Q.Which of the following is not true about enantiomers? (A) They have the same density. (B) They have the same melting or boiling point. (C) They have the same specific rotation. (D) They have the same chemical reactivity.

Concept: Enantiomers are non-superimposable mirror-image molecules. Their physical properties (density, melting/boiling point) are identical in an achiral environment, but they interact differently with plane-polarised light and with other chiral species.

Reasoning:

1. Enantiomers have identical physical properties like density and melting/boiling point — so (A) and (B) are true.
2. Specific rotation is equal in magnitude but opposite in sign for enantiomers — so (C) is false. …

Enantiomers are like your left and right hands — mirror images that cannot be superimposed. In chemistry, this “handedness” (chirality) leads to a fascinating split: most physical properties are identical, but interaction with polarised light is opposite. That’s the key to spotting the wrong option here.

Let’s check each statement one by one.

1. Density — Enantiomers have the same density.

   Density depends on molecular mass and packing in the bulk state. Since enantiomers are identical in every way except spatial arrangement at a chiral centre, their crystals or liquids pack identically. So density is the same. Option (A) is true.

2. Melting or boiling point — Again, these are bulk physical properties.

   In a pure sample, enantiomers have identical melting and boiling points. (A mixture of equal amounts, a racemate, may have a different melting point — but the question asks about enantiomers themselves, not their mixture.) So (B) is true.

3. Specific rotation — This is the trick.

   Specific rotation $[\alpha]$ measures how much a compound rotates plane-polarised light. Enantiomers rotate light by exactly the same magnitude but in opposite directions — one is (+) and the other is (−). So they do not have the same specific rotation; they have equal but opposite values. Option (C) is false — and that’s what we’re looking for.

4. Chemical reactivity — This needs a careful nuance. …