When two or more substances mix completely to form a single, uniform substance, the resulting mixture is called a solution. Every solution consists of a solute, the substance being dissolved, and a solvent, the substance doing the dissolving.
The amount of solute a solvent can hold at a specific pressure and temperature determines the solution’s state. Temperature plays a significant role in this capacity, as the solubility of most solid solutes increases as the solvent becomes warmer.
What Defines an Unsaturated Solution?
An unsaturated solution is defined by its capacity to readily dissolve additional solute. This state indicates that the solvent currently holds less than the maximum amount of dissolved substance possible under the existing conditions of temperature and pressure. Because the solution has not reached its limit, the solvent molecules have plenty of available space to incorporate more solute particles. When table salt is stirred into water, the resulting mixture is unsaturated as long as the salt disappears completely.
The concentration of solute in this state is lower than the solubility limit at that specific temperature. If a small crystal of the solute is added, it will simply dissolve without leaving any solid material behind. This ability to accept more material is the distinguishing characteristic of an unsaturated solution, making it the most common and stable state encountered in everyday mixtures.
The Saturation Baseline: Saturated Solutions
A saturated solution represents the precise boundary where the solvent is holding the maximum amount of solute possible under the specific temperature and pressure. At this point, the solubility limit has been reached. If any more solute is added, it will not dissolve and will remain as a solid on the bottom of the container. This state is sometimes referred to as the solubility equilibrium point for the given system.
Within a saturated solution, a process called dynamic equilibrium is established. This means that while no net change in the amount of dissolved solute is observed, the processes of dissolving and crystallizing occur simultaneously. The rate at which solute particles enter the solution is exactly equal to the rate at which dissolved particles return to the solid phase. Therefore, the concentration of the dissolved material remains constant, creating a stable chemical system.
Completing the Spectrum: Understanding Supersaturation
Supersaturation describes a temporary condition where a solution contains a greater concentration of dissolved solute than the saturation limit allows for that temperature. This is achieved by manipulating solubility, usually by first raising the temperature of the solvent. Since the solubility of most solids increases with heat, the warmer solvent can accommodate an excess amount of solute beyond its normal capacity.
After the extra solute has fully dissolved, the solution is then carefully cooled back down without any disturbance. If the cooling is done slowly, the excess solute particles remain in the solution, even though the temperature is now below the point where they should precipitate. The resulting mixture is highly unstable because it is holding more solute than it should, making it thermodynamically unfavorable.
The instability of a supersaturated solution is demonstrated by introducing a trigger, such as a small solid piece of the solute, known as a seed crystal. The seed crystal provides a surface template for the excess dissolved material, causing rapid crystallization out of the solution until the concentration drops back to the stable saturated level. Any physical disturbance, like stirring or shaking, can also initiate this rapid precipitation, demonstrating the non-equilibrium nature of the supersaturated state.