I read this sentence in a description to a podcast from https://www.eslpod.com/website/index_new.html
Batter, “batter,” when we talk about cooking is a liquid, made usually with eggs, and flower, and perhaps milk, and you combine these things together and you get a thick liquid, which we call batter
Why do I need to use a before liquid? What makes liquid countable?
As Catija mentions in the comments, "liquid" may be used both ways, as a non-count noun or as a count noun.
Many nouns that are otherwise non-count (let's use "X" to stand for an example noun) may be used as count nouns with the implication being "a specific type of noncount noun X".
This seems to apply to most words for states of matter. We can speak of "liquid," "gas," "fluid" but we can also speak of "a liquid," "a gas," "a fluid", as in "water is a liquid", "carbon dioxide is a gas".
Other examples: "red wine" (uncountable) vs. "a red wine" (countable), "vapo(u)r" (uncountable) vs. "a vapo(u)r" (countable).
A good learner's dictionary should list such nouns as being able to be used in both ways, countable and uncountable.
This might not be a very satisfying answer, but in general it's not easy to predict if a word will be countable or uncountable just from the meaning. "Fruit" is generally non-countable, while "vegetable" is countable. "Peas" is countable, but "corn" is non-countable. "Furniture," "silverware" and "clothing" are non-countable, although we can have singular items of any of these.
In a comment below another question, John Lawler wrote:
Virtually all mass nouns can be used as if they were count nouns under certain circumstances, and vice versa.
Another question that might be of use: When can you pluralize uncountable nouns? (As far as I know, any nouns that can be pluralized can also be preceded by the singular indefinite article "a".)
Sometimes words are left out when the meaning is clearly understood. This is especially true with many uncountable nouns, also called mass nouns, and countable ones (count noun). You need to look up the word in a dictionary to check if it can be used either way. For example,
I would like a cup of espresso, please = I'd like an espresso, please.
He drank fourteen pints of beer last night = He drank fourteen beers last night.
There are many types of wine I enjoy = There are many wines I enjoy
Batter, "batter", [...] is a type of thick liquid = Batter [...] is a (thick) liquid
coffee
[MASS NOUN]
1. A hot drink made from the roasted and ground bean-like seeds of a tropical shrub: a cup of coffee
1.1. [COUNT NOUN] A cup of coffee: we went out for a coffee
Noncountable nouns have countable uses, e.g.,
I'd like to order a beer.
Beer, being a liquid, is noncountable; however, "a beer," means a serving of beer, which is countable.
But this is independent of whether noncountable nouns take articles. They may do so even when they're used in a noncountable context:
The company produces a beer without alcohol.
This does not mean that the company produces only a single serving.
The beer served at the party was non-alcoholic.
Nor does this mean that the hosts served only a single serving.
We're not talking about individual servings here, but about noncountable amounts of liquid. The first sentence talks about a general category (thus using the indefinite article), and the second talks about a specific classification (thus using the definite article).
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.
