(i) Sulphur is not of volatile nature. Upon heating it melts. Therefore, sublimation process could not succeed.
(ii) The student should have dissolved the impure sample in carbon disulphide. It is a liquid in which sulphur completely dissolves while iron does not. From the solution, can be recovered with the help of crystallization process.
(iii) The process of sublimation is useful only if one of the constituents present in the mixture can sublime while the others do not undergo sublimation.