An open container of soda loses fizz when heated. The carbon dioxide that creates carbonation is more soluble at low temperatures; when the temperature of the solution rises, the carbon dioxide returns to its gaseous state and escapes into the atmosphere.
Temperature is only one factor that affects the carbonation level of soda. Pressure is the other contributor; carbon dioxide remains in solution in a sealed can or bottle of soda because the container is under pressure. A sealed container traps the carbon dioxide, allowing the gas to dissolve in the soda once the temperature lowers. If the can is opened before a sufficiently low temperature is reached, the carbon dioxide escapes just as it does when an open container is heated.
The same principle affects other carbonated products. Alka-Seltzer tablets fizz because they release carbon dioxide when dissolved in water. This reaction occurs more quickly in warm water; the carbon dioxide bubbles rise to the surface without dissolving. Cold water loses its carbonation over a longer period of time, due to some of the released carbon dioxide forming a solution. Changing the temperature of an Alka-Seltzer solution is a popular classroom experiment used to demonstrate how different conditions affect solubility.