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Now I am writing a scientific paper, and I want to write a sentence as follows:

A has a chemical composition so similar to that of B that it also has a high potential.

As you can see, the first 'that' is used to substitute for 'chemical composition', and the second 'that' is used to introduce the following clause.

However, I am so worried because it seems to confusing due to the repetition of 'that'. (Because I am not a native, I cannot make a sentence with assurance...)

So, I tried to use a comma before the second 'that', but I don't know if it will be a correct sentence.

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    I've seen a few questions on ELL worrying about whether it's okay to use the same word twice in a sentence. (See this questions, for example.) Usually, those fears are unfounded. There's nothing wrong with your sentence with two instances of that. – J.R. Sep 6 '18 at 9:31
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    Clarity is always best: "...similar to the composition of B, which has a high potential as well." Use which, but not that, when the information introduced is unnecessary for identifying the thing or person described. In this case, which has to be set off with a comma. – Sara Sep 6 '18 at 9:43
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    @Sara I think not. The second that here is not a relativizer but a subordinator marking the complement of so; it is not B but A which "also has a high potential". – StoneyB Sep 6 '18 at 9:52
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    Thanks a lot@StoneyB I really didn't see it that way. – Sara Sep 6 '18 at 9:59
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    @Sara That[!] is an example of why I like to be very sparing in the use of that :) – StoneyB Sep 6 '18 at 10:08
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There are times when a comma may be added for the sake of readability. For example:

Those that know, do. Those that understand, teach.

                                 – Aristotle

However, I think your sentence reads fine and needs no comma. Moreover, sentences that contain the word that two times are not uncommon. As a matter of fact, sometimes the two that's appear consecutively:

Don't be seduced into thinking that that which does not make a profit is without value.

                                 – Arthur Miller

If you still insist your sentence seems awkward, here's what I might do instead of trying to fix it with a comma:

A also has a high potential, because it has a chemical composition so similar to that of B.

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    Some more helpful reading: When should I accept my answer? and Are thanks for answers unnecessary? Cheers. – J.R. Sep 6 '18 at 10:02
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    So, you're saying that that "that" that follows "that" is not wrong? – Acccumulation Sep 6 '18 at 18:40
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    If you really want to use a comma to split up the long sentence, change the connecting words to something like "A has a chemical composition that is very similar to B, and therefore it also has a high potential." – alephzero Sep 6 '18 at 20:04
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SUPPLEMENTAL to J.R.'s answer:
A comma would not be appropriate before the second that: this that marks the complement of so ... and consequently should not be disjoined from it.

As J.R. remarks, the double use of that is not really problematic. It's really two different words, demonstrative that and subordinator that. They are only historically and orthographically the "same" word. In Present-day English speech they aren't even pronounced the same way: the demonstrative is always stressed, and the subordinator is always de-stressed and pronounced with a reduced vowel.

I confess, however, that I personally find the use of two orthographic ‹that›s in different senses a little jarring in written English when they're so near one another. My own solution would be to recast the sentence to eliminate the first—perhaps something like this:

A's chemical composition is so similar to B's that it also has a high potential.

  • You don't even need the genitive. "The chemical composition of A is so similar to B, that A also has a high potential." – Stian Yttervik Sep 6 '18 at 13:35
  • @StianYttervik you don't need the comma in your example either - and IMO (native British English speaker) it would be better without the comma. – alephzero Sep 6 '18 at 20:06
  • @alephzero yeah on second glance I agree. Not native speaker =P – Stian Yttervik Sep 6 '18 at 21:16
  • I don't like @StianYttervik's suggestion; it implies that A and the chemical composition of A are the same thing. – Anton Sherwood Sep 6 '18 at 22:53
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    @Sungil They are all fully understood. The subjects in these sentences are "the color of A" and "the color of B" and they are connected by a verb (being) which is a linking verb and the full verb would be "to be similar". That is transitive - that is why I defined 2 subjects. Now, the discussion here is how to represent these 2 subjects, and you are technically not changing the grammar - the structure of the sentence. It is allowed to omit subjects when they are known. "The man is similar" (to whom? if it is known, if it was discussed just before, the sentence is OK) – Stian Yttervik Sep 7 '18 at 10:26
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Your instinct to use a comma before the second that is a good one.

Commas do not have hard and fast rules, and as J.R. points out, there are indeed times when a comma may be added for the sake of readability.

I, however, respectfully disagree with J.R.'s opinion about the use of a comma before your second that. I think that doing so makes your sentence clearer, and more easily understood. And that is the point of good writing; not being so obsessed with rules like "Don't start a sentence with 'And'", and "Don't use a comma unless it's absolutely necessary".

My rule is this: If a comma helps me to convey meaning, and if it helps my writing sound more conversational, I almost always use one.

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