inconceivable vs impossible

inconceivable is not a noun while impossible is a noun.

Word	Noun	Adjective	Verb	Adverb
inconceivable	No	Yes	No	No
impossible	Yes	Yes	No	No

As adjectives, inconceivable and impossible are synonyms defined as:

Other synonyms of inconceivable include out of the question, unimaginable.

inconceivable (noun)	impossible (noun)
	something that cannot be done

inconceivable (adjective)	impossible (adjective)
totally unlikely	not capable of occurring or being accomplished or dealt with
	(used of persons or their behavior) not acceptable or reasonable
	totally unlikely