KEY2CHEM

Solutions

A solution is made of a solute (substance being dissolved) and a solvent (substance doing the dissolving). A solute will dissolve appreciably in a given solvent if the intermolecular forces between solute and solvent are relatively similar. This rule is often referred to as “like dissolves like”. For example, $$\require{mhchem}\ce{NaCl}$$ will dissolve in $$\require{mhchem}\ce{H2O}$$ because the cumulative ion-dipole interactions are stronger than the individual ionic forces and hydrogen bonds holding either $$\require{mhchem}\ce{NaCl}$$ or $$\require{mhchem}\ce{H2O}$$ together, respectively.

Example 1.

Which solute is expected to be most soluble in $$\require{mhchem}\ce{H2O}$$?

A. $$\require{mhchem}\ce{CH3OH}$$

B. $$\require{mhchem}\ce{CO2}$$

C. $$\require{mhchem}\ce{CCl4}$$

Solution

C. $$\require{mhchem}\ce{CH3OH}$$

$$\require{mhchem}\ce{CH3OH}$$ and $$\require{mhchem}\ce{H2O}$$ can hydrogen bond to one another, while the other solutes, which are nonpolar, can only display weaker dipole-induced dipole intermolecular forces as the strongest type of intermolecular force with $$\require{mhchem}\ce{H2O}$$.

Example 2.

Which diagram best describes the interaction of solute with solvent with $$\require{mhchem}\ce{MgCl2}$$ is dissolved in $$\require{mhchem}\ce{H2O}$$?

A.

B.

C.

Solution

A.

$$\require{mhchem}\ce{MgCl2}$$ is an ionic compound and $$\require{mhchem}\ce{H2O}$$ is a polar molecule (specifically, capable of hydrogen bonding). The strongest type of interparticle force will be ion-dipole interaction, with positively-charged $$\require{mhchem}\ce{Mg^2+}$$ cation attracted to the partial negative ($$\delta^-$$) of the oxygen atom in water. The negatively-charged $$\require{mhchem}\ce{Cl-}$$ anion is attracted to the partial positive ($$\delta^+$$) of the hydrogen atoms in water molecules.

Example 3.

Which solute is expected to be most soluble in carbon tetrachloride ($$\require{mhchem}\ce{CCl4}$$)?

A. $$\require{mhchem}\ce{C6H6}$$

B. $$\require{mhchem}\ce{H2O}$$

C. $$\require{mhchem}\ce{NaCl}$$

Solution

A. $$\require{mhchem}\ce{C6H6}$$

$$\require{mhchem}\ce{CCl4}$$ is a nonpolar solvent with dispersion forces as the strongest type of interparticle force. Other nonpolar substances are expected to be most soluble in $$\require{mhchem}\ce{CCl4}$$.