Take a look at your hands. Fingerprints and blemishes notwithstanding, your left and right hands probably look pretty much the same: four fingers, one thumb, a palm, and a back. Now try to overlay your left hand on your right and make everything line up. You can’t do it. Line up the palms so they’re both facing down and your thumbs point in opposite directions. Line up the thumbs, and your palms face one up, one down. Your hands are mirror images of each other, but they’re not superimposable, not identical. The term for that phenomenon is chirality, which, appropriately enough, is derived from the Greek word for hand, χειρ (kheir).

Molecules, like hands, can be chiral; the different-handed, mirror-image versions of a single molecule are called enantiomers. Enantiomers have identical physical properties. Their melting points coincide, as do their boiling points. They have the same spectral fingerprints...

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