Some bath products fizz and make bubbles when added to water. Ever wonder why? Try this experiment to find out. (Adult supervision recommended.)
A bath bomb is really showing how acids and bases react when mixed together. In this experiment, the baking soda is the base and the citric acid is, well, the acid. But the reaction of this acid-base combination can be controlled by the other ingredients in the bath bomb.
The cornstarch and Epsom salts both act as liquid absorbers to help keep the baking soda and citric acid from reacting with each other when liquids are added. They are also helpful if you live in a place with high humidity. The borax acts as both a preservative and an emulsifier, stabilizing the acid and the base and keeping them from reacting with each other before entering the water.
Water is the catalyst for the reaction to occur because it dissolves the solids and allows the ions in the acid and the base to move and collide with each other, causing a chemical reaction to occur. A catalyst is something added to a chemical mixture that speeds up the chemical reaction time. This reaction forms carbon dioxide, a gas, which rises to the surface of the bathwater in the form of bubbles. The oils and the fragrances are useful as a liquid to help form the bath bombs as well as leave the skin smooth and scented.
For further study, try these other fun spa science projects: