Define a pure substance.

Define a pure substance.

• A. A substance with variable composition
• B. A mixture of different components
• C. A substance that has multiple phases
• D. A uniform substance made up of only one type of particle, which can be an element or a compound.

Answer: (D)

A pure substance has a definite, uniform composition and consists of only one type of particle. This means it is either an element, made of a single kind of atom, or a compound, made of molecules that always combine in the same fixed ratio. Examples include a pure element like oxygen gas or gold, and a pure compound like water or carbon dioxide. Because the composition is fixed and the substance is uniform throughout, all samples share the same properties. A mixture, in contrast, contains more than one substance with a variable overall composition, so its properties can vary.

A pure substance has a definite, uniform composition and consists of only one type of particle. This means it is either an element, made of a single kind of atom, or a compound, made of molecules that always combine in the same fixed ratio. Examples include a pure element like oxygen gas or gold, and a pure compound like water or carbon dioxide. Because the composition is fixed and the substance is uniform throughout, all samples share the same properties. A mixture, in contrast, contains more than one substance with a variable overall composition, so its properties can vary.