A bath bomb is a hard-packed mixture of chemicals which effervesces

Effervescence (chemistry)

Effervescence is the escape of gas from an aqueous solution and the foaming or fizzing that results from a release of the gas. The word effervescence is derived from the Latin verb fervere preceded by the adverb ex, which means to boil...

 when wet. They are used to add scent and colour to bathwater.

Composition

Bath bombs' primary ingredients are a weak acid and a bicarbonate base. These are unreactive when dry, but react vigorously when dissolved in water to produce their characteristic fizzing over a period of several minutes; carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 bubbles on bathers' skin gives a tickling sensation some find pleasant. A common chemical mixture is citric acid

Citric acid

Citric acid is a weak organic acid. It is a natural preservative/conservative and is also used to add an acidic, or sour, taste to foods and soft drinks...

 and sodium bicarbonate

Sodium bicarbonate

Sodium bicarbonate or sodium hydrogen carbonate is the chemical compound with the formula Na HCO3. Sodium bicarbonate is a white solid that is crystalline but often appears as a fine powder. It has a slightly salty, alkaline taste resembling that of washing soda . The natural mineral form is...

:

C₅H₇O₅COOH + NaHCO₃ → C₅H₇O₅COO^-Na⁺ + H₂O + CO₂

The other ingredients in bath bombs can vary considerably. Most have scented ingredients — such as essential oils — in their composition to impart a pleasant fragrance to the water, and some are brightly coloured. Solid objects, such as glitter

Glitter

Glitter describes an assortment of very small pieces of copolymer plastics, aluminum foil, titanium dioxide, iron oxides, bismuth oxychloride or other materials painted in metallic, neon and iridescent colors to reflect light in a sparkling spectrum...

, can be added.

Production

Bath bombs are generally spherical but can be found in a variety of shapes, such as tablets or lumps. Shops offer a wide range of bombs, but they can also be made at home.