Carbonated water, also known as sparkling water, fizzy water, seltzer, and water with gas, is plain water into which carbon dioxide gas has been dissolved, and is the major and defining component of most soft drinks. The process of dissolving carbon dioxide gas is called carbonation. It results in the formation of carbonic acid (which has the chemical formula H2CO3). Carbonated water, also known as soda water, can be produced in the home by "charging" a refillable seltzer bottle by filling it with water and then adding carbon dioxide. Club soda may be identical to plain carbonated water or it may contain a small amount of table salt, sodium citrate, sodium bicarbonate, potassium bicarbonate, potassium citrate, potassium sulfate, or disodium phosphate, depending on the bottler. These additives are included to emulate the slightly salty taste of homemade soda water. The process can also occur naturally to produce carbonated mineral water, such as in Mihalkovo in the Bulgarian Rhodopes.