The short answer to this question is no.
The long answer is this.
While baking powder and baking soda look the same and both basically do the same job of helping our baked goods to rise beautifully, they do this under very different circumstances and conditions.
So, baking soda is pure bicarbonate of soda or sodium bicarbonate. Because it’s a base, it needs moisture and acid to react, so ingredients like buttermilk or yoghurt are used when baking with baking soda. They also balance the flavour of the baking soda. The baking soda reacts with the acidic ingredient and forms bubbles. These are what expand in the heat of the oven and cause your cakes and other baked goods to rise. The reaction between the baking soda and acidic ingredient happens immediately, so if your recipe calls for baking soda, you need to be using the batter straight away or whatever you’re baking is going to flop. ( sad but true)
Baking powder on the other hand does contain sodium bicarbonate but also other things as well. Namely, the acids that it needs to react with, and because the baking soda needs moisture to react, there’s also a drying agent like cornflour mixed in as well. Most baking powders are ” double acting”. This means that there will be a reaction at room temperature first and then while in the oven. The more bubbles created, the fluffier the texture of the cake or whatever it is you may be baking. Important to remember , is also that baking powder has a shelf life of 6 months, so be sure to check the expiry date on the packaging.
So the big question… Can I substitute baking powder for baking soda and vice versa if I’m too lazy to go out and buy it? Well…substituting either will affect the flavour, texture and even colour of your baked goods, but here are some cheats from About.com 🙂