speculate vs question

speculate is not a noun while question is a noun.

Word	Noun	Adjective	Verb	Adverb
speculate	No	No	Yes	No
question	Yes	No	Yes	No

As verbs, question is a hyponym of speculate; that is, question is a word with a more specific, narrower meaning than speculate:

Other hyponyms of speculate include premeditate, theologise, theologize, introspect, bethink, cogitate, wonder, puzzle, consider, study.

speculate (noun)	question (noun)
	uncertainty about the truth or factuality or existence of something
	an instance of questioning
	the subject matter at issue
	a formal proposal for action made to a deliberative assembly for discussion and vote
	a sentence of inquiry that asks for a reply
	an informal reference to a marriage proposal

speculate (verb)	question (verb)
reflect deeply on a subject	pose a question
to believe especially on uncertain or tentative grounds	challenge the accuracy, probity, or propriety of
talk over conjecturally, or review in an idle or casual way and with an element of doubt or without sufficient reason to reach a conclusion	place in doubt or express doubtful speculation
invest at a risk	pose a series of questions to
	conduct an interview in television, newspaper, and radio reporting