Ok, so surprisingly, it a physical reaction rather than a chemical one. Microscopically, mentos have many scratches and craters. When the soda is made, carbon dioxide (which is normally is and "wants" to be a gas) is dissolved into the soda. When the (microscopically) rough surface touches the soda, it releases the carbon dioxide at an extremely fast rate, making soda bubbles filed with carbon dioxide go everywhere. (This also should explain why shaking soda fizzes and why it fizzes on your tongue.) Hope this helps!