What do you add to cream of tartar make bicarbonate soda?

You don't add anything to cream of tartar to make baking soda (bicarbonate soda).

Cream of tartar and baking soda are two different ingredients.

* Baking soda (sodium bicarbonate) is a base.

* Cream of tartar (potassium bitartrate) is an acid.

They are often used together in baking because the acid in cream of tartar reacts with the base in baking soda to produce carbon dioxide gas, which creates the rise in baked goods.