why we need to placing the cover completely on top of the crucible after heating?

Question

water of hydration(Hydrated salt)

Answer 1

At the time of cooling, crecible will absorb moisture from the atmoshere , also external foreign particles should not enter crucible, we have to cover crusible completely whiling cooling of the crucible.

Answer 2

You heat the crucible and cover separately to drive off the water of hydration. Once it is gone, or supposedly gone based on the lab instruction, you need to cover the crucible to prevent the salt from picking up water from the atmosphere.