text: why cant u always burn a hydrated salt and evaporate the water? in other words..why wont the method be suitable? you never "burn" it as you may drive off water but end up reacting it with oxygen and contaminating it.  the sample is gently heated over a flame. but sometimes this is not enough to drive off all water. some salts will never be completely dry and can retain one water molecule of crystallisation per molecule of chemical (eg feso4.h2o). also, some salts are very sensititve to heat and may decompose at a certain temperature, turning into gases that will escape.