1)Why is it necessary to heat the alum until its mass does not change by more than 2mg?

Question

2)Why its it necessary to store the alum in a disccator while it cools before measuring the mass?

( systhesis and analysis of alum crystals experiment)

Answer 1

1. By reaching constant weight you know you have driven off all of the water it contains.

2. If allowed to stand in air the alum will reabsorb, from the air (which contains some water), some of the water diven off by previous heating. Cooling it in a desiccator keeps it from picking up moisture.