|MadSci Network: Chemistry|
Well, as you drop the salt into the soda, the salt crystals give the carbon dioxide gas a nucleation site or a place to form bubbles. The faster the bubbles form, the faster the CO2 can escape from the beverage.
Reason: sodas contain more CO2 than can really be dissolved in water or beverage. Carbonated soft drink (CSD) manufacturers use very cold temperatures to super-saturate the liquid and/or inject the CO2 gas into the beverage. Either way, the excess insoluble CO2 wants to escape the beverage as soon as you open the bottle or can. Adding salt just accelerates this process because it gives the gas more surface area on which to form bubbles.
You can find the solubility of CO2 in water in a chemistry reference, like Perry's Chemical Engineers' Handbook for more information. Or you can try an experiment where you add salt to hot and cold sodas. You should be able to see first-hand how temperature will affect how fast the CO2 escapes.
Try the links in the MadSci Library for more information on Chemistry.