Final answer:
Molecules can be classified as isomers, identical molecules, or neither, based on their atomic arrangements. Aliphatic hydrocarbons are categorized as alkanes, alkenes, or alkynes, while enantiomers are nonsuperimposable mirror images. Cis-trans isomerism is seen in alkenes and cyclic compounds.
Step-by-step explanation:
The question relates to identifying and classifying molecules based on their structures and characteristics in organic chemistry. When classifying pairs of molecules, they can be identified as isomers, identical molecules, or neither, based on how their atoms are arranged and connected. Isomers are compounds with the same molecular formula but different structural arrangements of atoms.
For part 4 and 6 of the question, you are asked to determine whether each molecule is an aliphatic or aromatic hydrocarbon. Aliphatic hydrocarbons can further be classified into alkanes, alkenes, and alkynes, depending on the type of bonding between the carbon atoms (single, double, or triple bonds, respectively). Aromatic hydrocarbons, on the other hand, are characterized by having a stable ring of carbon atoms with alternating single and double bonds.
The answer to part 2 is that molecules whose structures are nonsuperimposable mirror images are called enantiomers. This is a specific type of stereoisomers.
Regarding cis-trans (geometric) isomers covered in the Concept Review Exercises, these are isomers that differ in the spatial arrangement of groups around a double bond or a ring structure. Alkenes with a double bond and cyclic compounds can exhibit cis-trans isomerism, where 'cis' means the same side and 'trans' means the opposite side. The given molecular formula CH₂CH₂, however, does not show isomerism because it represents ethene, which has no substituents to create cis-trans isomerism.