What defines a mixture in physical science?

A mixture is defined as a combination of two or more substances that maintain their individual properties. In a mixture, the components can be physically separated, and each substance retains its own chemical identity and characteristics. For example, if sand is mixed with salt, both the sand and the salt can be identified separately, and they do not change their chemical nature as a result of being combined.

The other choices represent different concepts in physical science: a single substance that cannot be broken down describes a pure substance or an element, while a homogeneous solution that appears uniform refers specifically to solutions where the components are completely dissolved, forming a single phase. Finally, a material with a fixed composition indicates a pure substance or compound, which has a specific chemical formula and cannot be separated into its component elements by physical means. Thus, only the first choice accurately defines a mixture.