24 junio, 2024

Supersaturated solution: concept, characteristics, preparation, examples

Which is a supersaturated solution?

The supersaturated solution It is one in which the solvent has dissolved more solute than it can dissolve at saturation equilibrium. All have saturation equilibrium in common, with the difference that in some solutions it is reached at lower or higher solute concentrations.

The solute may well be a solid, such as sugar, starch, salts, etc.; or from a gas, such as CO2 in carbonated drinks. Applying molecular reasoning, the molecules of the solvent surround those of the solute and seek to open space between themselves to be able to house more of the solute.

Thus, there comes a time when the solvent-solute affinity cannot overcome the lack of space, establishing the saturation equilibrium between the crystal and its surroundings (the solution). At this point, no matter how much the crystals are ground or shaken: the solvent can no longer dissolve any more solute.

How to «force» the solvent to dissolve more solute? Through an increase in temperature (or pressure, in the case of gases). In this way, the molecular vibrations increase and the crystal begins to yield more of its molecules to the solution, until it dissolves completely; This is when the solution is said to be supersaturated.

The upper image shows a supersaturated solution of sodium acetate, whose crystals are the product of the restoration of the saturation equilibrium.

Theoretical aspects

Saturation

Solutions can be made up of a composition that includes the states of matter (solid, liquid or gas); however, they always have a single phase.

When the solvent cannot completely dissolve the solute, another phase is observed as a consequence. This fact reflects the equilibrium of saturation; But what is that balance about?

The ions or molecules interact to form crystals, occurring more likely as the solvent can no longer hold them apart.

On the surface of the crystal, its components collide to adhere to it, or they can also be surrounded by solvent molecules; some come out, others adhere. This can be represented by the following equation:

Solid <=> dissolved solid

In dilute solutions the «equilibrium» is shifted far to the right, because there is so much space available between the solvent molecules. On the other hand, in concentrated solutions the solvent can still dissolve solute, and the solid that is added after stirring will dissolve.

Once equilibrium has been reached, the particles of the added solid as soon as they dissolve in the solvent and others, in solution, must «come out» to open space and allow their incorporation into the liquid phase. Thus, the solute goes back and forth from the solid phase to the liquid phase at the same rate; When this happens, the solution is said to be saturated.

supersaturation

To force the equilibrium towards the dissolution of more solids, the liquid phase must open up molecular space, and for this it is necessary to stimulate it energetically. This causes the solvent to admit more solute than it normally can at room temperature and pressure.

Ceased the contribution of energy to the liquid phase, the supersaturated solution remains metastable. Therefore, in the face of any disturbance, it can break its equilibrium and cause the crystallization of excess solute until it reaches saturation equilibrium again.

For example, given a solute that is highly soluble in water, a certain amount of it is added until the solid cannot dissolve. Then heat is applied to the water, until the dissolution of the remaining solid is guaranteed. The supersaturated solution is removed and allowed to cool.

If the cooling is very abrupt, crystallization will occur instantly; for example, by adding a little ice to the supersaturated solution.

The same effect could also be observed if a crystal of the soluble compound were dropped into water. This serves as a nucleation support for the dissolved particles. The crystal grows by accumulating the particles of the medium until the liquid phase is stabilized; that is, until the solution is saturated.

Characteristics of supersaturated solutions

In supersaturated solutions, the limit has been exceeded at which the amount of solute is no longer dissolved by the solvent; therefore, this type of solution has an excess of solute and presents the following characteristics:

They can exist with their components in a single phase, such as in aqueous or gaseous solutions, or occur as a mixture of gases in a liquid medium.
Upon reaching the degree of saturation, the solute that is not dissolved will easily crystallize or precipitate (form a disorganized, impure solid without structural patterns) from solution.
It is an unstable solution. When excess undissolved solute precipitates, there is a release of heat that is proportional to the amount of precipitate. This heat is generated by local shock or on site of the molecules that are crystallizing. Because it stabilizes, it must necessarily release energy in the form of heat (in these cases).
Some physical properties such as solubility, density, viscosity, and refractive index depend on the temperature, volume, and pressure to which the solution is subjected. For this reason, it has different properties from its respective saturated solutions.

How is it prepared?

There are variables in the preparation of solutions, such as the type and concentration of the solute, the volume of solvent, the temperature or the pressure. By modifying any of these, a supersaturated solution can be prepared from a saturated one.

When the solution reaches a state of saturation and one of these variables changes, then a supersaturated solution can be obtained. In general, the preferred variable is temperature, although it can also be pressure.

If a supersaturated solution is subjected to slow evaporation, the solid particles meet and may form a viscous solution, or a whole crystal.

Examples and applications

-There is a wide variety of salts with which supersaturated solutions can be obtained. They have been used industrially and commercially for a long time and have been the subject of extensive research. Among the applications, sodium sulfate solutions and aqueous potassium dichromate solutions stand out.

-Supersaturated solutions formed by sugary solutions, such as honey, are other examples. Candies or syrups are prepared from these, having a vital importance in the food industry. It should be noted that they are also applied in the pharmaceutical industry in the preparation of some medicines.

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