Baking soda (not to be confused with
baking powder) is sodium bicarbonate (NaHCO3) that is added to baked goods to
make them rise. Recipes that use baking soda as a leavening agent also contain
an acidic ingredient, such as lemon juice, milk, honey or brown sugar.
When you mix together the baking
soda, acidic ingredient and liquid you'll get bubbles of carbon dioxide gas.
Specifically, the baking soda (a base) reacts with the acid to give you carbon
dioxide gas, water and salt.
This works the same as the classic
baking soda and vinegar volcano except instead of getting an an eruption the
carbon dioxide fizzes to puff up your baked goods. The reaction occurs as soon
as the batter or dough is mixed, so if you wait to bake a product containing
baking soda the carbon dioxide will dissipate and your recipe will fall flat.
The gas bubbles expand in the heat of the oven and rise to the top of the
recipe, giving you a fluffy quickbread or light cookies.
Waiting too long after mixing to bake
your recipe can ruin it, but so can using old baking soda.