Racemic mixtures and meso-forms

Here we see four forms of tartaric acid, each of which would seem to be optically active.

The dark red form and the dark blue form are enantiomers, and are optically active.

The two green forms are not enantiomers; although the two mirror images can not be placed directly over one another, the molecules do have an axis of symnmetry (shown by the light blue line) so they are not chiral. Nor are they optically active. They are called meso-forms of the compound.

A racemic mixture is a solution containing both optically active isomers in equal amounts, so that the solution is not optically active.