A chemical solution is a homogeneous mixture found throughout science and daily life, from the air we breathe to the beverages we drink. Understanding solutions begins with recognizing them as a specific type of mixture where components are blended at the molecular level. Unlike simple mixtures where substances remain distinct, a solution is characterized by a uniform composition that appears as a single substance.
What Defines a Chemical Solution
A chemical solution is defined formally as a homogeneous mixture of two or more substances in a single physical state. The term “homogeneous” means that the composition and properties are identical throughout the mixture, even when viewed under a microscope. The particles of the dissolved substance are extremely small, typically between 0.1 and 2 nanometers, which prevents them from settling out over time. This characteristic size ensures the solution is stable and will not separate upon standing, unlike suspensions or heterogeneous mixtures. Furthermore, the components of a true solution cannot be separated by simple physical methods like filtration.
The Roles of Solutes and Solvents
Every solution is composed of at least two parts: a solute and a solvent. The solvent is the substance that does the dissolving and is typically the component present in the greater amount. The physical state of the solvent usually determines the physical state of the resulting solution. The solute is the substance that gets dissolved into the solvent, and it is generally present in a lesser amount than the solvent. Water is frequently referred to as the “universal solvent” because its molecular polarity allows it to dissolve a wide array of chemical compounds.
Categorizing Solutions by Physical State
Solutions are categorized based on the physical state of the final mixture, which is determined by the state of the solvent. These categories demonstrate that not all solutions are liquids, despite the common example of salt water. There are nine possible combinations of solute and solvent states, but they fall into three main solution types: liquid, solid, and gas.
Liquid Solutions
Liquid solutions are the most common type, where the solvent is a liquid. Common examples include a solid dissolved in a liquid, such as table sugar dissolving in water. A liquid can also dissolve another liquid, like ethanol mixed into water to create rubbing alcohol. Furthermore, a gas can dissolve in a liquid, as seen with carbon dioxide gas dissolved under pressure in water to create carbonated beverages.
Solid Solutions
Solid solutions are mixtures where the final solution is in a solid state. The most familiar example is an alloy, which is a solid dissolved in another solid. Brass, for instance, is a solid solution where zinc is dissolved within copper. Another example involves a gas dissolved in a solid, such as hydrogen gas absorbed into the crystal lattice of the metal palladium.
Gas Solutions
Gas solutions have a gas as the solvent and are formed when the components mix uniformly at the molecular level. Air is the most ubiquitous example of a gas solution, consisting primarily of nitrogen gas acting as the solvent. Oxygen, carbon dioxide, and other trace gases are dissolved in the nitrogen solvent. Since all gases mix freely and completely with one another, all mixtures of gases are considered homogeneous solutions.
Understanding Solution Concentration
Concentration describes the amount of solute dissolved in a specific amount of solvent or total solution. Solutions are qualitatively described as either dilute or concentrated based on this ratio. A dilute solution contains a relatively small amount of solute, while a concentrated solution contains a large amount of solute.
The concept of saturation relates to the maximum amount of solute a solvent can hold at a given temperature. An unsaturated solution is one where more solute can still be dissolved. Once the solvent has dissolved the maximum possible amount of solute, the solution is considered saturated. If a saturated solution is carefully heated and cooled slowly, the extra solute can remain dissolved temporarily, creating a supersaturated solution. Supersaturated solutions are unstable and the excess solute will crystallize out if the solution is disturbed.