Which of the following best describes chemical compounds?

Chemical compounds are best described as pure substances formed when two or more different elements chemically combine. This definition is essential because it emphasizes the unique characteristics of compounds compared to other forms of matter. When elements combine chemically, they undergo a process that results in the formation of new chemical bonds; this process transforms the individual properties of the elements into properties of the compound that are distinct from those of the individual elements.

For instance, when hydrogen and oxygen combine to form water, the resulting compound has different properties (like being a liquid at room temperature) than either hydrogen (a gas) or oxygen (also a gas). This transformation is key to understanding why compounds are categorized as pure substances—they have a consistent composition and fixed ratios of their constituent elements.

The other options describe different concepts: mixtures involve physical combinations of substances rather than chemical bonds, solid structures composed solely of ions may refer to ionic compounds but do not encompass all types of compounds, and simpler substances that cannot be broken down are more accurately termed elements, not compounds. Each of these alternatives lacks the completeness and specificity that captures the essence of what a chemical compound is.