Crack cocaine is cocaine hydrochloride processed with ammonia or sodium bicarbonate (baking soda) and water, and heated to remove the hydrochloride thus producing a form of cocaine that can be smoked. The term "crack" refers to the crackling sound heard when the mixture is smoked (heated), presumably from sodium bicarbonate.

The purpose of the baking soda is to make the crack smokable - the baking soda alone does not get you high and is thus not a controlled substance. To make crack, you first must have cocaine, so we just keep cocaine illegal.

This is often the case with chemistry. Two things apart can be fine but when put together make something dangerous or two things together can be fine but are dangerous apart.