This is the entry from Oxford Advanced Learners' Dictionary where it's stated to be both countable and uncountable. There is no any example however. Oxford Advanced Learner’s Dictionary, 8th edition
li • quid noun, adjective BrE / lɪkwɪd / NAmE / lɪkwɪd /
noun
word origin
thesaurus
example bank
[ uncountable , countable ]
a substance that flows freely and is not a solid or a gas, for example water or oil
She poured the dark brown liquid down the sink.
the transition from liquid to vapour
see also washing-up liquid
© Oxford University Press, 2010
In the example bank section, I've found the following sentence. We can make conclusions now, I guess.
Immiscible liquids such as oil and water do not mix.
As can be seen, the very word is used in plural which means it has singular form used with the article A as well. I guess, If the topic is about the different kinds of the substance, then it's accepted to use the word as a countable noun.