Which of the following best describes carbon dioxide (CO_2)?

• A. It is a mixture because it contains more than one element.
• B. It is a pure substance because it has a fixed composition and distinct chemical properties.
• C. It is a heterogeneous mixture because its components are not uniformly distributed.
• D. It is an element because it is made up of only one type of atom.

Answer: (B)

Verified step by step guidance

1. Understand the definitions of the terms involved: an element is a substance made of only one type of atom; a pure substance has a fixed composition and distinct chemical properties; a mixture contains two or more substances physically combined; a heterogeneous mixture has components that are not uniformly distributed.
2. Analyze the composition of carbon dioxide (CO_2): it consists of carbon and oxygen atoms chemically bonded in a fixed ratio (1 carbon atom to 2 oxygen atoms).
3. Recognize that because CO_2 has a fixed chemical formula and consistent properties, it is not a mixture but a pure substance.
4. Note that CO_2 is not an element because it contains more than one type of atom; elements consist of only one kind of atom.
5. Conclude that the best description of CO_2 is that it is a pure substance due to its fixed composition and distinct chemical properties.