THE CHEMISTRY OF BAKING POWDER
Many food products such as bread, sponge cakes and buns have a honeycomb structure which contains many bubbles. During cooking these bubbles are formed by a gas, and the mixture ‘rises’. In some cases the gas is air which is whipped into the mixture before cooking and which expands during cooking. In other cases the gas is carbon dioxide. This can be formed either from the fermentation of sugar aided by yeast (as in making bread) or by chemicals that react to form carbon dioxide.
The most common chemical used for this purpose is sodium hydrogencarbonate, NaHCO3 (more commonly called sodium bicarbonate, bicarbonate of soda or just ‘bicarb’). This can form carbon dioxide in two ways:
h On heating
2 NaHCO3 (s) → Na2CO3 (s) + CO2 (g) + H2O (l)
h On reacting with an acid, such as hydrochloric acid (HCl)
NaHCO3 (s) + HCl (aq) → NaCl (aq) + CO2 (g) + H2O (l)

The gaseous product of the reaction is the same as with hydrochloric acid - carbon dioxide- but it is produced much more slowly. Potassium hydrogentartrate is a solid and this means that it is possible to mix it with the sodium hydrogencarbonate without the two reacting – they only react in the presence of water. This dry mixture is the basis of baking powder. The reaction is:

One problem with the use of potassium hydrogentartrate is that it is very soluble in water. So as soon as it becomes wet ( when milk is added in a cake recipe, for example ) it dissolves and reacts. This risks all the gas escaping while the cake mix is still liquid and before it goes in the oven. Most baking powders nowadays are so-called ‘double acting’. This means that, along with the sodium hydrogencarbonate, they use a mixture of potassium hydrogentartrate and calcium dihydrogendiphosphate (CaH2P2O6), which is also a solid acid. The potassium hydrogentartrate dissolves and reacts almost immediately (which make the dish ‘rise’ on mixing) while the calcium dihydrogendiphosphate is slower to dissolve and will not react until the mixture is in the oven and the gas bubbles are trapped by the cake as it bakes.
NOTE: Both calcium dihydrogendiphosphate and potassium hydrogentartrate are acidic salts. The hydrogen atoms marked in red in the formula of calcium dihydrogendiphosphate below are acidic. That is, the salt can dissociate, losing the red hydrogen atoms as H+ ions.

Source: article from the book of Kitchen Chemistry, written by Ted Lister and Heston Blumenthal, published by Royal Society of Chemistry, London in 2005.
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