If I used baking soda instead of baking powder..will it ruin my cake? :-(
Yes, Baking Powder has a acidic powder mixed with it to create air bubbles, a rising action.
Baking Soda does not have this.
You could sub if you had some creme of tarter to initiate the soda/acid reaction.
Or (depending on the cake recipe) some sour cream, lemon juice, or buttermilk.
Still it would be a guess. Unless you have creme of tarter. The Google has the proportions for that sub..it's less baking soda..and addition of creme of tarter. Butter Milk, or Sour Cream can also sub as a partner with the soda/acid mix.
I agree with Sam 1148,baking soda needs an acid to activate it. Even if you add some acid, you probably won't get the same results. Most commercial baking powders are double acting, meaning they have an acid that reacts upon mixing with liquid, and another that reacts at higher temperature.
I rarely use baking soda. So it sits the cupboard for ages. And when I need it...it's flat.
Until. I learned to keep one jar un-opened. (it goes stale quickly after opening). That way I can 'proof' the older stuff. Put a bit in water..if it fizzes up like the fresh stuff does, it's good. You have to do this on a freshly opened jar to judge the amount of fizz you get because even the old stuff will fizz a bit..but not enthusiastically.
Soda will not give you equivalent results to powder. As others have said, soda needs an acid to activate it, while powder works without one. In addition, soda will increase the sodium content in your finished product, since baking soda is bicarbonate of sodium (or sodium bicarbonate). May not be an issue health-wise, or might be, but could also end up making your finished dish overly salty as well as not leavened properly.
I learned from Edna Lewis in The Gift of Southern Cooking to make homemade baking powder. It's 2 parts cream of tartar, 1 part soda. It works very well.
I just did this with the Genius banana bread recipe! It's no been baking for 10 minutes over the normal time and is still mushy on the inside of the bread. Needless today I will never make this mistake again...