Baking soda (sodium bicarbonate) and washing soda (sodium carbonate) aren't just different because of more/less absorbed water. That's the difference between anhydrous/monohydrate/decahydrate forms of sodium carbonate. They're different compounds.
Originally Posted by jnanian
But jnainan is correct that baking soda will decompose on heating, slowly, starting at 70c, more quickly at 200c, into the anhydrous form of sodium carbonate. Left open after cooling, it will eventually absorb some water and convert to the monohydrate form, the most stable.