What’s the difference?
They can be rather confusing, having similar uses and names but in short, baking soda is a pure substance of 100% sodium bicarbonate whereas baking powder contains baking soda but another ingredient too, cream of tartar, a similar looking white powder that has stabilising properties and is acidic.
To create the carbon dioxide pockets in the dough for rise, the bicarbonate of soda needs to react with an acid. Cream of tartar is acidic already, so doughs/batters that called for baking powder will rely on any liquid to activate the cream of tartar, creating an acid that will react with the baking soda already in the baking powder.
Baking soda (bicarb) has nothing else in it, so it relies on an acid to kickstart it. These tend to be things like buttermilk or cocoa powder which is why red velvet cake, for example, tends to call for bicarbonate of soda.
When using baking soda, the acidic element is neutralised when carbon dioxide is produced in the bake. What if acid is required for flavour? You won’t retain a distinctive taste and tang in savoury muffins if you only use baking soda. Combining the two will give a rise and also maintain flavour.
Sometimes, baking soda can give a metallic and plain odd tangy flavour and some things need lots for a good rise. This is why some recipes call for both baking soda and powder, to help give more lift without the weird taste, provided the balance of the two leaveners is precise.