What term is used to describe chemical substances made up of two or more elements in fixed ratios that cannot be separated by physical means?

The term that describes chemical substances composed of two or more elements in fixed ratios that cannot be separated by physical means is "compounds." Compounds are distinct entities formed through chemical reactions, where the elements lose their individual properties and create a new substance with different characteristics.

For example, water (H2O) is a compound made from two hydrogen atoms and one oxygen atom, combined in a fixed ratio. It cannot be separated into hydrogen and oxygen through physical means alone; a chemical process, such as electrolysis, is required.

In contrast, other options like elements refer to pure substances that consist of a single type of atom and cannot be broken down into simpler substances by chemical means. Molecules represent two or more atoms bonded together, which can be elements or compounds. Lastly, mixtures consist of two or more substances that are physically combined and retain their individual properties, allowing them to be separated by physical means.