Why is it necessary to heat the residue to constant mass?

Question

another question: What can we observe when Magnesium is heated?

how about when Iron is added to Sulfur, and then it was heated?

thank you so much!

Answer 1

Residue contain often a part of solvent.If you do not eliminate it , your measurements will be wrong

Answer 2

The residue is a compound under investigation.  Some compounds absorb water from the atmosphere.  Sometimes the residue contains water or another solvent, but all we want is the mass of the residue.

By heating the residue to constant mass, we can be sure that no absorbed water or solvent remains in the residue.  It is important to make sure that the residue (after heating) is cooled to room temperature before measuring the mass.

If Magnesium metal is heated hot enough in an inert atmosphere, it will melt.  If heated in an oxygen containing atmosphere, it will burn (usually forming Magnesium oxide):

2 Mg + O2  ---> 2 MgO

If Iron is added to Sulfur at room temperature, not much happens.  If it is heated a chemical reaction takes place forming Iron (II) sulfide:

Fe + S ---> FeS

or

8 Fe (s) + S8 (s)  ---> 8 FeS (s)