Vinegar reacts with baking soda to produce carbon dioxide and water. This reaction absorbs heat, causing a drop in temperature. This type of reaction is an endothermic reaction. (An exothermic reaction means giving off heat).
Chemical reactions occur all of the time, for some examples -- digestion, cooking food, a sunburn, a permanent to curl your hair, fireworks, rust -- just to name a few.
We breathe in oxygen and breathe out carbon dioxide; plants breathe in carbon dioxide and breathe out oxygen!
The fizz in a soda is carbon dioxide that the manufacturers add to the drink.