A solution represents a homogeneous mixture where one substance, the solute, uniformly disperses throughout another, the solvent. This blend results in a single phase.
The Concept of Solubility
Solubility refers to the maximum amount of a specific solute that can dissolve in a given quantity of solvent at a particular temperature. For instance, more sugar can dissolve in water than salt under identical conditions.
Temperature plays a significant role; for most solid solutes in liquid solvents, solubility typically increases as temperature rises because the solvent molecules gain kinetic energy, aiding dissolution. Conversely, the solubility of gases in liquids generally decreases with increasing temperature but increases with greater pressure.
Identifying Unsaturated Solutions
An unsaturated solution contains less solute than the maximum amount that can dissolve in the solvent at a given temperature. Such solutions often appear clear and do not show any undissolved particles.
To determine if a solution is unsaturated, a straightforward test involves adding a small amount of the solute. If the added solute completely dissolves into the solution after stirring, it confirms the solution is unsaturated. For example, if you add a spoonful of sugar to a glass of water and it disappears entirely, the sugar water is an unsaturated solution.
Identifying Saturated Solutions
A saturated solution holds the maximum amount of solute that can dissolve in a specific amount of solvent at a given temperature. At this point, the solution has reached its dissolving capacity. Any additional solute introduced will not dissolve and will instead remain as a solid.
A practical method to identify a saturated solution involves adding more solute to it. If, after thorough mixing, the added solute settles at the bottom of the container and does not dissolve, the solution is saturated. This undissolved material is a key indicator that the solution can no longer incorporate more of that specific substance. For instance, if you keep adding sugar to water until some sugar crystals persistently rest at the bottom, even after stirring, the solution has become saturated.
Identifying Supersaturated Solutions
A supersaturated solution is an unstable state containing more dissolved solute than a saturated solution would normally hold at the same temperature. These solutions are typically prepared by dissolving a solute in a heated solvent, where solubility is often higher, and then carefully cooling the solution without allowing the excess solute to crystallize out. This careful cooling traps the extra dissolved solute, making the solution inherently unstable.
The most distinct way to identify a supersaturated solution is by disturbing it. Adding a tiny “seed crystal” of the solute, scratching the inside of the container, or even a slight bump can trigger a rapid and often dramatic crystallization of the excess solute. The previously clear solution will suddenly become cloudy as the dissolved material precipitates out of the solution, typically leaving behind a saturated solution once the crystallization is complete. Common examples include the sodium acetate solutions found in reusable hand warmers, where bending a metal disc initiates crystallization and releases heat. Another familiar instance is the formation of rock candy, where sugar crystals grow from a supersaturated sugar solution.