sodium carbonate
Also Known As:
washing soda, soda crystals, soda ash
What it is:
Additive
Sodium salt of carbonic acid. It most commonly occurs as a crystalline heptahydrate, which readily effloresces to form a white powder, the monohydrate; and is domestically well known for its everyday use as a water softener. It has a cooling alkaline taste, and can be extracted from the ashes of many plants. It is synthetically produced in large quantities from table salt in a process known as the Solvay process.
Further reading:
http://en.wikipedia.org/wiki/Sodium_carbonate