Why do enantiomers have identical physical properties but different optical activities?

Enantiomers have identical physical properties because they have the same molecular structure, but different optical activities due to their mirror-image arrangement.

Enantiomers are a type of stereoisomer, specifically, they are mirror images of each other. This means that they have the same molecular formula and the same sequence of bonded atoms, which is why they share identical physical properties such as boiling point, melting point, and solubility. These properties are determined by the overall structure of the molecule and the types of atoms involved, which are the same in both enantiomers.

However, the spatial arrangement of atoms in enantiomers is different. While they are mirror images of each other, they are not superimposable. This is similar to your left and right hands - they are mirror images but cannot be perfectly overlaid on each other. This difference in spatial arrangement is what leads to different optical activities.

Optical activity refers to the ability of a substance to rotate the plane of polarised light. When plane-polarised light passes through a solution of an optically active substance, it is rotated either to the right (dextrorotatory) or to the left (levorotatory). The direction of rotation depends on the spatial arrangement of the atoms in the molecule.

In the case of enantiomers, one will rotate the plane of polarised light to the right, and the other will rotate it to the left. This is because the arrangement of atoms in the molecule interacts with the light in a different way, causing it to be deflected in a different direction. This difference in optical activity is the only physical property that distinguishes enantiomers from each other.

In summary, enantiomers have identical physical properties because they share the same molecular structure, but their mirror-image spatial arrangement causes them to interact differently with plane-polarised light, resulting in different optical activities.