A solution is a fundamental type of mixture, present everywhere from the air we breathe to seawater. It is a combination of two or more substances that intermingle so completely that the resulting material appears to be a single substance. The concept of a solution is central to chemistry and biology because it describes how materials are dissolved and transported at a microscopic level. Understanding the characteristics of a solution helps distinguish it from other mixtures, such as suspensions or colloids.
The Defining Structure of Solutions
The defining characteristic of a true solution is its homogeneity, meaning the mixture possesses a uniform composition and appearance throughout. This uniformity exists at the molecular level, ensuring that any sample taken from the solution will have the identical proportion of components. A solution is composed of two main parts: the solute and the solvent.
The solvent is the substance present in the greatest amount, and it is responsible for dissolving the other components. The substance being dissolved is the solute, and it is typically the minor component. Solutes and solvents can exist in any state of matter—solid, liquid, or gas—though the final physical state of the solution is usually determined by the solvent.
This uniform distribution differentiates a solution from a heterogeneous mixture, like a mixture of sand and water. In a true solution, the solute particles are fully dispersed and intermingled with the solvent particles, forming a single phase. The components are considered “soluble” if they can form this homogeneous mixture.
Observable Physical Traits
The molecular homogeneity of a solution gives rise to several distinct, observable physical traits. The solute particles are exceptionally small, typically less than 1 nanometer (nm) in diameter. These particles are individual atoms, ions, or molecules, making them invisible to the naked eye or even with a standard light microscope.
Due to this minuscule particle size, a true solution is stable, and the solute particles will not settle out over time. This stability contrasts sharply with a suspension, where larger particles eventually fall to the bottom. Because the particles are so small, they pass easily through standard filter paper, meaning the components cannot be separated by simple filtration.
Another observable trait is the solution’s interaction with light; they are generally clear and transparent, though they may be colored. When a beam of light passes through a true solution, the light passes through without scattering, known as the absence of the Tyndall effect. This lack of scattering occurs because the solute particles are too small to deflect the light waves, which helps distinguish solutions from colloidal mixtures.
Quantifying Solution Composition
Solutions are characterized by their concentration, which is a quantitative measure of the amount of solute dissolved in a given amount of solvent or total solution. Concentration allows for a precise description of the mixture, moving beyond qualitative terms like “dilute” or “concentrated.” A dilute solution has a small quantity of solute, while a concentrated solution contains a large quantity of dissolved solute.
A common way to express concentration is through percentage, such as percentage by mass or percentage by volume. Percentage by mass is calculated by dividing the mass of the solute by the total mass of the solution and multiplying by 100. This calculation provides a clear, quantitative figure for the mixture’s composition.
The maximum amount of solute that a solvent can dissolve at a specific temperature defines its solubility. When a solution contains this maximum possible amount of dissolved solute, it is called a saturated solution. A supersaturated solution is an unstable condition where the solution temporarily holds more dissolved solute than is normally possible.