What is baking soda?

To answer the first part of the question “what is baking soda”. Baking soda and Bicarbonate of Soda is the same ingredient.

When baking soda is combined with moisture and an acidic ingredient (e.g. yogurt, chocolate, buttermilk, honey), the resulting chemical reaction produces bubbles of carbon dioxide that expand under oven temperatures, causing baked goods to rise. The reaction begins immediately upon mixing the ingredients, so you need to bake recipes which call for baking soda immediately, or else they will flop.

What is Baking Powder

Baking powder contains sodium bicarbonate, but it includes the acidifying agent already (cream of tartar), and also a drying agent (usually starch). Baking powder reacts in two phases and can stand for a while before baking. With double-acting powder, some gas is released at room temperature when the powder is added to dough, but the majority of the gas is released after the temperature of the dough increases in the oven.

If a recipe calls for a specific ingredient, be it soda or powder, it would be advised to stick to the recipes requirements. I think it is always recommended to have both baking soda and baking powder in your cupboard, as in some instances a recipe might require both ingredients