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Why are supersaturated solutions considered a mysterious phenomenon in the scientific community?
In physical chemistry, supersaturation occurs when the concentration of a solute in a solution exceeds the solubility of that solute at equilibrium. This phenomenon mostly applies to solid solutes in liquids, but it can also apply to liquids and gases dissolving in liquids. A supersaturated solution is in a metastable state and may return to equilibrium by dissociating excess solute, adding solvent to dilute it, or increasing the solubility of the solute.
The study of supersaturated solutions dates back to early experiments with sodium sulfate, a salt that was ideal for early research because its solubility in water decreases as temperature rises.
Over time, scientists gradually realized that the crystallization process of supersaturated solutions does not rely solely on stirring as previously believed, but requires the addition of solid "seeds" to start the crystallization process. This concept was expanded by the famous chemist Gay-Lussac, who pointed out the influence of the kinematics of the salt ions and the characteristics of the container on the supersaturated state. Henry Lowell later proposed that nuclei in the solution and the walls of the container catalyzed the crystallization process.
Examples and mechanisms of supersaturation
When the temperature of a saturated solution is changed, the solution of a compound in a liquid becomes supersaturated. Generally, as the temperature decreases, the solubility decreases and the excess solute quickly separates from the solution in the form of crystals or amorphous powder. However, there are cases where the opposite occurs. Recrystallization is a process used to purify chemical compounds. It involves heating a mixture of impurities and solvent until the compound is dissolved, then filtering out the remaining impurities, and then lowering the temperature to briefly supersaturate the solution.
These processes illustrate the behavior of supersaturated solutions and indicate that tiny crystals or glass particles can act as nuclei to induce crystallization.
Interaction of Gas Solutes and Liquids
The solubility of a gas in a liquid increases as the pressure of the gas increases. When the external pressure is reduced, the excess gas is released from the solution. For example, carbonated drinks fizz because carbon dioxide dissolves in the liquid under high pressure. When the bottle is opened, the pressure decreases and the gas is released in the form of bubbles. For divers, when the body is supersaturated with gases, surfacing may lead to the risk of decompression sickness.
Supersaturated Applications
In the pharmaceutical field, supersaturated solutions have practical applications. By creating a supersaturated solution of a drug, the drug can be taken in liquid form. This supersaturated state of the drug can promote absorption in the body, and even drugs taken in crystalline form may form a supersaturated state in the body. This phenomenon is called internal supersaturation, and using tools to identify supersaturated solutions, marine ecologists can study the activity of organisms and populations to determine the biological productivity of a region.
As for atmospheric science, the existence of supersaturation has been recognized since the 1940s. When the troposphere is supersaturated with water vapor, the chance of water freezing increases. This reveals the importance of supersaturation phenomena in meteorology and environmental science.
When water molecules are in a supersaturated state, the relative humidity can sometimes reach over 100%, a phenomenon that scientists have yet to explore in depth.
Supersaturated solutions not only inspire scientists' research enthusiasm, but also reveal the logic behind many mysterious phenomena in nature. What are the laws and operating mechanisms of reactions in these extreme states?
