The magic bit really is dead simple. But since I failed chemistry so many years ago, it bares laying out, plain as can be =P
Sodium bicarbonate (baking soda, or NaHC03) neutralizes acid, and in the process creates sodium (Na), water (H20), and carbon dioxide (C02). It does this fairly quickly, and the C02 escapes in so many little bubbles. Fizz.
Now, in order for the NaHC03 to mix with an acid, and cause this sort of chemical reaction, the acid needs to be wet. Two dry ingredients may butt up really close to each other, but they ain’t mixing. So you can mix the two as dry, and save the fizz-to-be for a rainy day. Just drop in water when you’re ready.
Getting the NaHC03 and your acid of choice to compact into an easy to store and carry block of longer lasting fizz, rather than a loose powdery mess, requires the application of just a little bit of liquid (usually H20, or some sort of oil); not enough to cause everything to fizz away, just enough to make it a little gummy. This takes less liquid than you may think, and is the trickiest part of this whole shebang. Once done, you should be able to squeeze the mess into a ball, and it will keep its shape. Let it dry out for a couple of days, and you’ll have a slightly brittle, but hard rock of fizzing action.
Which is fun in and of itself, but what if you could use this rock as a vehicle for hidden goodies?!
Anything that is mixed in to the dry ingredients before you form your ball will also be slowly released into the water as the ball fizzes away. Powdered milk, glitter, food colouring, essential oils, aspirin.
Yep, aspirin! And you just made Alka Seltzer.
All of the bath bomb recipes that I’ve been able to find use citric acid for the other half our of happy partnership. Which begs the question: why? Don’t know.
It is cheap (my local water shop sells it for $3/kilogram), and fairly benign to work with. Could be as simple as that.
Incidentally, baking powder is baking soda which already has its acidic partner blended in, ready to react. Usually, a dry acid salt such cream of tartar (KHC4H4HO6), sodium aluminum sulfate (Na2SO4 . Al2(SO4)2), and/or monocalcium phosphate (CaH4(PO4)2). So there =P