Sea water contains chlorides, bromides and iodides of sodium, potassium, magnesium and calcium. NaCl is the most abundant (2.5% by mass) among all of these. Certain forms of marine life contain iodine in their systems. Various sea weeds, contain upto 0.5% of iodine and chile saltpetre contains upto 0.2% of sodium iodate. NaCl and carnallite, etc., are present in the deposits of dried up seas.