Solutions and the Role of Molecular Polarity

Solutions can be gaseous, liquid, or solid. In general, the physical state of the solvent determines the physical state of the solution. We focus on liquid solutions because they are by far the most common and important. 

From cytoplasm to tree sap, gasoline to cleaning fluid, iced tea to urine, solutions in which the solvent is a liquid are familiar in everyday life. Water is the most prominent solvent because it is so common and dissolves so many ionic and polar substances. But there are many other liquid solvents, and their polarity ranges from very polar to nonpolar. 

Liquid-Liquid and Solid-Liquid Solutions Many salts dissolve in water because the strong ion-dipole attractions that water molecules form with the ions are very similar to the strong attractions between the ions themselves and, therefore, can substitute for them. The same salts are insoluble in hexane (CgH ) because the weak ion-induced dipole forces their ions could form with the nonpolar molecules of this solvent cannot substitute for attractions between the ions. Similarly, oil does not dissolve in water because the weak dipole-induced dipole forces between oil and water molecules cannot substitute for the strong H bonds between water molecules. Oil does dissolve in hexane, however, because the dispersion forces in one substitute readily for the dispersion forces in the other. Thus, for a solution to form, like dissolves like means that the forces created between solute and solvent must be comparable in strength to the forces destroyed within both the solute and the solvent. 

In Table 13.2, the models show the relative change in size of the polar and nonpolar portions of the alcohol molecules. In the smaller alcohols (one to three 

Solubility of a Series of Alcohols in Water anci Hexane 

Water solubility decreases dramatically for alcohols larger than three carbons, and those with chains longer than six carbons are insoluble in water. For these larger alcohols to dissolve, the nonpolar chains have to move between the water molecules, substituting their weak attractions with those water molecules for strong H bonds among the water molecules themselves. The —OH portion of the 

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Intermolecular Forces in Solution 393 Liquid Solutions and the Role of Molecular Polarity 394... 

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