When to use "Adj+Noun" & when to Use "Compound Noun"

Why saying "beautiful girl" but not "beauty girl"

And, "Chemical Lesson" or "Chemistry Lesson"?

  • 2
    Possibly because one studies chemistry, not chemical, hence the compound noun form. – Mick Sep 30 '16 at 3:28
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    When you see two nouns like chemistry and lesson, the first is a noun adjunct that modifies the second. We actually do see the phrase Beauty girl in English, and that phrase follows the same rule. As for chemistry lesson, please use your dictionary! What does "chemistry" mean? – P. E. Dant Sep 30 '16 at 3:33
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    Just some thoughts. Saying that 'a beautiful girl' is a girl who's beautiful works (but a beauty girl is a girl who's beauty doesn't). Trying to say that 'a chemical lesson' is a lesson that's chemical doesn't work, so we wouldn't say a chemical lesson. – Damkerng T. Sep 30 '16 at 11:04
  • @DamkerngT. A counter example might be "glamorous girl" instead of "glamour girl". We do say "glamour girl" but we wouldn't say "a girl who is glamour". It's a tricky thing to find a rule for. – ColleenV parted ways Nov 9 '16 at 12:24

Why do we say "beautiful girl" but not "beauty girl"?

Think of the sentence combination "The girl is beautiful." + "She is coming this way now." You can put them together by using Adj+Noun":

The beautiful girl is coming this way now.

And, "Chemical Lesson" or "Chemistry Lesson"?


I have to go to my Chemistry lesson now or I have to go to my Chemistry class now.

This is the same as saying I have to go to my class about Chemistry now or I have to go to my class in Chemistry now.

So, Chemistry Lesson is an example of using a Compound Noun.

Let's see if we can make it work as an Adj+Noun". Hmm. Let's try the following two simple sentences: "I learned a lesson about Chemistry in the Kitchen today. I learned that if you forget to put the milk back in the refrigerator, it goes sour and you have to throw it out."

In other words, I learned a valuable chemical lesson today.

That was a bit of a stretch. It's much more common to use "Chemical lesson" as a Compound noun.


It depends on what you are saying. When you say 'beautiful girl' you are describing a girl who is beautiful. Hence adjective + noun is used. But if you say "chemical lesson" to mean studying/learning the subject of Chemistry, then it's wrong.

When you say "I have to finish my chemistry Lessons" it means you have lessons related to the subject of Chemistry. But if you say "I have to finish my chemical Lessons" it means you have lessons related to a chemical or chemicals.

Further examples:

Thanks for the chemical lesson sadiq. (means, sadiq taught you something related to chemicals.)

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