A pure substance is defined as:

A pure substance is defined as:

• A. A substance with fixed composition and the same intrinsic properties throughout.
• B. A substance with variable composition
• C. A mixture of two or more substances
• D. A substance with impurities only

Answer: (A)

A pure substance has fixed composition and uniform properties throughout. This means every portion of the material contains the same chemical species in the same proportions, so its physical and chemical properties—like melting point, boiling point, density, and refractive index—are consistent everywhere in the sample. That consistency is what sets a pure substance apart from mixtures, where the composition can vary from one portion to another and the properties can change accordingly. For example, pure water is H2O everywhere, while saltwater contains both salt and water and can vary in the amount of salt. An impure sample also breaks the rule, since impurities are additional substances mixed in.

A pure substance has fixed composition and uniform properties throughout. This means every portion of the material contains the same chemical species in the same proportions, so its physical and chemical properties—like melting point, boiling point, density, and refractive index—are consistent everywhere in the sample. That consistency is what sets a pure substance apart from mixtures, where the composition can vary from one portion to another and the properties can change accordingly. For example, pure water is H2O everywhere, while saltwater contains both salt and water and can vary in the amount of salt. An impure sample also breaks the rule, since impurities are additional substances mixed in.