Baking powder is a mixture of cream of tartar and sodium bicarbonate. In baking, when liquid is added, cream of tartar (an acid) reacts with sodium bicarbonate (an alkali), and releases bubbles of carbon dioxide, causing the cake to rise. Most commercial brands are double–acting: they produce bubbles when activated first by liquid, then by heat. To make a single–acting version, mix 1/2 teaspoon of cream of tartar and 1/4 teaspoon of sodium bicarbonate to equal about 1 teaspoon of baking powder in a recipe. Don’t be too heavy–handed when using baking powder as too much will taint the cake with a slight soapy flavour. Replace every 6 months—to see if it’s still active, stir some into a cup of hot water—it should bubble vigorously. Bicarbonate of soda is also a rising agent, but needs an acid such as buttermilk, yoghurt or sour cream to activate it. Bicarbonate of soda starts acting as soon as liquid is added, so cook the mixture quickly.