Weigh a clean, dry evaporating dish using the scale. Record the mass (Md) of the dish.
Use a clean, dry spatula to spoon the hydrated sample into the evaporating dish. Place the dish and its contents on the scale. Measure the combined mass (Mt) of the hydrated sample and the dish.
Obtain the mass of the hydrated sample (Ms) by subtracting the mass of the dish from the combined mass: Ms = Mt -- Md. For example, if an evaporating dish weighs 50 g when empty, and 90 g when filled with an hydrated sample, then the mass of the sample is 40 g = Mt -- Md = 90 -- 50.
Rest the evaporating dish on a wire gauze heating plate on top of a tripod. Place a lighted Bunsen burner set on a low flame below the tripod. Gently heat the evaporating dish to encourage loss of water from the sample. Many hydrated samples change color during the process of heating and drying. Use a clean, dry glass rod to stir the sample until all traces of the original hydrated sample color are removed from the sample. For example, copper sulfate crystals are usually bright blue, course crystals in the hydrated form, but become very pale blue or white and powdery in the anhydrous form.
Cool the evaporating dish to room temperature, and weigh the dish with its contents again. Associate this mass with the symbol Mtn. The mass of the dehydrated or anhydrous sample (Msn) is: Msn = Mtn -- Md. For example, is an evaporating dish weighs 50 g and the dish and its anhydrous contents weigh 75 g, then the mass of the anhydrous sample is 25 g = Mtn -- Md = 75 -- 50.
Calculate the mass of the water lost (Mlost) from the sample during heating by finding the difference between the hydrated sample mass (Ms) and the dehydrated sample mass (Msn). Use the formula Mlost = Ms -- Msn. For example, if a hydrated sample weighs 40 g, and the same sample weighs 25 g after heating, then the water lost during heating is 15 g = Ms -- Msn = 40 -- 25.