baking soda (sodium bicarbonate) and tartaric acid
Baking powder is a leavening agents, which means it is added to baked goods before cooking to produce carbon dioxide and cause it to 'rise'. Baking powder contains baking soda, but the two substances are used under different conditions.
Carbon dioxide is released.
Baking powder is baking soda plus acid, and is used in recipes that don't have acid--acid being what makes baking soda leaven things. I've used baking powder in recipes that call for baking soda, and it didn't hurt them; maybe they were a bit fluffier but that's okay.
No it is not. Borax and baking soda are two very different substances.
I think you mean baking soda, not baking powder. Anyway, that's not a very good answer. The correct answer isn't that interesting. The answer is that these two substances simply don't react. It takes an acid, vinegar, to react with the base, bicarbonate. An acid neutralizes the base and carbon dioxide is given off. I guess the short, best answer is that alcohol is not an acid.
Baking powder has two active ingredients: Cream of tartar (tartaric acid) and baking soda (sodium bicarbonate).
If you mean baking powder, then just mix two parts cream of tartar with one part baking soda.
Yes, but one should reduce the amount of salt in the recipe. When one does not have baking soda, it is best to use a cookie recipe that calls for baking powder, because the two ingredients are not identical.
liquid and powder
No you can't. Baking powder is a leavening agent while cornstarch is a thickening agent. The same applies to baking soda, which will also make doughs "rise," whereas cornstarch will not.
Baking soda is already an ingredient of baking powder so by mixing the two you would be simply changing the proportion of baking soda to baking powder. It will make no real difference to the outcome if you make up the volume of raising agent the recipe calls for with any proportion of baking powder to baking soda. As a safeguard you may like to add a teaspoon of white vinegar or a tablespoon of yoghurt or buttermilk to a cupcake recipe made with baking soda - the acid in the vinegar/yoghurt/buttermilk will react with the Soda to release carbon dioxide which will make you cupcakes rise.
Baking powder is a mixture of baking soda, starch and an acid (often tartar) to activate the baking soda. Since baking soda is already in the baking powder, it is possible you will not need any additional baking soda. Baking powder and baking soda are used to "raise" or puff up the pastry - too little and it will not raise properly...too much and it will taste like soda. If you do not have a specific recipe, you will need to experiment.