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Right, there’s baking powder and there’s bicarbonate of soda, but what’s the difference? Well, they are both raising agents, and they’re both used in cakes, but they have very different properties and react completely differently. Baking powder actually has bicarbonate of soda in it, as well some other good stuff, so it’s a great overall raising agent. But. If a recipe calls for bicarb alone, do not substitute it with baking powder, or vice versa. The recipe probably won’t work, and you may cry. If you want the proper sciencey explanation for why this is, then read on.
Bicarbonate of Soda is also called baking soda. It has a distinct strong flavour and is usually mixed with acidic ingredients to neutralise the flavour. Baking soda is used in baking to give a good rise. It reacts with heat and produces a carbon dioxide causing the baked goodies to expand and rise.
Baking powder already contains acid so does not need to be mixed with acidic ingredients. In fact baking powder is part baking soda and part acid. Baking powder is used in baking to give a good rise. It reacts to heat or moisture.