Hydrated Crystals?

Question

why is it necessary to let the evaporating dish cool before measuring the mass.
why should the mass of the evaporating dish be measured immediately after it cools, and not later?
thanks=)

Answer 1

If you put a hot object on a balance, air currents created by the hot object will cause your mass to be incorrect.

If you let it sit too long, the anhydrous crystals might absorb water from the air and so your mass of anhydrous crystals would be incorrect.