Phosphorus pentachloride (PCl5) is a chemical compound composed of one phosphorus atom and five chlorine atoms. It is a yellowish-white solid that fumes in moist air due to its reaction with water vapor. PCl5 is highly reactive and can hydrolyze rapidly in the presence of moisture to form phosphoric acid and hydrochloric acid. It is commonly used as a reagent in organic chemistry for the conversion of carboxylic acids to acyl chlorides, which are useful intermediates in the synthesis of a variety of organic compounds. PCl5 is also used in the production of pesticides, plasticizers, and flame retardants. It has strong irritant properties and can cause severe burns upon contact with the skin, eyes, or respiratory tract. Therefore, it must be handled with great care and under proper safety precautions.