Why must the mass of an anhydrous salt be measured immediately upon cooling?

Question

In an experiment in which I had to heat a hydrate (copper sulfate xhydrate) to form an anhydrous salt, I had to measure the mass of the evaporating dish, hydrate, and anhydrous salt. Any idea why the anydrous salt has to be measured immediately after it is cooled?

Answer 1

Well,
I am chemist and work with anahydrous Sodium Sulfate all the time. 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.

Hope this helps.

The Chemist.
Wee Man.

Answer 2

For the best answers, search on this site https://shorturl.im/awJJM

The fact that the question specifies an "anhydrous salt" implies that the salt spontaneously absorbs water [from the air] and becomes hydrated. When the salt becomes hydrated, the weight of the water absorbed effects the weight of the overall substance. In order to get a proper measurement of the weight of the salt (and not the salt + water), the measurement should be taken as soon as possible so as not to allow water being absorbed.

Answer 3

i believe it is because the salt begins to absorb moisture from the air, and then becomes hydrated