The bubbles in a carbonated soft drink are produced when carbonic acid decomposes to form carbon dioxide and water. In a closed system this equilibrium exists as Why does a carbonated soft drink lose?

A carbonated soft drink loses its carbonation over time because carbon dioxide slowly escapes from the liquid into the surrounding atmosphere, as the partial pressure of carbon dioxide inside the bottle is higher than outside. This process is accelerated by factors like agitation, temperature, and the surface area of the liquid exposed to the air.