Bath bombs fizz due to a chemical reaction that occurs when baking soda and citric acid come into contact with water. Baking soda, or sodium bicarbonate, dissolves quickly in water, and the positively charged sodium (Na+) breaks apart from negatively charged bicarbonate (HCO3-). Citric acid, which is a weak acid, reacts with the bicarbonate to produce carbon dioxide (CO2) gas bubbles. The carbon dioxide forms bubbles in the soap and bath water, and rushes to the surface with a delightful fizz. The reaction helps to break up the bath bomb, releasing fragrances, detergents, and oils. The cornstarch in a bath bomb slows down the reaction. If there are perfumes or scented oils in the bath bomb, they are released into the air with the carbon dioxide bubbles. To make a bath bomb, mix baking soda, citric acid, and cornstarch together in a bowl. The ratio of baking soda to citric acid should be 2:1.5. Bath bombs should be used relatively quickly, as the citric acid and sodium bicarbonate will oxidize over time, causing them to become less reactive in the water.