Carbonated water is water

Water

Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 into which carbon dioxide

Carbon dioxide

Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 gas under pressure has been dissolved, a process that causes the water to become effervescent

Effervescence

Effervescence might refer to one of the following:*Effervescence is the escape of gas from an aqueous solution.*Collective effervescence is a perceived energy formed by a gathering of people....

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Carbonated water is the defining ingredient of carbonated soft drinks

Soft drink

A soft drink is a non-alcoholic beverage that typically contains water , a sweetener, and a flavoring agent...

. The process of dissolving carbon dioxide in water is called carbonation

Carbonation

Carbonation is the process of dissolving carbon dioxide in water. The process usually involves carbon dioxide under high pressure. When the pressure is reduced, the carbon dioxide is released from the solution as small bubbles, which cause the solution to "fizz." This effect is seen in carbonated...

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In the US, carbonated water was known as soda water until WWII, due to the sodium salts it contained. These were added as flavoring and acidity regulators with the intent of mimicking the taste of natural mineral water

Mineral water

Mineral water is water containing minerals or other dissolved substances that alter its taste or give it therapeutic value, generally obtained from a naturally occurring mineral spring or source. Dissolved substances in the water may include various salts and sulfur compounds...

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During the Great Depression

Great Depression

The Great Depression was a severe worldwide economic depression in the decade preceding World War II. The timing of the Great Depression varied across nations, but in most countries it started in about 1929 and lasted until the late 1930s or early 1940s...

, it was sometimes called "two cents plain," a reference to its being the cheapest drink at soda fountain

Soda fountain

A soda fountain is a device that dispenses carbonated drinks. They can be found in restaurants, concession stands and other locations such as convenience stores...

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