While visiting my parents, Madeline and I made bath fizzies. The effervescence is caused by the baking soda and citric acid reacting, but the reaction only happens at any appreciable level once dissolved. In addition to the acid and base, there’s also epsom salt, some food coloring, some oils (essential and olive).

If memory serves, we used this recipe, but any recipe is a variation on the above theme. I find it difficult to add water slowly enough to keep it from reacting while mixing. I’ve tried using a spray bottle with the hope of distributing it relatively evenly from the beginning, but I’m not sure how much that helps.