A bath bomb’s fizz is a reaction between baking soda (sodium bicarbonate) and citric acid. The two are present together inside a bath bomb in powdered form so it isn’t until they hit the water that they are able to combine and start fizzing.
The fizzing reaction between baking soda and citric acid is the same reaction that causes bread to rise, or you may also be familiar with vinegar and baking soda rockets that many of us made as children.
Some bath bombs will fizz more than others, some will sink, some will float, but all use the same process. The fizz can be helped along by adding the bath bomb to a warm bath as close to the water spout as possible so the water can help to move the bath bomb around.