[Rutherford]

A white solid substance X underwent a series of tests in which samples of X were calcined in a flow of various gases and the resultant solid residues were weighed. The experimental data are summarized in the attached table. In all tests the mixture on exit contained, aside from the initial gases, the same unknown gas Y. In test No. 5, a reddish-brown substance Z condensed on the colder parts of the apparatus. Identify the lettered compounds.


This one seemed to me very interesting but I don't know how to conclude what compounds could it be, so I would need some hints for the beginning.
Test No. 5 seems the most interesting. The substance must have been dissolved in HCl so it is probably a carbonate, but why are HCl and Cl₂ used together?

[Arkcon]

Well, this is a problem worthy to be one of Borek:'s problem of the week.  There's a lot to do here:  For starters, you may have to determine if the weight changes are meant to be perfect, or meant to have (simulate) experimental error.  But I agree, experiment # 5 is very interesting.  Its completely decomposed in HCl + Cl₂ ... but it condenses into a reddish-brown solid.  That is a big hint ... if you're into chemistry trivia.  At any rate, you could try to find the identity of that condensed compound.

[Rutherford]

I don't know so much the colors of substances. The first compounds that occured to my mind were silver-dichromate and iron(III)-hydroxyde, but it need to be a liquid or gas so it can form a solid. I can't think of any liquid or gas that makes a reddish-brown solid and that it has to be formed from X, HCl and Cl₂. I'd need more hints.