This is a sample of 4 grams of solid phosphorus pentachloride, sealed in an ampoule, which protects it from moisture. Phosphorus pentachloride is exceedingly moisture sensitive and it is one of the strongest chlorinating agents, which can replace -OH groups from organic molecules by -Cl atoms, e.g. it can convert organic acids of structure R-C(O)OH to the acid chlorides R-C(O)Cl.

Theoretically, phosphorus pentachloride should be a white crystalline solid. In practice, this compound has a light yellow/green color, due to partial decomposition to PCl₃ and Cl₂, which are trapped in the crystal lattice. When phosphorus pentachloride is added to water, then it looses its green color and a colorless solution of phosphoric acid and hydrochloric acid is obtained. With a small quantity of water, the colorless liquid POCl₃ is formed and gaseous hydrogen chloride.