GUM THAT FOAMS

Mad Dawg chewing gum is a practical joker’s dream come true. It is noticeably sour when someone first starts to chew it, but the big surprise comes about ten chews later when brightly colored foam oozes from the person’s mouth. Although the effect is dramatic, the cause is simple acid–base chemistry.

The foam consists of sugar and saliva churned into a bubbling mess by carbon dioxide released from the gum. The carbon dioxide is formed when sodium bicarbonate (NaHCO₃) present in the gum is mixed with citric acid and malic acid (also present in the gum) in the moist environment of the mouth. As NaHCO₃ dissolves in the water of the saliva, it separates into its ions:

The bicarbonate ion, when exposed to H⁺ ions from acids, decomposes to carbon dioxide and water.

The acids present in the gum also cause it to be sour, stimulating extra salivation and thus extra foam.

Although the chemistry behind Mad Dawg is well understood, the development of the gum into a safe, but fun, product was not so easy. In fact, early versions of the gum exploded because the acids and the sodium bicarbonate mixed prematurely. As solids, citric and malic acids and sodium bicarbonate do not react with each other. However, the presence of water frees the ions to move and react. In the manufacture of the gum, colorings and flavorings are applied as aqueous solutions. The water caused the gum to explode in early attempts to manufacture it. The makers of Mad Dawg obviously solved the problem.

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