Baking soda (aka sodium bicarbonate) is a leavening agent used in baking cakes, cookies, bread and other pastries. Baking soda needs an acidic ingredient to get into reaction with. This reaction creates carbon dioxide, which allows your baked pastry to rise. Therefore, when a recipe calls for baking soda it also calls for some type of acid such as yoghurt, lemon juice, vinegar, buttercream, apple juice, etc. When the reaction comes together with the heat of the oven, the rising effect is intensified.

Baking soda and baking powder are confused most of the time. Even though they are both leaveners and can sometimes be used interchangeably, they are chemically different products. The leavening power of baking soda is 3-4 times of baking powder. Baking soda requires an acidic element to get into reaction whereas baking powder has already an acidic compound in itself.

Tips:
The chemical reaction begins as soon as the ingredients are mixed with baking soda. Therefore, you need to be really fast to bake your recipes calling for baking soda. Otherwise, the reaction will go down and whatever you are baking might fall flat.

If your recipe calls for an acidic ingredient, you can substitute baking powder with baking soda. Use half a dessertspoon of baking soda instead of 2 dessertspoons of baking powder.

Baking soda expires in about 3 months. As it gets old, its effectiveness starts to die. Therefore, you need to replace your baking soda every 3 months.

Good idea:

If you have baking soda waiting on the shelf for a while, there is a method to test whether it is still fresh or not. To test it, in a bowl mix half a teaspoon of baking soda with 3 tablespoons of white vinegar. Stir it lightly. If it is fresh, it will react rapidly and bubbles will occur. If there is no reaction, you need to throw it away and buy a new, fresh package.