Bath bombs contain sodium carbonate, a base, and a mild solid acid such as citric acid (from lemons) or tartaric acid (from grapes). It is CO2, carbon dioxide, generated by a reaction between the base and the acid when the product is placed in water, that makes the bubbles. Another product which bubbles in water for the same reason is Alka-Seltzer. The bubbling effect of the acid-base reaction in soap bombs is added to by the effect of the bubble bath type soap also used in the formulation.