When a soft drink bottle is opened, CO₂ comes out. This change is classified as reversible or irreversible?

The reaction of CO2(gas state) <===> CO2(aqueous state) is an equilibrium reaction. Hence this reaction is reversible.

When left open, it is exposed to warm air. Since gas CO2 to aqueous CO2 is a exothermic reaction, the rise in temperature promotes a backwards reaction.

So most of the aqueous CO2 escapes as gas CO2.

To get the fizz back, you can put the drink and the CO2 in a sealed container. Cooling it will make the CO2(gas) go back to an aqueous phase.

Also, if you're in Antarctica, you can leave a coke bottle open for as long as you want and it won't lose its kick!