ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

Isomers are compounds that share the same molecular formula but differ in their structural arrangement or spatial orientation, resulting in distinct properties. This diversity in structure leads to the existence of various types of isomers, including structural isomers (where the connections among atoms differ) and stereoisomers (where the spatial arrangement of atoms differs without changing the connectivity).

While it is true that isomers can have different physical properties, such as boiling points, melting points, and solubilities, this is not universally true for all isomers. In fact, some isomers might exhibit very similar physical properties, making option A inaccurate as a blanket statement.

Identical molecular weights among isomers is also true; isomers share the same molecular formula, which means they will always have the same molecular weight. However, this is not as distinguishing a feature as the structural differences highlighted, making option C less relevant in identifying what isomerism is fundamentally about.

Furthermore, isomers can exist in various physical states, including solids, liquids, and gases, which contradicts the assertion in option D. Therefore, while several of the provided statements contain elements of truth, only the statement regarding the same molecular formula and different structures accurately captures the essence of isomerism.