Sodium carbonate and sodium bicarbonate have similar-sounding names and many chemical similarities, but they're used very differently. Sodium carbonate has household and industrial applications but is used very rarely in food and cooking. Sodium bicarbonate, on the other hand, is common household baking soda -- it has a variety of uses, including baking and cleaning.
Sodium carbonate is a relatively basic salt, where "basic" is the chemical opposite of acidic. It has the formula Na2CO3 and goes by the colloquial name "washing soda." Because of the caustic nature of sodium carbonate, it has few applications in food, though a few select food items -- including certain pretzels -- require the use of a strong base such as sodium carbonate. One of its most common applications is in the industrial manufacturing of glass.
With the chemical formula NaHCO3, sodium bicarbonate is structurally very similar to sodium carbonate. It's much less caustic, however, and is only a very mild base. It has many common household applications, including as a cleaning agent and as a leavening agent in baked goods. You can also ingest sodium bicarbonate in water as a mild home remedy for acid stomach; it neutralizes stomach acid, relieving symptoms of heartburn and producing the harmless compounds carbon dioxide and water.
In the Body
Sodium carbonate has virtually no physiological importance; your body neither utilizes nor produces it. On the other hand, sodium bicarbonate is quite important to physiological function. You naturally have bicarbonate in your bloodstream, explain Drs. Reginald Garrett and Charles Grisham in their book "Biochemistry." It acts as a buffer, meaning that it helps regulate the acidity of your blood. Your cells produce the waste product carbon dioxide, which combines with water in the bloodstream to produce bicarbonate.
Sodium bicarbonate and sodium carbonate act similarly in food, though sodium carbonate reacts much more strongly. Both react with acid, resulting in the production of a compound called carbonic acid, which has the chemical formula H2CO3. Carbonic acid then decomposes into carbon dioxide and water, explain Drs. Mary Campbell and Shawn Farrell in their book "Biochemistry." The carbon dioxide bubbles produce leavening action. The major difference between sodium bicarbonate and sodium carbonate in food is that while sodium bicarbonate doesn't significantly change a food's acidity, sodium carbonate makes the food very basic.