|MadSci Network: Biochemistry|
That's a very interesting question! I had to take a little extra time and look at quite a few resources to come up with a decent educated guess on that!
The kind of soda you drink (also called pop, soft drink or carbonated drink) can be of many different flavors, and nowadays many different kinds of drink, but it mostly refers to the group of drinks that are "fizzy" when you look at them and taste them.
Carbonation is carbon dioxide dissolved in water. In soft drink cans and bottles, the carbon dioxide is often put into the liquid under pressure so there is a lot more gas dissolved in the liquid than possible at regular air pressure. The bubbles you see (and feel in your mouth and your throat) are caused by the carbon dioxide gas slowly coming out of solution and rising to the surface of the liquid.
When a soft drink is left out at room temperature with no pressurized cover (like a cap on a bottle), eventually most of the carbon dioxide will come out of solution. When there is no "extra" carbon dioxide in the liquid, then it no longer has bubbles and has become "flat".
But let's move on to that question about the "burning sensation".
It turns out that carbon dioxide and water together also undergo a chemical reaction to form a very weak (dilute) solution of carbonic acid.
CO2 + H2O <=> H2CO3
Weak acids usually taste sour, like a lemon, but there is so much sugar in soft drinks, and so little acid from the carbonation, that it is sometimes difficult to tell.
After much consideration, my best educated guess is that when you burp, a little liquid comes up in that burst of air from your throat and that liquid has a little carbonic acid in it. The tissue in your sinuses is VERY sensitive, so the acid makes it feel like it is burning.
Soft drink -- Facts, Info, and Encyclopedia http://www.absoluteastronomy.com/encyclopedia/s/so/soft_drink.htm
Carbonation -- Facts, Info, and Encyclopedia http://www.absoluteastronomy.com/encyclopedia/c/ca/carbonation.htm
Another scientist's opinion with an interesting experiment http://www.ucdmc.ucdavis.edu/pulse/scripts/99_00/fyi_drinking_pop.html
An experiment that has a nice little intro discussing the chemistry of soda http://www.pasco.com/experiments/chemistry/march_2002/home.html
A newsgroup topic that covers a lot of related stuff http://yarchive.net/med/soft_drinks.html
Keep on asking questions! That's what makes a good scientist!
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