Sodium carbonate (also known as washing soda, soda ash and soda crystals), Na2CO3, is the water-soluble sodium salt of carbonic acid.It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate. Pure sodium carbonate is a white, odourless powder that is hygroscopic (absorbs moisture from the air), has an alkaline taste, and forms a strongly alkaline water solution.