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To express heating, we usually use $\Delta$ over reaction arrows like \begin{align} \ce{CaCO3 ->[\Delta] CaO +CO2}. \end{align} Why do we use $\Delta$ to mean heating during chemical reactions? I thought up with one idea that $\Delta$ symbolizes fire. But actually I have no information about the origin of this usage. Does anybody know the origin or reason of this usage of $\Delta$?

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  • $\begingroup$ Good question, I would like to know how widespread the triangle is as symbol for heating $\endgroup$ Commented Dec 2, 2018 at 18:48
  • $\begingroup$ It looks like a Bunsen burner flame. It used to be below the arrow. $\endgroup$
    – jimchmst
    Commented Jan 19 at 20:06
  • $\begingroup$ adding heat does not mean an increase in temperature it means to define a temperature and run the reaction at that temperature. Reactions are usually run at constant T and P. $\endgroup$
    – jimchmst
    Commented Jan 21 at 10:47

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The retrieval of this symbol history poses a considerable challenge, as it is scarcely mentioned in textbooks, Google Books and Google Scholar. The current application of the triangle symbol, distinct from the capital Greek letter delta, can be traced back to the mid-1700s in pre-modern chemistry. This is evidenced in Torbern Bergman's 1735-1784 work, "A Dissertation on Elective Attractions," accessible via the Internet Archive. At the end of the book, originally written in Latin, there is long list of symbols with explanations as shown below. In this context, the inverted triangle represented water, while the upright triangle symbolized the "matter of heat," aligning with symbolic alchemy. Wikipedia has a nice page on Alchemical Symbols. For these reasons, it will be impossible to assign a single person behind this notation.

enter image description here

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  • $\begingroup$ Here are the tables: chymist.com/Torbern%20Bergman.pdf $\endgroup$
    – Karsten
    Commented Jan 21 at 23:34
  • $\begingroup$ And here the entire translated thesis: archive.org/details/adissertationon00berggoog/page/n93/mode/1up $\endgroup$
    – Karsten
    Commented Jan 21 at 23:39
  • $\begingroup$ Apparently, arrows weren't used until about 100 years later: chemistryviews.org/details/ezine/2746271/… $\endgroup$
    – Karsten
    Commented Jan 22 at 2:05
  • $\begingroup$ The reference establishes the triangle as a reaction condition (at high temperature, typically in a crucible), distinct from reactions in aqueous solution (designated by a triangle with tip pointing downward). $\endgroup$
    – Karsten
    Commented Jan 22 at 9:25
  • $\begingroup$ @Karsten, The history of arrows is also interesting. Thanks for the links $\endgroup$
    – ACR
    Commented Jan 22 at 12:53
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It can be mentioned that the triangle was used as the Alchemical symbol for fire in ancient times. However I don't know if it has been in continuous use as a reaction-symbol that way since, or if it fell out of use and the modern symbol is taken from Delta and is similar only by random coincidence.

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    $\begingroup$ This explanation makes much more sense than the link to $\Delta H$, which could mean exothermic or endothermic, and is only loosely connected to heat transfer. I would be nice to find a reference, though. $\endgroup$
    – Karsten
    Commented Jan 19 at 17:07
  • $\begingroup$ Another related link, I'll let somebody else judge on its reputability eoht.info/page/%CE%94 $\endgroup$
    – Ian Bush
    Commented Jan 19 at 22:37
  • $\begingroup$ @Karsten: Alchemist's symbol for fire. $\endgroup$
    – user55119
    Commented Jan 19 at 22:39
  • $\begingroup$ @user55119 I have found reputable sources documenting the alchemists symbol for fire. The question is whether this is a coincidence or the symbol with the arrow is meant to symbolize fire. $\endgroup$
    – Karsten
    Commented Jan 20 at 1:13
  • $\begingroup$ @Karsten - I wonder if it would be more likely to get a documented answer at History of Science SE? I tried to flag it to be migrated there, but it's too old to migrate. Maybe as a moderator you can override that (if you agree)? $\endgroup$
    – Andrew
    Commented Jan 20 at 12:49
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Gosh folks! Way back when the only convenient laboratory heat source was a candle or Bunsen burner the symbol for heating was an isosceles triangle placed below the reaction arrow. It symbolized the shape of a candle or BB flame. With the advent of typewriters and computer fonts a symbol was needed; the equilateral triangle was the best available so the isosceles morphed into an equilateral. That it happens to be a Greek delta and is chemspeak for change is a coincidence. My only source is that I am old enough to have lived thru all this nonsense.

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Δ symbolizes difference. For chemical reactions, it's usually difference between energy that reactants already have and energy needed for reaction. Then Δ = $E_a$ - activation energy of the reaction. Activation energy is like a "potential barrier" for reaction to happen. And Δ means that we've added this amount of energy so energy of reactants changed by Δ (difference).

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