Is the word solvent in "solvent mixture" used as a noun or an adjective?

Solution 1:

Primarily, solvent in "solvent mixture" is a noun. More precisely, it is known as a noun adjunct.

an optional noun that modifies another noun; it is a noun functioning as a pre-modifier in a noun phrase. For example, in the phrase "chicken soup" the noun adjunct "chicken" modifies the noun "soup".
(WP)

Btw, a noun adjunct is almost always in the singular, except where possible ambiguity requires otherwise. In the given case, a mixture necessarily contains more than one ingredient, but still we use the singular because the purpose is to indicate the nature (to modify the following noun -- "what kind of") of the thing and not the thing itself.

For more details, see "Adjectives versus Noun Adjuncts [duplicate]" and "Is this noun used as an adjective?" right here on this site.

HTH.