What steps could one take in order o be reasonably sure that all of the NH4Cl had been sublimed away when heat
Answer:
Keep heating until a constant weight is acheived over a good period of time. NaCl and SiO2 have extremely high boiling points, so you'll get rid of the other before them.
You could also try a glass rod dipped in NaOH. Bring it close to the subliming mixture. If you see fumes of NH4OH, then you are not yet done. Of course, there may be very less fumes - which may go unnoticed. So, the better but more tedious way would be the weight measurement...
When heating that mixture the only compound that will change states is the ammonium chloride. First you should weigh the mixture and then heat. When you think that you have heated it enough and you no longer see the sublimation vapor then hold a piece of litmus paper above the mixture. When ammonium chloride sublimes it is converted to ammonia and HCl vapors. When sodium chloride or silicon dioxide is heated it is neither acidic or basic. If the limus paper turns either red or blue then it is still subliming and you should continue to heat it. When you see no color change, it has all sublimed away. Allow the mixture to cool and weigh to constant weight.
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