Question

Why is carbonic acid added to soft drink?

Answer 1

its create a buffer to stabilizes the carbonic acid so that the fizz remains in the drink

Answer 2

Soft drinks contain Carbon-di-oxide gas dissolved in water under high pressure. Carbon-di-oxide combines with water to form Carbonic acid, it is a reversible reaction. 

CO2 + H2O gives H2CO3.

When the bottle is opened the pressure is released and carbonic acid form Carbon-di-oxide gas, which makes the soft drink fizzy.
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