Here there are some facts that will help you to understand how they work.

WHAT IS THE DIFFERENCE?

Both baking soda and baking powder are leavening agents. These agents react in baked goods, like cakes, cookies and other desserts, producing carbon dioxide bubbles, which make the dough "rise". However, the two substances are used under different conditions.

BAKING SODA

Baking soda is pure sodium bicarbonate. It is a base, and it needs an acidic ingredient to react with.

When baking soda is combined with moisture and acidic ingredients, like yogurt, buttermilk, brown sugar, honey or lemon juice, bubbles of carbon dioxide, water and salt are produced. The bubbles expand under oven temperatures, making baked goods rise.

This reaction starts immediately when mix the ingredients. It is very important to bake immediately when recipes call for baking soda.

BAKING POWDER

Baking powder contais sodium bicarbonate, in other words, baking soda. Also, it includes an acidifying agent called cream tartar, and a drying agent, usually starch. It reacts with water, milk, eggs and other water-based liquid ingredients, producing carbon dioxide bubbles.

Nowadays, the most common baking powder is double acting. Bubbles appear at two different times and conditions. The first reaction happens immediately upon adding a water-based liquid ingredient. The second leavening occurs in the oven, when the mixture is heated.

It is important to bake right away the dough, before the bubbles disappear, and also, avoid over-mixing, so the bubbles don't stir out of the dough.

USING BAKING POWDER INSTEAD OF BAKING SODA

Be careful! Baking soda is 4 times stronger than baking powder. So, if your recipe calls for baking soda and you use baking powder, make sure that you are using the correct amount of powder.

If you substitute baking soda for baking powder, you will have to use 1 teaspoon of baking powder per one cup of flour. The general rule is 1/4 teaspoon of baking soda per cup of flour.
The ingredients in baking powder have an effect on the taste of the goods. The mixture might taste bitter, but this isn't necessarily bad.

If your recipe calls for baking powder and you don't have it, you will need to mix 2 teaspoons of cream tartar and 1 teaspoon of baking soda. If you don't use the homemade baking powder immediately it will clump together, so to avoid it, add 1 teaspoon of cornstarch and store the mixture in a airtight container.

DO THEY EXPIRE?

Yes! You can always buy and replace them. However, you can test baking soda and baking powder. And if they are homemade, before use them, you should test them to make sure that they are still fresh.

TESTING

          * To test baking soda, drip 3 tablespoon of vinegar or lemon juice onto 1/2 teaspoon of baking soda, and stir. It should bubble quickly and vigorously. If it doesn't, you should replace your baking soda.