Notes to Solubility Tests
1. Groups I, II, III (soluble in water). Test the solution with pH paper. If the
compound is not easily soluble in cold water, treat it as water insoluble but
test with indicator paper.
2. If the substance is insoluble in water but dissolves partially in 5% sodium
hydroxide, add more water; the sodium salts of some phenols are less soluble
in alkali than in water. If the unknown is colored, be careful to distinguish
between the dissolving and the reacting of the sample. Some quinones
(colored) react with alkali and give highly colored solutions. Some phenols
(colorless) dissolve and then become oxidized to give colored solutions.
Some compounds (e.g., benzamide) are hydrolyzed with such ease that careful
observation is required to distinguish them from acidic substances.
3. Nitrophenols (yellow), aldehydophenols, and polyhalophenols are suffi-
ciently strongly acidic to react with sodium bicarbonate.
4. Oxygen- and nitrogen-containing compounds form oxonium and ammonium
ions in concentrated sulfuric acid and dissolve.
5. On reduction in the presence of hydrochloric acid, these compounds form
water-soluble amine hydrochlorides. Dissolve 250 mg of tin(II) chloride in
0.5 mL of concentrated hydrochloric acid, add 50 mg of the unknown, and
warm. The material should dissolve with the disappearance of the color
and give a clear solution when diluted with water.
6. Most amides can be hydrolyzed by short boiling with 10% sodium
hydroxide solution; the acid dissolves with evolution of ammonia. Reflux
100 mg of the sample and 10% sodium hydroxide solution for 15–20 min.
Test for the evolution of ammonia, which confirms the elementary analysis
for nitrogen and establishes the presence of a nitrile or amide.