Much information can be obtained about an unknown substance by performing simple solubility and chemical tests.

Determining the solubility behavior of an unknown in water, 5% sodium hydroxide solution, 5% sodium bicarbonate solution, 5% hydrochloric acid solution, and cold concentrated sulfuric acid will yield three kinds of information.

First, the presence of a functional group is often indicated. For instance, because hydrocarbons are insoluble in water, the mere fact that an unknown is partially soluble in water indicates that a polar functional group is present.

Second, solubility in certain solvents often leads to more specific information about the functional group present. For example, an unknown may be insoluble in water, but soluble in 5% sodium hydroxide solution. In this case, the solubility of a water insoluble unknown in basic solution is a strong indication that an acidic functional group (like a carboxylic acid) is present.

Third, information about molecular size and shape can often be obtained. For example, in homologous series of compounds, the members with fewer than five carbon atoms are water soluble, whereas the higher homologs are not. Additionally, in homologous series of compounds, chain branching and positioning of functional groups toward the center of the molecule increases water solubility.