Preparation of Deuterium

Ordinary (normal) water already contains a tiny natural proportion of heavy water -- about $1.6 \times 10^{-4}\%$ D₂O. The key to concentrating this trace into usable deuterium is a kinetic difference: protium water (H₂O) dissociates at the electrodes faster than heavy water (D₂O) does. So when a sample of ordinary water is electrolysed, ordinary hydrogen gas is liberated much faster than deuterium gas, which means the D₂O left behind in the liquid becomes progressively more concentrated as electrolysis continues.

$2\,D_2O \xrightarrow{\text{Electrolysis}} 2\,D_2 + O_2$ …