What is oversaturated solution
A. Unsaturated solution
B. Supersaturated solution
C. Saturated solution with sediment (precipitation)
Oversaturation is the name for a metastable state of a solution or of vapor. Under normal conditions, such an excess of the equilibrium state would be prevented by a phase transition. In the case of oversaturated systems, however, this does not occur at the expected equilibrium point of the phase diagram. The temperature range in which oversaturation can occur is also known as the Ostwald-Miers range (after Wilhelm Ostwald and Henry Alexander Miers).
A supersaturated solution contains more of the solute than its solubility corresponds to at that temperature.
Making a supersaturated solution
When a saturated solution slowly cools down while standing still, a supersaturated solution is initially formed, before the excess of the dissolved substance precipitates.
Elimination of the "too much" dissolved substance
The "too much" substance that has remained in solution is excreted by
- Evaporation of the solvent,
- Rub with a glass rod on the inner wall of the glass vessel in which the solution is stored or through
- Adding a crystal of the substance that is dissolved in the liquid - "seeding". Crystallization then begins at the sharp edges of the seed crystals.
Supersaturated solutions are used, for example, to purify substances by means of recrystallization or even to obtain crystals (including single crystals) by means of crystallization.
Supersaturated steam has a higher density than steam in the thermodynamic equilibrium between steam and condensate. Especially with regard to water vapor in the air, if there are no condensation nuclei (aerosols) in the laboratory test (cloud chamber), a supersaturation of a maximum of approx. 800% is shown. Under atmospheric conditions, maximum supersaturations of 100% can be observed, with supersaturations of only a few percentage points generally occurring.
Production of supersaturated steam
When steam is slowly cooled in thermodynamic equilibrium between steam and condensate, supersaturated steam is produced.
Decomposition of supersaturated steam using clouds as an example
Rain can be removed from undercooled cloud fields by “inoculating” with germs from z. B.
bring to excretion. This may be desirable to prevent hailstorms.
Subcooling of pure substance melts
A widespread application, in which a wide Ostwald-Miers area is used, are hand warmers or heat cushions with sodium acetate trihydrate filling. It is liquefied at a melting temperature of 58 ° C, e.g. in the microwave. Even at temperatures far below the melting point - in some cases down to −20 ° C - the material remains liquid as a supercooled melt in a metastable state, since the salt dissolves in its crystal water; the water molecules form a kind of crystal lattice of their own, which dissolves first.
Decomposition and heat recovery
If a metal plate (similar to the one in a cracking frog) is pressed into the heat pad, this triggers crystallization. The cushion warms up again to the melting temperature, whereby the complete crystallization and thus the release of the latent heat can extend over a longer period of time. Possible triggers for crystallization are:
- the pressure wave that is triggered by pressing the metal plate,
- the resulting release of microscopic crystallization nuclei that get stuck in small cracks in the metal with each crystallization.
Date of the last change: Jena, the: 24.11. 2020
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