If a question asks how to separate insoluble ions, soil, etc. from water, is distillation an acceptable answer? (The given answer is "filtration")
Distillation would be a partially acceptable, but not optimal, answer because it implies you understand the question and know a process that should achieve the desired result; upon heating and vaporization, the water should theoretically be separated from any insoluble substances. Filtration is a better answer, however, because the process of distillation normally refers to a mixture of multiple liquids rather than liquids and insoluble solids. Distillation specifically requires heating and cooling according to the various boiling points of the liquids in the mixture, and since we presume there are no liquids in this mixture other than water, distillation is unnecessary (it may also result in reactions that we would prefer to avoid because we can't control exactly how the energy is being added to the molecules themselves). Filtration requires no additional input of energy and also ensures we don't accidentally vaporize any volatile compounds along with the water.
I think the legitimacy of this answer depends upon the context in which it is asked; speaking personally, I would give a student partial credit for this answer, but another teacher may not, for various reasons. I don't think my own definition of "acceptable" should necessarily apply to a problem assigned by someone else, and I always advise caution when seeking outside sources of authority on procedural qualification as opposed to the information itself.