Can I replace baking soda with bread flour in a cake recipe that already has baking powder? Google was NO HELP, and I need to make this birthday cake really today. Buying baking soda just isn’t a good use of my time or money, and I don’t have the time anyways...
No. Not at all.
Baking soda is a leavening agent. Together with baking powder it gives your cake lift and airiness. Bread flour is just flour with a higher protein content. If anything, it will make your cake more dense (though not in such small quantities as you would have for baking soda). If a recipe calls for both baking soda and baking powder, the recipe is balanced to rely on both and omitting one will cause the recipe to fail (to some degree). How it will fail will depend on the recipe, which you haven't included.
If you already have baking powder, and don't have the time to find baking soda, I recommend that you instead find a different cake recipe that uses ingredients you already have at home - specifically one that does not require baking soda at all. Many cake recipes use only baking powder, so this should not be overly difficult.
If you want to get creative, you may be able to replace the baking soda with baking powder but doing so may be difficult, as noted in this article from Bon Appetit:
Can I substitute baking powder for baking soda?
Because baking powder is made of baking soda combined with other ingredients, it's difficult to give a straight formula to convert it into pure soda, and substitutions are not recommended. Baking soda is much stronger than baking powder, so proceed with care. By trying to provide enough leavening, you may inadvertently end up with a chemical taste in the final product.
Because baking soda is a strong leavener, it's important to use the exact amount called for in the recipe. While not enough baking soda will give an inadequate rise and a heavy finished product, adding too much will cause baked goods to fall. This produces a heavy result, too.
So, as this says, not only is it difficult to substitute baking soda with baking powder, omitting it or even using too little can cause your cake to be heavy.
Also, since you mention bread flour, I hope you're not using bread flour for your cake instead of all purpose flour or cake flour. These three flours are quite different and will cause different outcomes if you use one when the recipe calls for another.
No, baking soda and flour serve different purposes in a cake. Baking soda is a leavener: it makes the cake rise. Flour gives the cake structure.
The way baking soda works is that it is a base, and when it reacts with an acid, it creates air bubbles which cause rising. Baking powder, on the other hand, contains both acid and base, and creates bubbles when it gets wet.
Likely, the recipe calls for both baking powder and baking soda because there is another acidic ingredient (such as lemon juice, buttermilk, or dark brown sugar) to balance the acid-base ratio. You may be able to reduce the acidic ingredient, remove the baking soda, and add a little more baking powder - but I can't say for certain without reading the whole recipe.