Carbonation is carbon dioxide gas dissolved in the beverage. Carbon dioxide gas is the same gas that you breath out. It has the chemical formula CO₂ and when it is dissolved in water, it combines with a water molecule to form carbonic acid.

The amount of carbon dioxide that can be dissolved in water depends on both the temperature and pressure of the system. More carbon dioxide will dissolve at higher pressures so carbonated beverages are kept in pressurized, sealed containers until they are to be served. When the container is opened, carbon dioxide starts to bubble out of the liquid. If you wait long enough, the beverage will "go flat" meaning that most of the carbon dioxide has bubbled out of it.

Like most gasses, carbon dioxide is more soluble in cold water than warm water. That means it is easier to carbonate cold beverages than hot beverages. If you heat up a soft drink, it will start to lose carbonation faster than if you kept it cold. Oxygen gas is also more soluble in cold water. If you take a cold water fish and put it in warm, tropical water it will die because it needs more oxygen than the warm, oxygen-deficient water can provide.