Which of the following best defines a chemical?

A chemical is best defined as an element, chemical compound, or mixture of elements or compounds. This definition encompasses all types of substances that have a defined composition and properties, including individual elements like hydrogen or oxygen, compounds such as water or sodium chloride, and mixtures that may contain various chemicals combined in different ratios. This comprehensive definition is essential in scientific discussions regarding reactions, properties, and behaviors of substances in various environments, especially in laboratory settings where precise identification and understanding of materials are crucial for safety and experimentation.

The other options provide limited or incorrect definitions. A compound composed of only two elements excludes many important substances that are considered chemicals, while the characterization of a liquid that can generate vapor overlooks solids or gases that are also classified as chemicals. Lastly, the idea that any substance that can be tested in a laboratory includes materials that may not be chemically defined, thus misrepresenting the nature of what constitutes a chemical.