11.5 Factors That Affect Solubility
Factors That affect Solubility
Solubility isthe amount of solute that can be dissolved in 100 g of solvent at a specific temperature. Several factors affect the solubility of a compound.
A solution is Saturated when the solute’s concentration is equal to solubility.
A solution is Unsaturated if the solute’s concentration is less than its solubility.
A solution with relatively low concentration of solute is called dilute, and one with a relatively high concentration is called Concentrated
Use this https://phet.colorado.edu/sims/html/concentration/latest/concentration_en.html and try the following experiment on this simulation.
Take about half liter of water
From the drop down menu select any compound and start spinking it in water. You can measure the concentration of the solution with the concentration measuring toggle button as well. Once the screen says saturated, it means the solution has maximum amount of solute that can be dissolved in this solution. If you add more solute now, it will just settle on the bottom of container.
Solutions may be prepared in which a solute concentration exceeds its solubility. Such solutions are said to be supersaturated.
Nature of Solute and Solvent:
As discussed previously, the principle of solution formation can be summarized as “Like dissolves Like.” This means that compounds with similar polarities, such as polar or ionic compounds, experience dipole-dipole attractions or hydrogen bonding with the solvent molecules that are as strong as those between molecules in the pure solute or pure solvent. Consequently, the two types of molecules can mix readily. In contrast, nonpolar compounds exhibit no significant difference in the strengths of solute-solute, solvent-solvent, and solute-solvent intermolecular attractions. Therefore, in general, a solution forms when two compounds of comparable polarities are mixed. In summary, a polar or ionic solute can dissolve in a polar solvent, whereas a nonpolar solute can only dissolve in a nonpolar solvent, thus the phrase “Like dissolves Like.”
In 2010 there was an oil spill in the gulf of Mexico. As in all oil spills, the oil stayed on top of the water, resulting in massive loss of marine life. Oil being non polar, did not dissolve in polar water.
Another excellent example of this phenomenon can be seen with vitamins. Since Vitamin A, D, E and K are nonpolar; they are fat-soluble vitamins. They can be stored in the fatty tissue of the body. However, vitamins B and C are polar and hence are water-soluble. They only get absorbed as needed, and any excess gets flushed out with urine.
———————————————————————————————————————
Learning Check:
- Which of the following will make a solution?
a) NaCl and oil b) Iodine in Water c) KMnO4 in water
Answer: c) KMnO4 in water, KMnO4 is an Ionic compound, and water is polar.
————————————————————————————————————-
Temperature:
While preparing sugar syrup, you may have noticed that more sugar can be dissolved in hot water than in cold water. This is an example of how temperature can affect solubility.
Fig. 11.11 This graph shows solubility of different solids and gases as a function of temperature
Source: www.openstax.org/
Usually, the solubility of solids increases with an increase in temperature as clear in the graph above. However, the solubility of gases decreases with an increase in temperature (Figure. 11.11). As the temperature increases, so does the kinetic energy of gas particles. Gas particles start moving faster and leave the solution; as a result, the solubility of gases goes down with increase in temperature.
We often hear about thermal pollution and global warming. As the temperature of the earth increases, so does the temperature of oceans. With the rise in ocean water temperature, the solubility of oxygen and other gases in water goes down. As a result, marine life that depends on this oxygen to breathe can no longer live and die.
A solution may be saturated with the compound at an elevated temperature (where the solute is more soluble) and subsequently cooled to a lower temperature without precipitating the solute. The resulting solution contains solute at a concentration greater than its equilibrium solubility at the lower temperature (i.e., it is supersaturated) and is relatively stable. Precipitation of the excess solute can be initiated by adding a seed or by mechanically agitating the solution. Some hand warmers, such as the one pictured in Figure 11.17, take advantage of this behavior.
Source: www.openstax.org/
Figure 11.17 This hand warmer produces heat when the sodium acetate in a supersaturated solution precipitates.
Pressure:
Have you ever wondered why soda goes flat after opening the bottle, but remain carbonated if you do not let the seal break?
Pressure does not affect the solubility of solids and liquids. However, it can significantly affect the solubility of gases.
Henry’s law explains the relationship between pressure and solubility of gases. As per Henry’s Law, as the partial pressure of gas above the surface of liquid increases, the solubility of gases also increases.
C = kP Eq. 11.6
Here C is the concentration of dissolved gas; k is Henry’s law constant, and P is the partial pressure of the gas.
In a sealed soda bottle , partial pressure of the carbon dioxide gas above the surface of liquid is very high and that keeps carbon dioxide dissolved in the liquid. However when we break the seal, the gas escapes and decreases the pressure of gas above the liquid. Hence, the solubility of carbon dioxide in soda goes down and slolwly all the dissolved carbon dioxide in the soda comes out and leaves.
Qw32wExample: If at 25 0C, the concentration of oxygen in water was found to be 0.0018 mols/L when the partial pressure of oxygen was 101.32 kPa. find the solubility of oxygen if the partial pressure of oxygen was increased to 250.2 kPa.
We are given C = 0.0018 mol/L, and initial partial pressure P = 101.32 kPa .
Since C = kP
0.0018 mol/L = k x 101.32 kPa
k = = 1.78 x 10-5 mol/L.kPa
New Partial Pressure = 250.2 kPa
New Concentration C = kP = 1.78 x 10-5 mol/L.kPa x 250.2 kPa = 0.0044 mol / L
———————————————————————————————————————
Learning Check:
- If the solubility of oxygen in water at 1.0 atm partial pressure is 1.4 x 10-3 mol/L at 20 0C, how many mg of Oxygen gas can be dissolved in one liter of water at 0.17 atm pressure?
Answer: 7.6 mg
(Hint – First find the new concentration in mol/L at 0.17 atm and then change this concentration in mol/L and then to mg/L)
———————————————————————————————————————
Watch this video for a deeper understanding of the factors that affect solubility.
Solutions of Liquids in Liquids
Some liquids may be mixed in any proportions to yield solutions; in other words, they have infinite mutual solubility and are said to be miscible. Examples of sun=ch liquids include Ethanol, sulfuric acid, and ethylene glycol (popular for use as antifreeze,).
Fig. 11.12
Source: www.openstax.org/
Water and antifreeze are miscible; mixtures of the two are homogeneous in all proportions. (credit:“dno1967”/Wikimedia commons)
Miscible liquids are typically those with very similar polarities. Consider, for example, liquids that are polar or capable of hydrogen bonding. For such liquids, the dipole-dipole attractions (or hydrogen bonding) of the solute molecules with the solvent molecules are as strong as those between molecules in the pure solute or in the pure solvent. Hence, the two kinds of molecules mix easily. Likewise, nonpolar liquids are miscible with each other because there is no appreciable difference in the strengths of solute-solute, solvent-solvent, and solute-solvent
intermolecular attractions. The solubility of polar molecules in polar solvents and of nonpolar molecules in nonpolar solvents is, again, an illustration of the chemical axiom “like dissolves like.”
Two liquids that do not mix to an appreciable extent are called immiscible. Separate layers are formed when immiscible liquids are poured into the same container. (Figure 11.13),
Gasoline, oil, benzene, carbon tetrachloride, some paints, and many other nonpolar liquids are immiscible with water. Relatively weak attractive forces between the polar water molecules and the nonpolar liquid molecules are not adequate to overcome much stronger hydrogen bonding between water molecules. Miscible liquids are of infinite mutual solubility, while liquids said to be immiscible are of very low (though not zero) mutual solubility.
Fig 11.13 : This NASA satellite image shows the oil slick from the Deepwater Horizon spill.
https://openstax.org/books/chemistry-2e/pages/11-5-colloids#CNX_Chem_11_05_oilspill
Two liquids, such as bromine and water, that are of moderate mutual solubility are said to be partially miscible.
Two partially miscible liquids usually form two layers when mixed. In the case of the bromine and water mixture, the upper layer is water, saturated with bromine, and the lower layer is bromine saturated with water. Since bromine is nonpolar, and, thus, not very soluble in water, the water layer is only slightly discolored due to the very small amount of the bright orange bromine dissolved in it. This small solubility of bromine is due to dipole-induced dipole interactions or polarizability of bromine molecules in the presence of water molecules.
Figure 11.15 Bromine (the deep orange liquid on the left) and water (the clear liquid in the middle) are partially miscible in the tube on right.
