Nothing wrong with it, and it's not an "evaporation" of salt, it's the reversible reaction between ammonia and acid that causes this phenomenon: on
heating NH4Cl partially decomposes to NH3 and HCL, which will combine again on cold to form a salt. It's commonly misunderstood btw., many think that
the salt sublimes, while the process is chemical in fact (as I read somewhere, don't ask for the refs. now). |