Why must the mass of the anhydrous salt be measured relatively soon after heating?

Sodium Sulfate LOVES water, when it collects enough water, it gets rock hard. Once the salt is dry and cooled, the heat from the salt can no longer emit heat energy to evaporate more water off. If you measure the mass of the salt while hot or warm, the mass will constantly change because its giving off something, like water. Once cool though, it can start to collect water again from the atmosphere and gain mass. This is why weighing a anahydrous salt at just the right time is essential. 

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