Tabla de Contenidos
A saturated solution is one that does not admit the dissolution of more solute. In other words, it is a solution in which the maximum concentration of solute that can be dissolved in that particular solvent and at a particular pressure and temperature has already been reached. These are solutions in which the solubility equilibrium has been established between the solute dissolved in the solvent and the solute in the solid state at the bottom of the container, in the liquid state either above or below the solvent (depending on the densities) or in a gaseous state.
solubility equilibrium
As just mentioned, a solution is saturated when the solubility equilibrium is reached. In the simplest case, this equilibrium can be represented by the following chemical equation:
Where S represents a molecular solute (which does not dissociate) and the subscripts indicate if it is pure and in a solid state, or if it is dissolved (ac means in aqueous solution, although it could be in any other solvent).
When you have molecular solvents as in this case, to obtain a saturated solution and equilibrium can be established, it is necessary that the concentration of the solute in the solution is equal to the equilibrium constant, Ks, and that there is still some solute left. in undissolved solid state.
In the case of ionic solutes such as salts, the general reaction looks like this:
where K ps is the solubility product constant, [M m+ ] eq represents the molar concentration of the cation M m+ in the saturated solution and [A n- ] eq represents the molar concentration of A n- in the saturated solution.
In this case, the condition that defines the saturated solution is that the product of the concentrations of the ions in solution (M m+ and A n- ) raised to their respective stoichiometric coefficients (nym) must be equal to the constant of the product of solubility. If the result is greater than K ps , the solution is supersaturated, and if it is less, it is unsaturated.
The equilibrium of the saturated solution is dynamic.
When a saturated solution is achieved, it appears that the solute is no longer dissolving in the solvent and that the dissolution process has stopped. However, this is not exactly so. In fact, as in most chemical equilibria, the solubility equilibrium is not a static equilibrium but a dynamic one, in which the forward reaction (dissolution of more solute) and the reverse reaction (precipitation of solute from solution) They are happening at the same rate. For this reason, no change is noted either in the net amount of solid solute or in the concentration of the solute in the solution.
Ways to obtain a saturated solution
There are three basic ways to obtain saturated solutions:
- Add solute until no more dissolves , no matter how vigorously the solution is shaken. This is the simplest method, although it can sometimes be very tedious since there are solutes that dissolve very slowly.
- The second way is to start from an unsaturated solution and start evaporating the solvent . As the total volume of the solution decreases without loss of solute, the concentration of the solute will increase until the maximum concentration (or solubility) is reached. At that moment the solute will begin to precipitate and from then on you will have a saturated solution.
- Another way is to dissolve more of the solute than the solvent can handle through heating . By allowing this solution to cool , a supersaturated solution will be obtained. For this reason, any disturbance, from a vibration to seeding a small crystal on the surface of the solution, will immediately trigger precipitation of excess solute. This precipitation will cease as soon as the saturation level is reached.
There is a fourth way to obtain saturated solutions from unsaturated solutions that consists of progressively modifying the medium or the solvent to reduce the solubility of the solute. This can be accomplished by adding an organic solvent, changing the pH, and in other ways as well.
Factors Affecting Solubility Equilibrium and Saturated Solutions
The nature of the solute and solvent
Each chemical compound has its solubility in each different type of solvent. For example, sugar is much more soluble than salt in water, so it will always be easier to saturate a solution with salt than with sugar. There are also cases in which it is impossible to obtain a saturated solution. Such is the case of solutes that are miscible with the solvent, such as solutions of ethyl alcohol and water, which can be mixed in any proportion.
Temperature
As seen just now, temperature plays an important role in saturated solutions, since an increase in temperature can increase solute solubility, dissolving all solid solute and turning a saturated solution into an unsaturated one.
On the other hand, the effect of temperature on the solubility of gases is just the opposite. Instead of increasing its solubility, high temperatures decrease it. Proof of this is the case of soft drinks. These lose most of their gases with increasing temperature.
pH
In those cases in which the solute has acid-base properties, the pH can play a very important role in determining its solubility. In general, any reaction that helps to further ionize the solute will increase its solubility, which can turn a saturated solution into an unsaturated one.
For example, if the solute is a weak acid such as benzoic acid and you have a saturated solution, adding sodium hydroxide that reacts with said acid and ionizes it will help dissolve more of the solute in the solution.
The pressure
Pressure affects gaseous solutes the most. Strongly increasing the pressure of gases above a solution can force the gas to dissolve in greater quantity in the solvent. This would be the equivalent of increasing the temperature for solid solutes. In the case of gases, as long as the solution and the gas are confined in a sealed container, no matter how much the pressure is, the solution will always end up gas saturated if given enough time.
common ion effect
The common ion represents an effect similar to that of pH. When it is desired to dissolve an ionic solute in a solution, it will dissociate and produce a certain concentration of its respective ions. If we try to dissolve the same ionic solute in a solution that already contains some of one of its ions, it will be more difficult to dissolve it than if we did it in the pure solvent. This is called the common ion effect and makes it easier to saturate solutions.
Examples of saturated solutions
Sealed fizzy drinks
All soft drinks, sodas, and carbonated beers are saturated solutions of carbon dioxide in water as long as the bottle or can is completely sealed.
The moment the bottle is uncorked, the equilibrium is lost and the solution suddenly becomes a supersaturated solution, so the gases begin to bubble, escaping.
The water on the shores of the dead sea
The Dead Sea is one of the saltiest lakes on earth, and on the shore you can see the crystallization of salt that comes from the lake water. This means that, in some parts, the water has been trapped in small puddles that, when they evaporate, become saturated with salt and begin to precipitate.
some types of honey
There are some types of honey that are more concentrated than others, and in some cases, they are so concentrated that the sugars they contain begin to crystallize in the bottle.
This shows that the solution was originally supersaturated, and that, after crystallization, it became a saturated solution.
References
Brown, T. (2021). Chemistry: The Science Center. (11th ed.). London, England: Pearson Education.
Chang, R., Manzo, Á. R., Lopez, PS, & Herranz, ZR (2020). Chemistry (10th ed.). New York City, NY: MCGRAW-HILL.
Flowers, P., Theopold, K., Langley, R., & Robinson, WR (2019). Chemistry 2e . Retrieved from https://openstax.org/books/chemistry-2e
Bubis, M. (1998). The Dead Sea – An Unusual Sea. Retrieved from http://sedici.unlp.edu.ar/bitstream/handle/10915/49306/Documento_completo.pdf
Honey and temperature (nd) Retrieved from https://www.latiendadelapicultor.com/blog/la-miel-y-la-temperatura/
