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Ask Dr. Knowledge

What’s the difference between baking soda and washing soda?

October 11, 2010

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What’s the difference between baking soda and washing soda? Are they interchangeable? They are similar but not quite the same and, for most purposes, not interchangeable. Washing soda, or sodium carbonate, is two sodium atoms attached to a carbonate group (a carbon atom and three oxygens).

Baking soda, or sodium bicarbonate (and sometimes called sodium hydrogen carbonate) has one of the sodium atoms replaced by a hydrogen atom. Both occur naturally as minerals and are often prepared in factories from more commonly occurring minerals like calcium carbonate (chalk) and sodium chloride (salt). In earlier times, sodium carbonate was extracted primarily from the ashes of seaweeds and certain land plants. Both sodas are alkaline, meaning they will neutralize acids, but washing soda is the stronger. Alkalis react with the chemicals in many stains, particularly those involving oil and grease, and help take them out. Washing soda is better at removing stains, but both can be used for this purpose.

Sodium carbonate is also used as a food additive (E500) to reduce acidity and help keep powders from clumping. It helps baked goods like breads rise by releasing carbon dioxide when it is heated or reacts with acids. It is, however, corrosive and has to be used in foods with great care.

For household cooking, baking soda is almost always used; it’s much milder and there’s usually no need to use strong alkalis in cooking, since the acids in items like fruits and vinegar aren’t strong. Sodium bicarbonate is commonly used to relieve acid indigestion and is added to toothpaste and mouthwash as a gentle mechanical abrasive and neutralizer of tooth-dissolving acids. Baking soda can also extinguish small electrical and grease fires, so it’s a good thing to keep near the stove.

Sodium carbonate has an interesting and not obvious use: making glass. Just melt it at high temperatures with sand and chalk and let it cool. Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to drknowledge@globe.com or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.