Washing soda and baking soda are both alkaline compounds, which reduce or neutralize acids. They are made from the same atoms, just in different amounts, which makes all the difference in how they are used.
Baking soda has a pH of 8 (7 is neutral) and is best known for its use as a leavening agent in cooking. It also can be used as a mild scouring agent, an antacid and an emergency kitchen fire extinguisher.
Washing soda is a caustic substance with a pH of 11. Because of this, you should wear gloves when handling it directly. Washing soda is used as a laundry additive and cleanser but never in food.
According to Dr. Dan Berger, a professor of chemistry at Bluffton College, the primary difference between washing soda (sodium carbonate) and baking soda (sodium bicarbonate) is that washing soda consumes twice as much acid as baking soda. (See Reference 2)
Some baking soda uses
Baking soda primarily is used in cooking as a leavening agent that causes cakes and cookies to rise. It also is used as a deodorizer by placing an open box in the refrigerator, sprinkling it on your carpet and then vacuuming it up or by sprinkling some in shoes. It can be safely handled by anyone.
Some washing soda uses
Washing soda most often is added as a booster to laundry detergent. It also can be mixed into a paste and used to cut grease, clean oil and remove wax or lipstick, and to scour a stained sink. Test washing soda paste on an inconspicuous spot before using because it could harm some finishes.
Baking soda and washing soda are useful around the house, but they are not interchangeable.