What type of reaction is the reaction between an acid and a carbonate? Consider the following reaction: 

$$\ce{2HCl + Na2CO3 -&gt; 2NaCl + H2O + CO2}$$

Is this a double displacement reaction or a decomposition reaction?
The first step is a proton transfer (acid-base reaction), from HCl to carbonate, to produce hydrogen carbonate ion. The second step is also a proton transfer, from HCl to hydrogen carbonate to produce carbonic acid. The third step (or maybe better described as a two-step acid-catalyzed process) is the dehydration of carbonic acid to carbon dioxide and water.