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I have no background in chemistry whatsoever. However, I'm very interested in sidewalk chalk composition. I read that chalk can be both either made out of gypsum or chalk (calcium carbonate). However, I'm confused, since a person that it is connected to a chalk (final product) manufacturer told me that the main ingredient in their product (chalk for kids) is sulfuric acid carbon.

As I said, my knowledge in chemistry is very reduced. I was just hoping that someone that knows well the subject can say if there can be a connection between the sulfuric acid carbon and gypsum.

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    $\begingroup$ It seems that most "chalk" today is really based on gypsum, $\ce{CaSO4\cdot 2H2O}$, rather than the mineral chalk which is sedimentary carbonate rock, $\ce{CaCO3}$, derived from marine organisms. $\endgroup$
    – MaxW
    Commented Apr 27, 2020 at 19:31
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    $\begingroup$ I couldn't find any reference to Sulfuric acid carbon as some sort of industrial product. I'd guess that Sulfuric acid carbon is their name for some sort of calcium sulfate product which they buy and is a byproduct of some other process. $\endgroup$
    – MaxW
    Commented Apr 27, 2020 at 19:37
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    $\begingroup$ There are probably at least 2 translation steps in the terminology, with the second being into English and the first from the chemical composition to everyday language. I can just about see how you could start from "mixed sulphate/carbonate" and get to "sulphuric acid, carbon" $\endgroup$
    – Chris H
    Commented Apr 28, 2020 at 8:13
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    $\begingroup$ @ChrisH: I suspect that there is a lot in what you say. In my native language, German, "Kohlensaurer Kalk" [literally translated "carbonic-acidic lime"] is an old-fashioned way of refering to CaCO3, similarly "Schwefelsaurer Kalk" [sulphuric-acidic lime] is CaSO4. $\endgroup$
    – cbeleites
    Commented Apr 29, 2020 at 12:30
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    $\begingroup$ In addition, we have a chinese whispers-like joke of how such terms can turn into meaningless jumble of perfectly normal words for NaHCO3: Doppelsohlenkauendes Nashorn [double sole chewing rhinoceros] instead of doppelt kohlensaures Natron. It is a bit of a tautology, since German "Natron" is already NaHCO3, doppelt kohlensauer: 2 (double) equivialents of carbonic acid (compare bicarbonate) [per equivalent of sodium]. But Natron in this naming system can stand for Na⁺ in general (or if you want, NaOH). So the tautology helps against this ambiguity. $\endgroup$
    – cbeleites
    Commented Apr 29, 2020 at 12:31

2 Answers 2

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I am afraid whoever told you "sulfuric acid carbon" does not know the basic terminology (must be from non-chemistry background). There is no such thing as "sulfuric acid carbon" to begin with. Any carbon obtained by charring with sulfuric acid would be jet black. Nobody would like to have a black chalk.

It is very easy to parse $\ce{CaSO4.2H2O}$ incorrectly.

I think he/she misread the calcium $\ce{Ca}$ symbol for carbon which is $\ce{C}$.

$\ce{SO_4^2-}$ can be easily misrepresented to be sulfuric acid.

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    $\begingroup$ +1 A weird but quite possible explanation! $\endgroup$
    – MaxW
    Commented Apr 28, 2020 at 0:34
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    $\begingroup$ It may also be that they used (in whatever language) the colloquial terms which were misunderstood and/or mistranslated by OP into English. $\endgroup$
    – cbeleites
    Commented Apr 29, 2020 at 12:49
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    $\begingroup$ Could you also add what the correct name would be? (Calcium sulfate?) $\endgroup$
    – Paŭlo Ebermann
    Commented Apr 29, 2020 at 17:22
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    $\begingroup$ Not all chalk is calcium sulfate I guess. But yes the you would call it (hydrated) calcium sulfate. $\endgroup$
    – ACR
    Commented Apr 29, 2020 at 17:36
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Rather pure chalk ($\ce{CaCO_3}$) can be found in the subsoil of huge regions of central France. This chalk can be used for writing on blackboard, but it is too soft. You quickly get whitened fingers and hands when using it in a classroom.

You can replace it with gypsum ($\ce{CaSO_4·2H_2O}$) which is made by mixing plaster with some water, to give a sort of paste, that could be moulded to form a stick useful for writing on a blackboard. But the stuff is too hard, and it will scratch the board.

The best piece of chalk to be used in classrooms is a appropriate mixture of these two white stuffs : $\ce{CaCO_3}$ and $\ce{CaSO_4·2H_2O}$

But in any case, sulfuric acid will never be used for making pieces of chalk or any other substance that may be handled by the hand. Sulfuric acid is a rather corrosive substance. It will punch skin and clothes. it is one of most corrosive substance known in the world.

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  • $\begingroup$ I don't think you answered the question. The big question here is what is Sulfuric acid carbon? $\endgroup$
    – MaxW
    Commented Apr 27, 2020 at 20:09
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    $\begingroup$ Sulfuric acid carbon does not exist. It may be considered as a mixture of carbon and sulfuric acid, which are two independent substances which can be mixed of course. But I don't see the interest in doing this mixture. This mixture will be corrosive like sulfuric acid. It will never be used for any game for children. Too dangerous. So what for ? $\endgroup$
    – Maurice
    Commented Apr 27, 2020 at 20:39
  • $\begingroup$ Maurice - Sorry, I obviously didn't make my point clearly. The OP asked about Sulfuric acid carbon because someone who worked at a plant where "chalk" was made said that was the main ingredient. I agree that it doesn't seem to be a recognized industrial name. I'm guessing that is just what they call the raw feedstock within that particular plant. i'm guessing also that sulfuric acid carbon is a byproduct of making sulfuric acid or using sulfuric acid in some other industrial process. In other words, the chalk company is buying some other company's industrial waste. $\endgroup$
    – MaxW
    Commented Apr 27, 2020 at 20:49
  • $\begingroup$ @MaxW: still, I think this answer may help OP to dig down to what was actually said and meant in whatever language it was said. $\endgroup$
    – cbeleites
    Commented Apr 29, 2020 at 12:10

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