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From a PDF containing teacher's notes on teaching limestone chemistry:

Heating the limestone (calcium carbonate) drives off carbon dioxide gas leaving behind lime, the base calcium oxide.

Do we need a comma before "leaving behind lime", or is it optional? Maybe the presence of "the base calcium oxide" makes it awkward to use a second comma in the sentence?

I'm not against the comma before "the base", I'm wondering whether we should use a comma before "leaving", like this:

Heating the limestone (calcium carbonate) drives off carbon dioxide gas, leaving behind lime, the base calcium oxide.

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The comma before leaving is desirable: it reflects the required spoken "reading" the sentence would take.

Without the comma, the participle phrase leaving behind lime &c is taken to be a restrictive modifier on gas, as if gas were the subject of leave; this would be equivalent to the carbon dioxide gas which leaves behind lime. In speech this meaning would be realized as a continuous phrase, with even stresses: 'gas 'leaving be'hind 'lime. . .

It's pretty clear, however, that leaving behind lime is a clausal modifier, that the subject of leave is the entire preceding clause: what leaves lime behind is the action of driving off the gas. In speech this distinction is realized with what is loosely called a "pause"—actually a lengthened syllable with a falling intonation— on gas, followed by a new phrase starting with leaving. That "pause" is what the comma represents.

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As written, the comma is needed, because "the base calcium oxide" defines "lime". If you read it as:

...lime the base calcium oxide.

it doesn't make any sense.

I think this was written strangely though, in that after defining limestone in parenthesis is the first phrase, lime was defined in a separate clause in the second phrase. I would prefer:

Heating the limestone (calcium carbonate) drives off carbon dioxide gas, leaving behind lime (the base calcium oxide).

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  • "lime the base calcium oxide" - I was not having in mind that comma, I was talking of the comma before "leaving". May 12, 2016 at 4:38
  • I would leave it in as it divides two separate actions. As I would say this sentence, I put a natural pause there.
    – user3169
    May 12, 2016 at 15:53

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