What defines a mixture?

A mixture is defined as an impure substance that consists of two or more different elements or compounds that are physically combined but not chemically bonded. This means that the individual components retain their own properties and can be separated by physical means, such as filtration, distillation, or evaporation.

In the correct choice, the emphasis is on the mixture being made from different elements or compounds mixed together. This highlights that no chemical reaction occurs between the components, which preserves their individual characteristics. For example, in a saltwater solution, both the salt and water retain their own properties even though they are mixed.

Other options describe concepts that do not align with the definition of a mixture. For instance, the notion of a substance made of only one type of atom corresponds to an element, which is not a mixture. Similarly, a statement about an impure substance made from different elements or compounds that are chemically joined describes a compound, which is formed through chemical reactions and thus does not fit the definition of a mixture. Lastly, describing a compound with a fixed composition again indicates a substance formed by a chemical bond, which rules it out as a mixture.