Can I use baking soda instead of baking powder?

The short answer: no, there's no substitute, and yes, baking soda is necessary. Baking soda, also known as sodium bicarbonate, has a number of functions in baking - it acts as a leavener, as well, as a browning agent. What does that mean? Think about a slice of bread: it’s full of little holes and pockets; and, it's not a perfectly smooth, flat surface. Those holes and pockets are caused by carbon dioxide, which baking soda gives off when it reacts with an acid. Remember those baking soda-and-vinegar volcanoes from science class? This is essentially the same thing. Baking soda, when it reacts with an acidic component in a recipe, fizzes and bubbles, creating an airier, lighter-textured bread than otherwise. However, there is such a thing as too much baking soda. If a baked good turns out dense and leaden, it's quite possible that excessive baking soda was the culprit. Why? If too much carbon dioxide is produced, the bubbles rise to the top of the bread and burst, instead of hanging out in the middle and making the bread airier. Baking soda's other job is to help browning by neutralizing the natural acidity present in food. If you see a recipe with a disparately large quantity of baking soda relative to the flour (the average ratio is 1/4 teaspoon per cup of flour), it's probably used to neutralize the acid and aid browning. Baking powder is similar to baking soda, though it's about a quarter as strong, and it's mixed together with exactly the right proportion of acid in order for it to activate. Though it may seem counterintuitive when a recipe calls for both, it makes scientific sense.

Nope..You can't susbstitute baking powder with soda..it doesn't work. Your cookies will be ruined so don't try it!! So wait up till you buy the powder and then bake. Some recipes call for both baking powder and soda but you can't replace one with the other.