Baking soda and baking powder

Yeast is slow. Cakes, pancakes and muffins rise in minutes, using chemical leaveners, which make CO₂ by a simple reaction instead of a living organism.

The key chemical is sodium bicarbonate, NaHCO₃, also called baking soda (bicarbonate of soda). Mixed with an acid in water it reacts:

NaHCO₃ + H⁺ → Na⁺ + H₂O + CO₂

The acid gives away a hydrogen ion (H⁺), the bicarbonate takes it and breaks apart into water and carbon dioxide gas. That is the fizz.

Baking soda needs an acid in the recipe. Everyday acidic ingredients include yoghurt, buttermilk, lemon juice, vinegar, honey, brown sugar and natural cocoa powder. If a recipe has soda and no acid, some soda stays unreacted.

Baking powder is a ready-made mixture: sodium bicarbonate, one or more dry acids, and starch (to keep the powder dry and separate). Add water and the acid dissolves and meets the soda, so no acidic ingredient is needed. Most baking powders are double-acting: one acid reacts when the batter gets wet, and another reacts only when the batter is hot in the oven. That gives a second burst of gas where it helps most.

Now count the gas. The molar mass of NaHCO₃ is about 84.0 g/mol. One gram is 1 ÷ 84.0 = 0.0119 mol. Each mole of bicarbonate that reacts with an acid gives one mole of CO₂. At 25 °C and 1 atmosphere, one mole of gas takes about 24.5 litres, so:

0.0119 mol × 24.5 L/mol ≈ 0.29 litres of CO₂ per gram of baking soda.

So 2 g of soda makes about 0.6 L of gas, roughly a small drinks bottle full. A real cake batter holds back part of it, and some gas is dissolved or lost, so this is the maximum.

Baking soda also reacts alone, with heat. Above roughly 80 °C it breaks down by itself:

2 NaHCO₃ → Na₂CO₃ + H₂O + CO₂

Note the difference: only half a mole of CO₂ per mole of bicarbonate, and it leaves sodium carbonate, which is strongly alkaline. Too much unreacted soda or carbonate gives a soapy or metallic taste and a yellow-brown crumb. Alkaline batter also browns faster, a clue we use in World 3.

Because the reaction starts when the wet and dry ingredients meet, bakers mix quickly and bake soon, so the gas is not wasted.