New attractions between hexane and water molecules do form, but because the new attractions are very different from the attractions that are broken, they introduce significant changes in the structure of the water. It is believed that the water molecules adjust to compensate for the loss of some hydrogen bonds and the formation of the weaker hexane-water attractions by forming new hydrogen bonds and acquiring a new arrangement. Overall, the attractions in the system after hexane and other hydrocarbon molecules move into the water are approximately equivalent in strength to the attractions in the separate substances. For this reason, little energy is absorbed or evolved when a small amount of a hydrocarbon is dissolved in water.
To explain why only very small amounts of hydrocarbons such as hexane dissolve in water, therefore, we must look at the change in the entropy of the system. It is not obvious, but when hexane molecules move into the water layer, the particles in the new arrangement created are actually less dispersed (lower entropy) than the separate liquids. The natural tendency toward greater dispersal favors the separate hexane and water and keeps them from mixing. This helps explain why gasoline and water do not mix. Gasoline is a mixture of hydrocarbons, including hexane. Gasoline and water do not mix because the nonpolar hydrocarbon molecules would disrupt the water in such a way as to produce a structure that was actually lower entropy ; therefore, the mixture is less likely to exist than the separate liquids. We can apply what we know about the mixing of ethanol and water to the mixing of two hydrocarbons, such as hexane, C 6 H 14 , and pentane, C 5 H 12 . When the nonpolar pentane molecules move into the nonpolar hexane, London forces are disrupted between the hexane molecules, but new London forces are formed between hexane and pentane molecules. Because the molecules are so similar, the structure of the solution and the strengths of the attractions between the particles are very similar to the structure and attractions found in the separate liquids.
When these properties are not significantly different in the solution than in the separate liquids, we can assume that the solution has higher entropy than the separate liquids. Therefore, when very similar liquids, like pentane and hexane, are mixed, the natural tendency toward increasing entropy drives them into solution. Exothermic changes lead to an increase in the energy of the surroundings, which leads to an increase in the number of ways that that energy can be arranged in the surroundings, and therefore, leads to an increase in the entropy of the surroundings. Endothermic changes lead to a decrease in the energy of the surroundings, which leads to a decrease in the number of ways that that energy can be arranged in the surroundings, and therefore, leads to a decrease in the entropy of the surroundings. Therefore, exothermic changes are more likely to occur than endothermic changes. We can use this generalization to help us explain why ionic compounds are insoluble in hexane. For an ionic compound to dissolve in hexane, ionic bonds and attractions between hexane molecules would need to be broken, and ion-hexane attractions would form. The new attractions formed between the ions and hexane would be considerably weaker than the attractions broken, making the solution process significantly endothermic. The tendency to shift to the higher entropy solution cannot overcome the decrease in the entropy of the surroundings that accompanies the endothermic change, so ionic compounds are insoluble in hexane. Ionic compounds are often soluble in water, because the attractions formed between ions and water are frequently strong enough to make their solution either exothermic or only slightly endothermic. For example, the solution of sodium hydroxide is exothermic, and the solution of sodium chloride is somewhat endothermic. Even if the solution is slightly endothermic, the tendency to shift to the higher entropy solution often makes ionic compounds soluble in water. The dividing line between what we call soluble and what we call insoluble is arbitrary, but the following are common criteria for describing substances as insoluble, soluble, or moderately soluble. If less than 1 gram of the substance will dissolve in 100 milliliters (or 100 g) of solvent, the substance is considered insoluble. If more than 10 grams of substance will dissolve in 100 milliliters (or 100 g) of solvent, the substance is considered soluble. If between 1 and 10 grams of a substance will dissolve in 100 milliliters (or 100 g) of solvent, the substance is considered moderately soluble. Although it is difficult to determine specific solubilities without either finding them by experiment or referring to a table of solubilities, we do have guidelines that allow us to predict relative solubilities. For example, this guideline could be used to predict that ethanol, which is composed of polar molecules, would be soluble in water, which is also composed of polar molecules. Likewise, pentane (C5H12), which has nonpolar molecules, is miscible with hexane, which also has nonpolar molecules. We will use the Like Dissolve Like guideline to predict whether a substance is likely to be more soluble in water or in hexane. It can also be used to predict which of two substances is likely to be more soluble in water and which of two substances is likely to be more soluble in a nonpolar solvent, such as hexane: Polar substances are likely to dissolve in polar solvents. For example, ionic compounds, which are very polar, are often soluble in the polar solvent water. Nonpolar substances are likely to dissolve in nonpolar solvents.
For example, nonpolar molecular substances are likely to dissolve in hexane, a common nonpolar solvent.
Two additional guidelines are derived from these: Nonpolar substances are not likely to dissolve to a significant degree in polar solvents. For example, nonpolar molecular substances, like hydrocarbons, are likely to be insoluble in water. Polar substances are not likely to dissolve to a significant degree in nonpolar solvents. For example, ionic compounds are insoluble in hexane.