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What is a supersaturated solution?

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

The solute may well be a solid, such as sugar, starch, salts, etc .; or from a gas, such as CO 2 in carbonated beverages. Applying molecular reasoning, the solvent molecules surround those of the solute and seek to open space between themselves to be able to hold 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, it doesn’t 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 dissolution, until it dissolves completely; this is when the solution is said to be supersaturated.

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

Theoretical aspects

Saturation

The solutions can be made up of a composition that includes the states of matter (solid, liquid or gaseous); 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 balance of saturation; But what is this balance about?

The ions or molecules interact to form crystals, occurring more likely as the solvent cannot keep them apart any longer.

On the surface of the glass, its components collide to adhere to it, or they can also surround themselves with solvent molecules; some come out, some stick. The above can be represented by the following equation:

In dilute solutions, the “equilibrium” is very far to the right, because there is a lot of 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 is 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 speed; when this happens the solution is said to be saturated.

Oversaturation

To force the equilibrium to the dissolution of more solid the liquid phase must open molecular space, and for this it is necessary to stimulate it energetically. This causes the solvent to admit more solute than it normally can under ambient temperature and pressure conditions.

Once the contribution of energy to the liquid phase has ceased, the supersaturated solution remains metastable. Therefore, in the event of any disturbance, it can break its equilibrium and cause the crystallization of the excess solute until reaching the saturation equilibrium again.

For example, given a solute that is very 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, adding a little ice to the supersaturated solution.

The same effect could also be observed if a crystal of the soluble compound were thrown into the water. This serves as a nucleation support for the dissolved particles. The crystal grows 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 in which the amount of solute is no longer dissolved by the solvent has been exceeded; therefore, this type of solution has an excess of solute and has the following characteristics:

  • They can exist with their components in a single phase, as in aqueous or gaseous solutions, or present as a mixture of gases in a liquid medium.
  • Upon reaching the degree of saturation, the solute that is not dissolved will crystallize or precipitate (forms a disorganized solid, impure and without structural patterns) with ease in the 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 the local or in situ collision of the crystallizing molecules. 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 than its respective saturated solutions.

How do you prepare?

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 is modified, 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 particles of the solid meet and can form a viscous solution, or a whole crystal.

Examples and applications

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

-Supersaturated solutions formed by sugary solutions, such as honey, are other examples. From these candies or syrups are prepared, 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 drugs.

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