Bicarbonate of soda, or baking soda, is an alkali which is used to raise soda breads and full-flavoured cakes such as gingerbread, fruit cake, chocolate cake and carrot cake. It needs an acid (as well as moisture) to activate it so is often combined with cream of tartar, yoghurt, buttermilk or milk.Bicarbonate of soda works by giving off carbon dioxide which expands the mixture. Once the mixture is cooked, the carbon dioxide is replaced by air leaving a light cake or bread.As with all raising agents, use the amount specified in the recipe. Adding extra bicarbonate of soda can result in a peaked or collapsed cake, a strong unpleasant flavour and a greenish tinge.Store it In an airtight container in a cool, dry place.