Why when heating and cooling three different separate substances in open containers does the mass change in two of them (one loss mass; the other gained mass) but stays the same in the third when it is known that mass is conserved in chemical reactions? Would it be because one gave off vapor, the second melted and the third was a solid that wasn't heated enough to give off vapor or melt?
The lost mass should be accountable through the vapor pressure of the volatiles increasing on heating the material.
The increased mass has to be a result of a chemical reaction of the open system with the surrounding atmosphere. The heating must initiate what is most likely an oxidation of the ...