The original post was a day or so ago and I don't remember all of the question. Have you identified the two white powders? If so, post that information and I can respond.
Well I know the first two answers. 2a) was sublimination- turning directly from a solid to a gas and I worked out that 2b) was zinc oxide- it turned yellow on heating and white on cooling.
Changing a solid to a gas by sublimation is a physical change because it was the same material before and after; i.e., the composition has not changed. I also think the ZnO is a physical change since the composition did not change. (Some oxides decompose on heating to give the metal + oxygen but I don't think ZnO is one of those).