Baking soda is used when there is an acidic ingredient in the recipe. For example, molasses, buttermilk, chocolate. Please note that baking is like a chemical experiment and ingredients and amounts especially leavening need to be exact to have the recipe work.
It is because they are two DIFFERENT things. Baking Powder contains baking soda but baking soda does NOT contain baking powder. It is sort of independent. Baking soda is also called "Sodium bicarbonate". Both baking powder and baking soda are used as leveling agents so, you can sometimes substitute (replace) yeast with either of them. Hope this helps! ;)
Baking powder is activated by heat and liquid, baking soda is activated by acid. You can replace baking powder with baking soda in your quick bread recipe but you have to make sure there is some acid content too. Use buttermilk, or replace part of the milk content with yoghurt or stir a tablespoon of cider vinegar or Orange Juice into the liquid before you add it.
a yeast dough uses yeast and a quick bread uses baking powder or baking soda.
Quick breads are leavened with baking powder or baking soda. Yeast breads are leavened with yeast, and require more time to rise.
Baking powder is used as leavening in baked goods such as cookies, cakes, quick breads, biscuits and pancakes. It creates gas in the batter that causes the product to rise.
They are chemical (non-yeast) leavening agent usually used in quick breads, cakes and cookies.
Sourdough is a type of bread made with yeast starter instead of dry yeast. Starter is for yeast bread and baking powder and soda are for quick breads and cookies. They are not interchangeable.
Both release carbon dioxide gas bubbles. These, in turn, cause baked goods to rise.
Quick breads such as muffins, unlike yeast breads, can be baked immediately after the ingredients are mixed. In other words, there is no need to wait for leavening to occur before baking the muffins because they rise during baking. Therefore, they are relatively quick to make. Yeast breads on the other hand require a lot of time for leavening before baking and are much more time consuming.Source: "Understanding Food" by Amy Brown
Leavening agents are the ingredients that make the baked goods rise so they are light and airy. It could be yeast, baking soda, or baking powder. Sometimes in cakes, the eggs are whipped into a froth to add air to the cake and this acts as a leavening agent.
A reason as to why quick breads collapse as they are cooling is that the batter was mixed too fast or too vigorously, which create unstable air bubbles in the mix. Other reasons can include that an ingredients, such as baking powder is old, or the quick bread is under-baked.
A well-made quick bread should be moist and hold together well. Not too dry, and not too moist or heavy.
Many baked goods that are not raised with yeast contain baking power; some of the most common are biscuits, muffins (USA), cookies, quick breads, cakes and crackers.
Baking powder is used in cakes since it is relatively 'taste free' and a quick/easy to use leavener. But baking powder is rarely used in breads - yeast is used as the primary leavener in breadmaking. (Yeast is what gives bread it's bread taste plus irregular air bubbles).