On the other hand, no amount of mental activity can superimpose the two trans -isomers. They are in fact mirror images of one another, like a left hand and a right hand. Their boiling points, spectra, densities, and many other physical properties are identical, but they are different compounds. By definition, a chiral compound is one with a non-superimposable mirror image. An achiral compound can be superimposed on its mirror image.
Enantiomers rotate the plane by equal amounts in opposite directions.
This property is easily detected and measured in a polarimeter and is used to determine the enantiomeric purity or enantiomeric excess of a given enantiomer in a mixture. If both enantiomers are present in equal amounts, which is the normal outcome of a synthesis using achiral reactants, the mixture is called a racemic mixture and does not rotate the plane of polarized light at all. It is a major program of many drug companies at the present time to offer enantiomerically pure drugs instead of racemic mixtures. In most cases, only one enantiomer is physiologically effective, and the other may be inert or even harmful.