxygen atom bonded to two hydrogen atoms ($H_2O$). The molecule is bent, and the electrons within it are not shared equally between the atoms. Oxygen exerts a stronger pull on the shared electrons, causing them to spend more time near the oxygen nucleus.
This uneven distribution of charge creates a polar molecule. The oxygen end develops a slight negative charge, while the two hydrogen ends develop slight positive charges. These opposite partial charges allow water to act like a tiny magnet, attracting and surrounding other charged particles.
Water’s polarity allows it to effectively pull apart the atoms or molecules of other substances. The negative oxygen end is attracted to any positively charged part of a solute, and the positive hydrogen ends are drawn to any negative parts. This mechanism of attraction disrupts the forces holding the solute’s structure together, making water an effective solvent.
The Governing Principle of Solubility
The most reliable rule for predicting what will dissolve in water is the principle of “like dissolves like,” meaning that polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes. Because water is a polar molecule, it can only dissolve substances that share a similar electrical nature.
Substances that dissolve readily in water are either ionic or highly polar themselves. Ionic compounds, such as table salt (sodium chloride), are composed of positively and negatively charged ions held together by strong electrical attraction. When salt is added to water, the water molecules swarm the salt crystal. The negative oxygen ends pull on the positive sodium ions, and the positive hydrogen ends pull on the negative chloride ions. These coordinated attractions overcome the internal forces of the salt, separating the ions and distributing them evenly throughout the water.
Polar covalent compounds, like common table sugar (sucrose) or glucose, also dissolve well because they contain multiple oxygen-hydrogen ($O-H$) groups. These groups create partial charges within the molecule, allowing them to form hydrogen bonds directly with the surrounding water molecules. The formation of these new attractions is strong enough to break the attractions holding the sugar crystals together.
In contrast, nonpolar substances, such as oils, fats, and waxes, do not dissolve in water because they lack the necessary electrical charges for water to interact with. Molecules like oil have a symmetrical charge distribution, meaning water’s polar nature cannot find a positive or negative end to attract. Water molecules are more attracted to each other through their strong hydrogen bonds than to the nonpolar oil molecules, causing the nonpolar substance to cluster together and separate into distinct layers.
How External Conditions Affect Dissolution
While molecular structure determines if a substance can dissolve, external conditions affect how much can dissolve (solubility) and how quickly the process occurs (dissolution rate). Temperature is a major factor, though its effect depends on the solute’s state. For most solid solutes, such as sugar, an increase in temperature generally increases solubility because the added heat provides energy to help break apart the solute’s structure.
The solubility of gases in water follows the opposite trend, decreasing as the temperature rises. When water is heated, the increased kinetic energy causes the water molecules to move faster, allowing dissolved gas molecules to escape the liquid more easily. This is why warm soda loses its fizziness faster than cold soda.
Pressure primarily affects the solubility of gases in liquids, having virtually no effect on solids or liquids. Increasing the pressure of a gas above a liquid forces more of the gas molecules into the solution. Soft drinks are bottled under high pressure of carbon dioxide to maximize the amount of gas dissolved. When the container is opened, the pressure drops, allowing the gas to escape rapidly.
Two other factors influence the rate of dissolution, but not the final solubility amount. Crushing a solid into a powder increases the surface area exposed to the solvent, allowing water molecules to interact with the substance more quickly. Stirring or agitating the solution also speeds up the process by continuously bringing fresh solvent into contact with the solute.