Solubility

Most chemical reactions take place in solution. That is, the solutes, or the reagents that you want to react, are uniformly mixed with, or dissolved in, the solvent. There are two types of solutions: single-phase solutions and multiple-phase solutions. In a single-phase, or homogeneous, solution all the solutes are soluble in the same solvent. Reactions take place best in single-phase solutions. This

uniform distribution allows a higher rate of reaction between the reactants because they have a greater amount of contact with each other. Single-phase solutions also allow you to control the solution concentrations and reaction conditions. Multiple-phase mixtures are mixtures in which the individual solutes are not soluble in the same solvent, thus, you must use more than one solvent. Each solvent then forms a different layer in the mixture. In multiple-phase mixtures, the reactants have less contact with each other so that the rate of reaction is generally slower than in a single-phase solution.

One compound is soluble in another as a result of the various intermolecular forces present in both compounds. These intermolecular forces are van der Waals forces, hydrogen bonding, and polarity. Molecular weight also plays a part in the solubility of a compound. The higher the molecular weight the lower the solubility. A compound dissolves in a solvent when the interactions between the compound and the solvent are either similar in strength or stronger than those interactions between molecules in the compound to be dissolved. There are also correlations between the molecular structure of the compounds and their solubility. A simple rule of thumb for estimating solubility is: A compound dissolves most easily in a solvent that is structurally similar to itself. The phrase “structurally similar” means that the solvent and the solute are similar types of molecules. For example, polar solutes or solvents mix with other polar solutes or solvents and nonpolar solutes or solvents mix with other nonpolar solutes or solvents, but polar solutes or solvents do not generally mix with nonpolar solutes or solvents.
Water is highly polar making it an excellent solvent for ionic compounds and small polar organic molecules. If an ethyl group

replaces one of the hydrogens in water, the result is ethanol, a solvent that has a substantially reduced polarity. Ethanol actually has both a polar and a nonpolar end. Although ethanol participates in hydrogen bonding and thus is a moderate solvent for salts and highly polar molecules, it also has van der Waals forces, making it a good solvent for a variety of organic molecules as well. Ethanol is too polar, however, to readily dissolve many low-polarity substances. Replacing both hydrogens in water with ethyl groups produces diethyl ether. Diethyl ether is only moderately polar. It acts as a hydrogen bond acceptor and so is a poor solvent for salts. However, it is a good solvent for a variety of both polar and nonpolar organic molecules.
Water is a poor solvent for low polarity compounds, such as gasoline. Gasoline, a mixture of alkanes, will not dissolve in water. All attempts to dissolve gasoline in water, or water in gasoline, result in a two-phase solution with the gasoline floating on the water. Gasoline won't dissolve in water for several reasons. (1) The molecular structures of the two substances are different. Gasoline, a mixture of hydrocarbons, is nonpolar, and water is highly polar. (2) The

molecular weights of the hydrocarbons are much larger in comparison to water. (3) The van der Waals forces between the molecules of gasoline are much stronger than they are in water. With water, primarily hydrogen bonding and secondarily dipolar attractions determine its intermolecular interactions. Both hydrogen bonding and dipolar attractions are stronger than van der Waals forces. Thus, gasoline does not dissolve in water because such a solution would reduce the number of stronger interactions. With gasoline and water, there is an unfavorable solvent-solute interaction.
Solubility is dependent on the temperature of the solution. In general, the higher the temperature the higher the solubility of a solute in a given solvent. For example, the solubility of benzoic acid in water is 1.7 g/L at 0oC; it increases to 68.0 g/L at 95oC. Figure 4.7 shows a plot of the solubility of benzoic acid in water versus temperature. Note that the solubility of benzoic acid increases rapidly as the temperature increases above75oC.