When the pressure of CO2 is high (in an unopened can), the amount of CO2 dissolved in the liquid is very high. When the pressures of CO2 is lowered a whole lot by opening the can, the equilibrium shifts to the left, CO2(aq) leaves the solution, and the drink goes flat.
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The equilibrium involved is:
CO2(g) <--> CO2(aq)
When the pressure of CO2 is high (in an unopened can), the amount of CO2 dissolved in the liquid is very high. When the pressures of CO2 is lowered a whole lot by opening the can, the equilibrium shifts to the left, CO2(aq) leaves the solution, and the drink goes flat.