What baking powder means
Baking powder is a raising agent used to make cakes, scones, muffins, pancakes, biscuits and other baked goods rise. It is a mixture of bicarbonate of soda, which is an alkali, dry acids, such as cream of tartar or other acid salts, and a starch, usually cornflour, which absorbs moisture and keeps the ingredients from reacting in the tub. When baking powder is mixed with liquid in a batter or dough, the acid and alkali react to release bubbles of carbon dioxide gas. Most modern baking powders release some gas when wet and more when heated in the oven, which gives a steady, reliable rise.
The bubbles expand in the heat of the oven, and the batter sets around them, giving a light, airy texture. Because it contains its own acid, baking powder works without needing acidic ingredients in the recipe, unlike bicarbonate of soda, called baking soda in the US, which needs buttermilk, yoghurt, lemon juice or another acid. Baking powder is also the ingredient that turns plain flour into self-raising flour. Too much can make bakes taste bitter or soapy and cause them to rise and then collapse.
It loses its strength over time once opened, so it is worth testing an old tub before using it. You will see baking powder in cake, scone and pancake recipes.