Baking soda and baking powder have the similar name but actually it is different compound. Many people mix it up even though they have different chemicals, even when they use it for cooking. This is the main difference you should know in order to be able to differentiate them.
Baking soda or also known as bicarbonate of soda or sodium bicarbonate is an ingredient that acts as a base mineral. When mixed with acidic ingredients it dissolves and creates carbon dioxide.
When used for cooking you should remember that you will need to add something acidic such as yogurt, brown sugar, lemon juice or tartar cream.
In comparison to baking powder, the baking soda is much stronger its compound that provides a taste of iron. This is recommended correlation for mixing baking soda: ¼ teaspoon of baking soda and 1 cup flour. You should keep to this mixture whenever you use it in your recipe.
On the other hand, baking powder is a mixture of acids such as baking soda, cornstarch and tartar cream. If used in cooking the two reactions will occur once it becomes wet and the second is when it is heated. This is the recommended use in a recipe: 1 teaspoon of baking powder per one cup of flour. Have in mind this correlation of mixture recommended when used it in recipe.
In case your recipe requires using both this is due to the fact that it requires to keep the balance because the carbon dioxide that is created as a result of the reaction with the baking soda it needs to leaven the volume.
Date of expiration is something that both baking soda and baking powder have. It is best to use them fresh rather than wait until the last two-three months.
There is a way you may test the effectiveness of them. Best way is to put ½ tablespoon of baking soda in white distilled vinegar (3 tablespoons) and stir it. If the mixture bubbles then you will know it is fresh.