Learn English – What part of speech is “turn” in the phrase “it’s the turn”? What’s its origin

You can rather easily argue that to belongs to its own part of speech. The only evidence you need is that it does not fit into any of the major part of speech categories. You will just construct grammatical tests showing that it is not a noun, not a verb, not a preposition, etc. Following examples suggest that to is not a verb or noun, and that other prepositions cannot precede verbs in their base form (respectively).

*Nobody shall to.
*The to is ready.
*It is hard to see with run.

Of course, you can go on with this type of argument with pretty much any high frequency word until you have so many parts of speech that the concept is no longer useful. Of possible interest is Croft's discussion of Word Classes in Radical Construction Grammar.

Money burns a hole in my pocket.

The Phrase Finder shows very old usages of the idiom, which clearly suggests a sense of urgency to get rid of something because it is supposedly too hot:

• "It was only a bit of change, but it was plainly burning a hole in his pocket." As though it were something hot, he wanted to pull the money out--and get rid of it by spending it. This can be used of almost anything that a new owner wants to use or spend right away.

  • In the 18th century it was sometimes expressed as "burning in one's pocket" or something similar. Two examples cited by the OED are 1740 MRS. DELANY Autobiog. & Corr. II. 165 The post brought me your letter, which burnt in my pocket. 1768 TUCKER Lt. Nat. I. 152 Children..cannot rest till they get rid of their money, or, as we say, it burns in their pockets."

• The more modern version appeared at least as early as the 19th century: "1857 TROLLOPE Three Clerks II. ix. 198 How was she to give him the purse? It was burning a hole in her pocket till she could do so." (Example quoted in the OED, s.v. burn.)