Baking soda
Unlike baking powder, baking soda is made of a single ingredient: a carbonate. It is classified as “E500” and is made from seawater or plain salt. It is a natural mineral.
As baking soda does not contain any acids itself, it will only work if it is used in a recipe already incorporating some other acid. This could for example be yoghurt, vinegar or lemon. If you are baking cookies, for example, this could be honey or refined brown sugar.
Tip: if you want to check if your baking soda is still good to use, drop a small spoon of the powder in warm water with some vinegar or lemon. If you see it bubbling, everything is fine.