I wouldn't put "The" if you want to generalize about sulphur. If however, your sentence follows another talking about sulphur, and that because of a certain chemical reaction it becomes a liquid, then yes, I would use "the sulphur". Does this make any sense?
The article a is necessary here.
Whether the is necessary depends on the context. If you are talking about what happens to various elements or chemicals, for example, it should be removed:
Carbon does blahblahblah.
If you are specifying the sulphur with respect to something else, then the article is required:
When you heat a solution of sulphur and some other stuff, the sulphur melts and the other stuff evaporates.