water is a liquid vs water is liquid [duplicate]
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 coffeeOxford Dictionaries