Distillation is a method of purification (or separation) of liquids, based on the evaporation of a liquid followed by the condensation of the vapor back to the liquid. In this experiment, a green aqueous solution of nickel sulfate (NiSO4) was distilled.
When a salt solution boils, water evaporates; the water vapor is condensed (liquefied) in a condenser. The dissolved salt does not evaporate from the boiling aqueous solution and remains in the distillation flask. The distillate, water in this experiment, drips into the still receiver.
Nickel sulfate in an aqueous solution is dissociated into ions:
NiSO4(aq) → SO42-(aq) + Ni2+(aq)
The presence of SO42--ions can be shown by the addition of barium chloride (BaCl2). The reaction of barium chloride with nickel sulfate (sulfate ions in particular) yields the white precipitate of barium sulfate (BaSO4):
NiSO4(aq) + BaCl2(aq) → BaSO4(s) + NiCl2(aq)
or in ionic form:
SO42-(aq) + Ba2+(aq) → BaSO4(s)
The distillate should not contain sulfate-SO42- or nickel-Ni2+-ions.
As no precipitate was formed upon the addition of barium chloride solution, it may be concluded that there were indeed no sulfate ions.
The presence of nickel ions-Ni2+ is hinted by the green color of the solution. Considering that the distillate was completely colorless, it may be concluded that neither were there nickel ions in the distillate.