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Chlorine(VII) oxide reacts with water to give the very strong acid, chloric(VII) acid - also known as perchloric acid. (source)

I guess we can use a instead of the here:

Chlorine(VII) oxide reacts with water to give a very strong acid, chloric(VII) acid - also known as perchloric acid.

After all, this is the first mention, and we're describing "chloric(VII) acid".

Chloric acid is a very strong acid.

I wonder what might have nudged the author towards the use of the here. Is there any difference in meaning?

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  • Forget about the first mentions "rule." Go read any magazine article on the internet and you will find that 70% of first mentions utilize the definite article.
    – GoDucks
    Feb 1, 2016 at 19:25
  • @GoDucks - I know, it's just when I'm busy with other things, I forget exactly why we use the in this or that "first usage" context. So I'm curious why is it used so now.
    – CowperKettle
    Feb 1, 2016 at 19:33
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    The difference in meaning in seems like emphasis to me. The is more emphatic and suggests that the author will go on to talk about this type of acid.
    – Era
    Feb 1, 2016 at 19:35
  • Because it is idiomatic to do so in scientific articles (among others), seeing as how the referent will be further defined and discussed. In this context, speaking definitely about something is deemed more desirable than speaking indefinitely about something.
    – GoDucks
    Feb 1, 2016 at 19:37
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    "the history" demands an explanation, as it (incorrectly) assumes the listener/reader knows what is being referred to. What history? The poet is aware of this; hence, the explanatory "this history..." in the next line.
    – Tᴚoɯɐuo
    Feb 2, 2016 at 11:08

3 Answers 3

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We often signal the topic of what we are going to talk about not with a/an but with the. The acid Chloric(VII) acid is the topic of the portion of the article following its first mention. The definite article is used because of the discourse prominence of the referent.

The definite article serves a variety of functions in discourse besides the previously mentioned identifiability/familiarity. It also denotes:

  • the discourse prominence of an entity;...

Source: Page 151 of Extension and its Limits. Search the text for "discourse prominence".

Consider also:

An especially clear illustration of a discourse prominent entity is one that is highly topical. Most interesting in this respect are entities entered into the discourse with an initial definite description in order to signal that they will be topics in the subsequent portion of text [my emphasis]

Source: pdf The definite article, accessibility, and the construction of discourse referents

by Richard Epstein.

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As always, the choice of article reflects the speaker's attitude with respect to the listener's ability to recognize what he is referring to.

A speaker who says the strong acid, chloric acid is, intentionally or unintentionally, projecting the attitude that he expects the listener to recognize it by name and to already be aware that the name is associated with a characteristic, namely "strong acid".

A speaker who says a strong acid, chloric acid is not projecting the attitude that he expects the listener to recognize the acid by name.

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  • -1 hardly as always. Also, if you downvoted my answer why don't you leave a reason, or do you just serially downvote anything I post?
    – GoDucks
    Feb 2, 2016 at 16:10
  • If one says Have you heard about the accident on Romano Avenue? the speaker is not "intentionally or unintentionally, projecting the attitude that he expects the listener to recognize it," (the accident) rather...
    – GoDucks
    Feb 2, 2016 at 16:14
  • ...he is signalling to the auditor that he knows the auditor cannot identify the referent and that he is now going to identify the referent for the auditor by making it the topic of what he wants or intends to talk about.
    – GoDucks
    Feb 2, 2016 at 16:17
  • It wasn't my downvote. At least I didn't recall downvoting it. To test, I just tried to dowvote your answer now, and the attempt was successful. So I hadn't. The listener may or may not have heard about that accident referred to as "the accident". But for the speaker to use the word the projects the speaker's attitude, namely, that people have heard about it and have been hearing about it (it is the topic of conversation and gossip) and the listener ought to become one of these people if he is not already one of them.
    – Tᴚoɯɐuo
    Feb 2, 2016 at 16:37
  • Well, obviously, I don't agree with that interpretation and it does not accurately describe usage/what's going on. You might give the pdf by Epstein a read.
    – GoDucks
    Feb 2, 2016 at 16:42
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It depends on how you interpret the statement.

Chlorine(VII) oxide reacts with water to give a very strong acid, (which is) chloric(VII) acid...

Here using an indefinite article is OK, considering that there are many strong acids, and chloric(VII) acid is just one of them.

Chlorine(VII) oxide reacts with water to give the very strong acid, chloric(VII) acid...

Here using the definite article is OK, considering that you are specifically naming chloric(VII) acid.

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