##Calcium nitride## mind you I would not want to eat the soup afterwards.
Boiling point elevation is proportional to the number of species in solution; it is a colligative property.
##KCl(s) rarr K^+ + Cl^-##
##SrBr_2(s) rarr Sr^(2+) + 2Br^(-)(aq)##
##Ca_3N_2(s) + 6H_2O rarr 3Ca^(2+) + 6HO^(-) + 2NH_3(aq) ##
Calcium nitride would give by far the most particles in solution on a per mole basis and of course the ammonia would speciate.
The only such contaminant I would put in my soup would be ##sodium chloride##. How would this affect the boiling point?
Methane. ##CH_4## is a volatile non-ionic species. You could use this to cook the soup.