Is plain flour and baking powder the same as self-raising flour?

Plain flour and baking powder are not the same as self-raising flour. Self-raising flour is a mixture of plain flour, baking powder and usually a little salt. Baking powder is a leavening agent, which means that it causes baked goods to rise. It is made of a combination of baking soda, an acid, and a drying agent. When baking powder is mixed with water, the acid and the baking soda react to produce carbon dioxide gas, which causes the dough or batter to rise.

Plain flour is simply finely ground wheat flour. It does not contain any leavening agents, so you need to add baking powder or another leavening agent to it when baking.