These 2 items are often confused since they are similar and look alike. They are sometimes used in the same recipe, but are very different. They both are leaveners and chemically different.
This item is also known as bicarbonate soda or sodium bicarbonate. It is a base mineral and when mixed with something acidic, it makes carbon dioxide. So, when a recipe needs baking soda it needs acid. When you use this in some recipe, also mix in something acidic, like brown sugar, lemon, tartar cream, molasses, yoghurt and buttermilk.
This is stronger than baking powder. But, too much baking soda does not mean more lifting and does not react with ANY acid so there might be leftover soda. When this is not neutral with the acid, the recipe will be metallic in taste.
To decide the best measurement, know the rule of thumb: use ¼ tsp baking soda for 1 cup flour.
The baking powder is a mix of baking soda and acids like tartar cream and cornstarch. When making this recipe, the 1st leavening happens when the baking powder wets. The 2nd leavening is when the powder is heated. So it has an acid already. This is the rule for the dosage: use 1 tsp baking powder for 1 cup flour.
Why the 2 items are needed for some recipes?
Some recipes need both items. They usually have some acid and the carbon dioxide from the acid and soda is not enough for more volume. So you have to use both items.
They can expire
Always use fresh baking powder and soda and change them every 3 months. Also in case you are unsure for the expiry date, check the effectiveness prior usage.
To test the powder, put 3 tbsp warm water in a bowl. Add ½ tsp powder and mix. If it reacts, it is fresh. To test the soda, put 3 tbsp white vinegar in a bowl and add ½ tsp baking soda, then mix. If it bubbles, it is fresh.
The last tip is to remember the differences; baking soda= single item, baking powder=a big poof in the oven.
Source and image source: unknownremedy.com