extempore vs impromptu

extempore is not a noun while impromptu is a noun.

Word	Noun	Adjective	Verb	Adverb
extempore	No	Yes	No	Yes
impromptu	Yes	Yes	No	Yes

As adjectives, extempore and impromptu are synonyms defined as:

Other synonyms of extempore include ad-lib, extemporaneous, extemporary, off-the-cuff, offhand, offhanded, unrehearsed.

extempore (noun)	impromptu (noun)
	an extemporaneous speech or remark
	a short musical passage that seems to have been made spontaneously without advance preparation

extempore (adjective)	impromptu (adjective)
with little or no preparation or forethought	with little or no preparation or forethought

extempore (adverb)	impromptu (adverb)
without prior preparation	without advance preparation