Carbonated water, also known as sparkling water, fizzy water and seltzer, 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).
In the past, soda water, also known as club soda, was 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 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.
Joseph Priestley first discovered a method of infusing water with carbon dioxide when he suspended a bowl of water above a beer vat at a local brewery in Leeds, England. The air blanketing the fermenting beer—called 'fixed air'—was known to kill mice suspended in it. Priestley found water thus treated had a pleasant taste and he offered it to friends as a refreshing drink. In 1772 Priestley published a paper entitled Impregnating Water with Fixed Air in which he describes dripping oil of vitriol (or sulfuric acid as it is now called) onto chalk to produce carbon dioxide gas, and encouraging the gas to dissolve into an agitated bowl of water.
In 1771 Swedish chemistry professor Torbern Bergman independently invented a similar process to make carbonated water. In poor health at the time yet frugal, he was trying to reproduce naturally-effervescent spring waters thought at the time to be beneficial to health.
Today, carbonated water is made by passing pressurized carbon dioxide through water. The pressure increases the solubility and allows more carbon dioxide to dissolve than would be possible under standard atmospheric pressure. When the bottle is opened, the pressure is released, allowing the gas to come out of the solution, thus forming the characteristic bubbles.
In the US, carbonated water was commonly known by the name of soda water until World War II. During the Great Depression, it was also referred to as two cents plain, a reference to its place as the cheapest drink available at the soda fountain. In the 1950s new terms such as sparkling water and seltzer water began to be used. The term seltzer water is a genericized trademark that comes from the German brand Selters, which is produced and bottled in Nieder-Selters, Germany.
Flavored carbonated water is also commercially available. It differs from sodas in that it contains flavors (usually fruit flavors such as lemon, lime, orange, or raspberry) but no sweetener.
Intake of carbonated beverages has not been associated with increased bone fracture risk in observational studies, and the net effect of carbonated beverage constituents on the amount of calcium in the body is negligible, leaving carbonated water as harmless as regular water.