What property of organic molecules did Pasteur discover by studying tartaric acid crystals?

By studying tartaric acid crystals, Pasteur discovered molecular chirality: molecules can exist in left- and right-handed forms that are mirror images but cannot be perfectly superimposed.

In the 1840s, Pasteur investigated tartrate salts and noticed that crystals in a sample of racemic tartrate had two subtly different shapes. He manually separated the right- and left-handed crystals and found that their solutions rotated polarized light in opposite directions. The crystals had the same chemical composition, but their internal arrangements were mirror-image forms.

His work, published in 1848, provided the first experimental demonstration that molecular structure could be asymmetric. It became a foundation of stereochemistry and helped explain why compounds with identical formulas can behave differently in biological systems. This is why chirality is sometimes illustrated with hands: a left hand and a right hand correspond, but cannot be placed exactly on top of each other.

The discovery is often confused with polarization or racemization. Polarization was an observable optical effect, and racemic acid was the mixture Pasteur separated; molecular chirality was the deeper structural property he revealed.