The carbonyl group of an aldehyde or ketone reacts with different types of nucleophiles through a nucleophilic addition reaction. When the nucleophile is water, the product obtained is a "gem"-diol or geminal diol. This functionalization is unstable and quickly reverts to the parent carbonyl group. The chemistry of "gem"-diols is not usually covered in depth in organic chemistry courses. Herein, we propose a two-step activity to study the isolation of different "gem"-diols: an organic laboratory practice designed for undergraduate students combined with an in-class activity. The approach focuses on the study of heterocyclic compounds with aldehyde moieties using solution and solid-state nuclear magnetic resonance combined with X-ray crystallography.