- What is an unsaturated solution?
- Effect of temperature
- Insoluble solids
- Examples
- Difference with saturated solution
- References
An unsaturated solution is one in which the solvent medium is still capable of dissolving more solute. This medium is generally liquid, although it can also be gaseous. Regarding the solute, it is a conglomerate of particles in solid or gaseous state.
And what about liquid solutes? In this case, the solution is homogeneous as long as both liquids are miscible. An example of this is the addition of ethyl alcohol to water; the two liquids with their molecules, CH 3 CH 2 OH and H 2 O are miscible because they form hydrogen bonds (CH 3 CH 2 OH-OH 2).
Source: Pixabay
However, if dichloromethane (CH 2 Cl 2) and water were mixed, they would form a solution with two phases: one aqueous and the other organic. Why? Because the molecules of CH 2 Cl 2 and H 2 O interact very weakly, so that one slides over the other, resulting in two immiscible liquids.
A tiny drop of CH 2 Cl 2 (solute) is enough to saturate the water (solvent). If, on the contrary, they could form an unsaturated solution, then a completely homogeneous solution would be seen. For this reason, only solid and gaseous solutes can generate unsaturated solutions.
What is an unsaturated solution?
In an unsaturated solution, the solvent molecules interact with such effectiveness that the solute molecules cannot form another phase.
What does this mean? That the solvent-solute interactions exceed, given the pressure and temperature conditions, the solute-solute interactions.
Once solute-solute interactions increase, they “orchestrate” the formation of a second phase. For example, if the dissolving medium is a liquid, and the solute a solid, the latter will dissolve in the former to form a homogeneous solution, until a solid phase appears, which is nothing more than the precipitated solute.
This precipitate is due to the solute molecules being able to group together due to their chemical nature, intrinsic to their structure or bonds. When this happens, the solution is said to be saturated with solute.
Therefore, an unsaturated solution of solid solute consists of a liquid phase without a precipitate. Whereas if the solute is gaseous, then an unsaturated solution must be free from the presence of bubbles (which are nothing more than clusters of gaseous molecules).
Effect of temperature
Temperature directly influences the degree of unsaturation of a solution with respect to a solute. This can be mainly due to two reasons: the weakening of the solute-solute interactions due to the effect of heat, and the increase in molecular vibrations that helps to disperse the solute molecules.
If a solvent medium is considered as a compact space in whose holes the solute molecules are housed, as the temperature increases, the molecules will vibrate, increasing the size of these holes; in such a way that the solute can break through in other directions.
Insoluble solids
However, some solutes have such strong interactions that solvent molecules are hardly able to separate them. When this is the case, a minimum concentration of said dissolved solute is sufficient for it to precipitate, and it is then an insoluble solid.
Insoluble solids, by forming a second solid phase that differs from the liquid phase, generate few unsaturated solutions. For example, if 1L of liquid A can dissolve only 1g of B without precipitating, then mixing 1L of A with 0.5g of B will generate an unsaturated solution.
Similarly, a range of concentrations between 0 and 1g of B also form unsaturated solutions. But when going from 1g, B will precipitate. When this happens, the solution goes from being unsaturated to being saturated with B.
What if the temperature is increased? If a solution saturated with 1.5g of B is subjected to heating, the heat will help dissolve the precipitate. However, if there is a lot of precipitated B, heat will not be able to dissolve it. If so, an increase in temperature would simply evaporate solvent or liquid A.
Examples
Source: Pixabay
Examples of unsaturated solutions are numerous, as they depend on the solvent and solute. For example, for the same liquid A, and other solutes C, D, E… Z, their solutions will be unsaturated as long as they do not precipitate or form a bubble (if they are gaseous solutes).
-The sea can provide two examples. Sea water is a massive dissolution of salts. If a little of this water is boiled, it will be noted that it is unsaturated in the absence of precipitated salt. However, as the water evaporates, the dissolved ions begin to clump together, leaving saltpeter stuck to the pot.
-Another example is the dissolution of oxygen in the water of the seas. The O 2 molecule crosses the depths of the sea far enough for marine fauna to breathe; despite the fact that it is poorly soluble. For this reason it is common to observe oxygen bubbles emerging to the surface; of which, a few molecules manage to dissolve.
A similar situation occurs with the carbon dioxide molecule, CO 2. Unlike O 2, CO 2 is slightly more soluble because it reacts with water to form carbonic acid, H 2 CO 3.
Difference with saturated solution
Summarizing the above just explained, what are the differences between an unsaturated and a saturated solution? First, the visual aspect: an unsaturated solution consists of only one phase. Therefore, there should be no presence of solid (solid phase) or bubbles (gas phase).
Also, solute concentrations in an unsaturated solution can vary until a precipitate or bubble forms. While in saturated, biphasic solutions (liquid-solid or liquid-gaseous), the dissolved solute concentration is constant.
Why? Because the particles (molecules or ions) that make up the precipitate establish an equilibrium with those that lie dissolved in the solvent:
Particles (from the precipitate <=> dissolved particles
Bubble Molecules <=> Dissolved Molecules
This scenario is not contemplated in unsaturated solutions. When trying to dissolve more solute in a saturated solution, the equilibrium shifts to the left; to the formation of more precipitate or bubbles.
Because in unsaturated solutions this equilibrium (saturation) has not yet been established, the liquid can "store" more solid or gas.
Dissolved oxygen is present around an algae on the seabed, but when oxygen bubbles emerge from its leaves, it means that gas saturation occurs; otherwise no bubbles would be observed.
References
- General chemistry. Teaching material. Lima: Pontifical Catholic University of Peru. Recovered from: corinto.pucp.edu.pe
- Helmenstine, Anne Marie, Ph.D. (June 22, 2018). Unsaturated Solution Definition. Recovered from: thoughtco.com
- TutorVista. (sf). Unsaturated Solution. Taken from: chemistry.tutorvista.com
- Chemistry LibreTexts. (sf). Types of Saturation. Recovered from: chem.libretexts.org
- Nadine James. (2018). Unsaturated Solution: Definition & Examples. Recovered from: study.com