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When do you use what word to express that something consists of something else?
Does a whole “compose” its parts?
Correct use of “consist”

Water comprises/composes/combines/consists two hydrogen atoms and one oxygen atom.

Can anyone let me know the correct usage in this sentence?

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Note that it is two hydrogen atoms and one oxygen atom that make one water molecule. –  neil Sep 17 '12 at 18:32
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marked as duplicate by RegDwigнt Oct 21 '12 at 10:05

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There are several possible correct usages.

You can say "Water comprises two hydrogen molecules and one oxygen molecule".

You can say "Water consists of two hydrogen molecules and one oxygen molecule".

You can say "Water is composed of two hydrogen molecules and one oxygen molecule".

You can probably say "Water is a combination of two hydrogen molecules and one oxygen molecule", though it sounds a bit off to me.

Note that they all have different structure, and subtly different semantics.

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“Water is a combination of two hydrogen molecules and one oxygen molecule” isn't “off”, merely verbose. One can also grammatically say “Water combines two hydrogen molecules and one oxygen molecule”, which does sound a bit off. –  jwpat7 Sep 17 '12 at 18:22
    
Yes, maybe it's just the verbosity that felt a bit strange to me. Maybe because "combination" is too vague a word. Dunno. –  Avner Shahar-Kashtan Sep 17 '12 at 18:23
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I agree with your grammar, but (as @neil states) the science would dictate: A water molecule comprises two hydrogen atoms and one oxygen atom. A water molecule consists of two hydrogen atoms and one oxygen atom. A water molecule is composed of two hydrogen atoms and one oxygen atom. –  JLG Sep 17 '12 at 18:50
    
I want best answer out of these. –  Sudhir Sep 17 '12 at 19:01
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There's no best answer out of these. I'm fond of "comprises", personally, but that's a personal stylistic choice, not a "wrong" or "right". Note @neil and JLG's clarifications that you're talking about water molecules, not water in general, and choose the one that sounds nicest to you. –  Avner Shahar-Kashtan Sep 17 '12 at 20:15
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The last time I had water, there were a lot more than two hydrogen atoms involved, and very many oxygen atoms. For this to make sense, you have to make it clear that you're talking about a single molecule of water, not water in general.

My preference for how to express this is the following.

A molecule of water consists of two hydrogen atoms and one oxygen atom.

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For clarity use composed of, perhaps. Comprise can be seen in two opposite senses--whether the whole comprises its parts, or the parts comprise the whole.

OED under 4b:

The house comprises box-room, nine bed-rooms, bath-room, etc.

but under 8b:

These essays together with those contained in this volume comprise the total of C. S. Lewis's essays on literature.

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