KEY2CHEM

Solubility and Interparticle Interactions

The solubility of substances in one another is described as “like dissolves like”. This means that substances with similar intermolecular forces will dissolve in one another. For example, NaCl is soluble in water because strong ion-dipole intermolecular forces are formed between the ions and the permanent dipole moments in the water molecules. Similarly, ethanol ($$\require{mhchem}\ce{CH3CH2OH}$$), or grain alcohol, is miscible (soluble in any proportion) with water due to strong hydrogen bonding interactions between the compounds. Conversely, nonpolar compounds, such as oils, are not very soluble in water. Nonpolar compounds display London dispersion forces as the strongest type of intermolecular force, resulting in weaker interactions relative to the interparticle forces between individual components.

In addition to intermolecular forces, entropy plays a role in forming solutions. Entropy is related to the number of ways that the system’s energy can be distributed, and increasing entropy is favorable. Generally, the formation of a solution from individual components increases the entropy of the system, which is favorable.

Example 1.

Which solute is expected to be most soluble in water?

A. $$\require{mhchem}\ce{CCl4}$$

B. $$\require{mhchem}\ce{CH3OH}$$

C. $$\require{mhchem}\ce{CO2}$$

Solution

B. $$\require{mhchem}\ce{CH3OH}$$

The solute that is most soluble will have intermolecular forces most similar to those in the solvent ("like dissolves like”). $$\require{mhchem}\ce{CH3OH}$$ and $$\require{mhchem}\ce{H2O}$$ will hydrogen bond to one another. The other solutes are nonpolar molecules, which could have dipole-induced dipole interactions with the solvent.

Example 2.

Which solute is expected to have the lowest solubility in hexane ($$\require{mhchem}\ce{C6H14}$$)?

A. $$\require{mhchem}\ce{C6H6}$$

B. $$\require{mhchem}\ce{I2}$$

C. $$\require{mhchem}\ce{CH3OH}$$

Solution

C. $$\require{mhchem}\ce{CH3OH}$$

Hexane is a nonpolar molecule with dispersion forces as the strongest type of intermolecular force holding its particles together. $$\require{mhchem}\ce{CH3OH}$$ is a polar molecule, while the other two solutes are nonpolar.

Example 3.

Which diagram shows the change in entropy during formation of a solution when $$\require{mhchem}\ce{NaCl}$$ is dissolved in water?  = $$\require{mhchem}\ce{NaCl}$$ and  = $$\require{mhchem}\ce{H2O}$$

A.

B.

C.

Solution

A.

Entropy generally increases during formation of a solution. The number of ways the system can be distributed (sometimes called “microstates”) increases in a solution relative to the separated components, as there are more options the particles can be in relation to one another.