Indole has a melting point of 52.5°C, a boiling point of 253-255°C (decomposition), 128-133°C (3.733×10Pa), a relative molecular mass of 117.15, and a relative density of 1.22. Indole is moderately polar, soluble in hot water, benzene, chloroform and petroleum ether, easily soluble in ethanol, ether and toluene, but insoluble in mineral oil and glycerin. Since the solubility of indole in hot water is extremely large, but it is very small in cold water, it is about 1:540 at 25°C. Therefore, water can be used as a good solvent for recrystallization and purification of indole. Indole is very weakly alkaline (pK=—3.5). It can volatilize with water vapor. It can turn red when placed in the air or exposed to light. It can be resinized under the action of acid.
