Simple chemistry reaction question?

I was doing an MCAT practice test and I got this question right, but I don't know the exact reaction behind it, so it was more or less of a guess.

In simple terms:

What gas evolves when you mix HCl with CO3^2- ?

I guessed CO2 and got it right, but I would like to know what exactly happens between a strong acid like HCl and a base like carbonate ion which leads to CO2 being produced. What is the exact reaction with all products?

Thanks.