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How can it float in the air ?

The balloon is filled with helium gas which is lighter than air. It is similar to when we blow air under water. The bubbles filled with air under the water would go up because air is much lighter than the surrounding water.

My question is about the phrases "under water" and "under the water". Is it okay to leave out the "the" in the first usage? And what are the rules to include or exclude "the" in this situation?

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up vote 5 down vote accepted

OP's cited example seems perfectly natural to me.

The first under water is a non-specific "adjectival phrase" - the writer isn't interested in any particular air or water, so in principle he could have simply joined the two words together as underwater. As this chart shows, Americans have enthusiastically adopted this one-word form in recent decades.

But in this particular instance, the next sentence is about the bubbles and their relationship with the water. I think it would be stylistically clumsy to use water as part of a "compound word", then use it (twice!) as a standalone word in the very next sentence.

Or, the writer might be an older Brit. We've been much slower to adopt the one-word form (but we're getting there).

When used as an adjective preceding a noun, the one-word form is well-established. The uncertainty only arises when it's used as a predicate adjective (as in OP's example, or my first chart showing the rise of American usage of breathe underwater).

In short - I'm quite prepared to believe the writer might ordinarily have used the one-word form, but being a competent writer, he deliberately chose not to in this particular case.

As regards "the rules" for using "the" in this situation - there aren't any, really. The writer could reasonably have dispensed with it before "bubbles" and/or the second instance of "water". He could even have added another one - "because the air is much lighter".

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Thanks for the detailed analysis. The answer is very clear and helpful ... – Stanley Jul 25 '12 at 6:19
Underwater breathing sounds perfectly fine, but I’m not sure I’m comfortable with breathing underwater. – tchrist Jul 25 '12 at 13:01
@tchrist: As my adjective preceding a noun link above says, in that "leading" context, the one-word form has been around a long time, so I'd expect everyone to be happy with it. Attitudes differ when it's a predicate adjective, but it seems clear to me it's more recent, and is rapidly gaining acceptability. – FumbleFingers Jul 25 '12 at 13:08

You can actually just say "underwater," without even a space or a hyphen. "Under the water" would typically only be used to refer to a specific body of water: e.g. Question: "where's the water proof phone?" "Answer: it's under the water in the kitchen sink!"

Basically you almost always use "under water" or "underwater" and not "under the water."

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Thanks for the answer. The joint form "underwater" did not come into my mind at all even though I should have seen it multiple times before. – Stanley Jul 25 '12 at 6:26

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