This is a basic question, but its one that I can't seem to find a definitive answer on.

Take the sentence: The sky is blue.

I am inclined to say that "blue" is a noun, for example, you could say (in a metaphor) the sky is an ocean. "an ocean" is a noun phrase, so I think "blue" is too. But I'm not positive.

In a similar fashion:

Dogs won't eat cat food.

I believe "cat" is an adjective. It is describing the food, the sentence doesn't change grammatically at all if you insert a well known adjective, like hot: Dogs won't eat hot food.

But again, I'm not positive.

  • In the NP "cat food", "cat" is a noun, not an adjective. Its function is that of attributive (pre-head) modifier. Nouns can be premodified by a wide range of items: "a big house", "another two exams", "a brick wall", "a sleeping child". But we don't want to call them all adjectives: "big" is an adjective, of course, but "two" is a determinative, "brick" is a nominal, and "sleeping" is a verb. – BillJ Oct 26 '16 at 9:20
  • What part of speech is 'clever' in 'Einstein was clever'? – Edwin Ashworth Nov 7 '16 at 23:55

It will help 1) to separate the lexical class of a word (i.e., its part of speech) from that word's function in a sentence and 2) recognize that one word may belong to more than one class. With regard to 2), blue is an adjective because it modifies nouns:

blue sky

It is also a noun because it names some thing:

Q: Which dress do you like?
A: I like the blue.

You may decide for yourself whether blue is two English words that happen to be spelled the same or whether it's one English word that happens to belong to two different lexical classes.

We determine which parts of speech a word belongs to by applying certain grammatical tests. For instance, adjectives have grades; nouns don't. So we say

That's the bluest sky I've ever seen

but not

*​That's the skiest thing I've ever seen.

Nouns take determinatives and nouns appear as objects of prepositions. Thus we can classify blue as a noun since we can say

I like the blue.
She was dressed in blue.

Notice that in general we can't say the same thing for adjectives:

*I like the happy.
*She was in happy.

We can be sure that cat is a noun. It names a thing, namely at type of animal that makes an excellent pet. It takes determinatives, is modified by adjectives, and shows up as the object of prepositions:

My neighbor talks to her blue cat.

It doesn't have a grade, i.e., we don't say that one cat is catter than the next. But the functional roles in English sentences aren't restricted to particular parts of speech. The role of noun modifier isn't restricted to adjectives. A noun may also be a noun modifier, and in that role such a noun is called an attributive noun:

cat food
cat house
cat person
and so on.

Just because attributive nouns function as noun modifiers doesn't make them adjectives.

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  • The + adjective (or past participle) is broadly used to describe a group of people such as "the young should respect the old", the poor, the rich, the educated, the sick, the good, the wicked. Depending on context, the happy could be understood that way. – user24743 Oct 26 '16 at 10:02

Blue can be used as both a noun and an adjective. You can talk about the colour blue (noun), and you can talk about blue sky (adjective).

Cat food is a compound noun. In this instance, you could replace it with cats' food (food prepared for cats) without too much loss of meaning.

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  • 1
    >Cat food is a compound noun. You can replace it with cat's food I'm not sure I agree here. Maybe, cats' food. Because cat's food implies one cat. While cat food as a whole is different. Are you sure its not being used as an adjective? – Jane Doe Oct 26 '16 at 7:49
  • I should have said "in this instance". I was just trying to explain the meaning of a compound noun (which you possibly already understood). – Mick Oct 26 '16 at 8:00
  • Oops! You're quite right. I misplaced the apostrophe. [hangs head in shame] – Mick Oct 26 '16 at 8:03
  • I think cat's food is the key point in distinguishing noun from adjective. – user24743 Oct 26 '16 at 10:04
  • @Rathony I always get into trouble when I try to do grammar. I know I should stick to vocabulary and idioms. – Mick Oct 26 '16 at 10:10

Adding to other great answers, you should note that just because you can replace a noun with an adjective as in "Dogs won't eat cat / hot food" doesn't necessarily mean they are the same part of speech.

  1. An adjective is supposed to be placed before a noun except for some exceptional cases such as something/nothing/anything and some adjectives mainly used as post-modifiers. Therefore, you can't change the sentence to "Dogs won't eat food hot." or "Dogs won't eat food for hot."

  2. A noun generally can change its form or position more freely. For example:

Dogs won't eat cat's food (as suggested by @Mick)

Dogs won't eat food for cats.

  1. Even though some words have the same form, it could be used both as an adjective or noun. For example:

A sleeping baby: Here sleeping is a present participle (of a verb) functioning as an adjective. Nobody uses a baby sleeping or a baby for sleeping. They sound unnatural.

A sleeping bag: Here sleeping is a gerund functioning as a noun. It indicates the purpose of a bag as in a bag for sleeping.

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The sky is blue.

Here blue is an adjective, and not a noun. I am going to test whether it's a noun or an adjective.

Noun vs Adjective -

  • Nouns are not gradable. But adjectives are gradable.

Blue -> bluer -> bluest

  • Nouns are inflectional in case of plural, but adjectives are not.

I got two boxes. They are blue in colour. [NOT blues]

  • Comparative and superlative adjectives can take part in partitive constructions, but nouns can't.

But Pachet, a senior researcher operating under the bluest of blue skies, has been given an opportunity to think about what I really wanted to do as a researcher.

However, blue can also be a noun, but in other meaning, and context.

The room was decorated in vibrant blues and yellows.

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  • 'But adjectives are gradable.' nuclear, more nuclear, most nuclear, very nuclear? Classifying (and extreme etc) adjectives are not gradable. // Count noun usages usually inflect for plurals. – Edwin Ashworth Nov 6 '16 at 16:23
  • @EdwinAshworth If one test is not applicable, go for the other tests. I didn't exactly get what you mean, though. – Man_From_India Nov 7 '16 at 17:00
  • '[A]djectives are gradable.' is demonstrably not always the case. / 'Nouns are inflectional in case of plural' I assume means 'one can tell a noun because it will have a distinct plural form'. Again, demonstrably not always the case. 'If one test is not applicable, go for the other tests.' ... but how do you test the tests for applicability? // A more reliable differentiator between a singular predicate common noun and predicate adjective would seem to be the presence or otherwise of a determiner, but even this seems fallible (The sky was a dark blue-grey). – Edwin Ashworth Nov 7 '16 at 23:52
  • @EdwinAshworth hmmm right. I have to come up with something better :-) – Man_From_India Nov 8 '16 at 0:35

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