Which one of the following statements about a compound is NOT correct?

Statement (c) is incorrect: a compound is a PURE substance, not an impure one. A compound has a fixed composition and definite properties, which are characteristics of pure substances. Mixtures, not compounds, are considered impure substances. Statement (a) is correct: compounds are formed when two or more elements combine chemically in a fixed proportion. Statement (b) is correct: a compound's properties differ from those of its constituent elements (e.g., water is liquid while H and O are gases). Statement (d) is correct: separating a compound into its constituent elements requires chemical or electrochemical means, not physical separation.