What is the difference between club soda and seltzer water?

According to Cook's Encyclopedia: Seltzer water is a flavorless, naturally effervescent water that takes its name from a region of Germany. Club soda, sometimes called soda water, was introduced in the later half of the 18th century when carbon dioxide was injected into water. Seltzer and club soda are essentially the same, except one is naturally effervescent and one is man-made.

Seltzer water is manufactured by pumping carbon dioxide gas into water under pressure until the CO2 dissolves in the water. When pressure is reduced (by opening the bottle), CO2 bubbles out of the water. This looks like boiling water where heat turns water from a liquid to a gas and the gas bubbles out of the liquid on your stove.

Club soda is also carbonated water as is seltzer water, however, it is manufactured by putting bicarbonate of soda into water in a sealed (pressurized) container. The result is salt (sodium chloride) and carbon dioxide (CO2). The reaction is the same one you see when you put a tablet of Alka Seltzer into a glass of water. In the open glass of water, the CO2 bubbles up and out because there is no pressure (other than atmoshperic pressure). In the factory pressurized container, the bubbles stay in the water turned club soda until the container or bottle is opened and pressure is reduced. Look at the ingredients label for Club Soda and Seltzer Water. The former has lots of Sodium (from Sodium Chloride, i.e. salt) and Seltzer Water has zero miligrams of Sodium.

Ginger Ale and many other soda pop flavors can be made by combining flavor with Club Soda. But look at all of the Diet-Rite flavors. They say no calories, no caffeine, no sodium. The reason they have no sodium is that the manufacturer pumps the CO2 into the flavored water rather than using the Alka Seltzer method to produce the bubbles.

This is one of the useful things I learned while practicing law—reducing real estate assessments and property taxes—in Cook County, Illinois.