In cartoons, etc., acid is generally shown as vividly green, as in this scene from the Simpsons:

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We even have the colour "acid green".

But when I used to use acids in chemistry class, they weren't bright poisonous green. They were mostly colourless, as, for example, this picture of sulfuric acid from Wikipedia:

enter image description here

So where does this idea of green acid come from? Are there acids which are actually green? Or perhaps acids are green when highly concentrated?

  • $\begingroup$ my guess it comes from slang 'acid' for LSD, which was a source of psychedelic art, making a lot of use of bright 'acidic' colors. $\endgroup$ – permeakra Jan 20 '16 at 18:03
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    $\begingroup$ BTW, acid green color is not as much a color of acid - it is a color used to depict any toxic or dangerous substance. Green is a common color-code for 'poisoned' state of a character. Not sure where it came from. $\endgroup$ – permeakra Jan 20 '16 at 18:07
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    $\begingroup$ According to one source, green is associated with 'dangerous' from the association of (radioactive and dangerous) radium paint used to make green-colored glow-in-the-dark watch faces. Perhaps 'dangerous' = 'glowing green' is applied to acid as well. Also, transparent wouldn't render too well in a cartoon :) $\endgroup$ – Todd Minehardt Jan 20 '16 at 18:07
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    $\begingroup$ I guess it was simply the one basic, bright color left: blue is for pure water, red is for blood which leaves green for acid. $\endgroup$ – Thawn Jan 20 '16 at 21:15
  • $\begingroup$ @Thawn, what colour would the grass be? ! :) $\endgroup$ – Technetium Jan 21 '16 at 2:42

The green is the color of bile which usually accompanies gastric acid. Acid in its original context referred to a bitter taste. Bitter tasting compounds, such as vinegar, is what led to the modern chemical definition. The chemical definition is less likely to have influenced the populace than the experience associated the somewhat bitter, acidic, taste of gastric acid accompanied by bile.

  • $\begingroup$ Good answer. However, I think you mean sour or acrid taste not bitter... $\endgroup$ – Thawn Feb 8 '16 at 14:15
  • $\begingroup$ Bitter is correct in this answer's context. In the early days of chemistry several acids, for example tannic and hydrocyanic acid, were discovered by extracting plant material which tasted bad, or the "bitter principle." Bitter, along with sweet, was regarded as one of only two types of tastes. This thinking, started by Aristotle in 350 BC, did not change until the early twentieth century when the number of accepted taste types expanded to four with a division between sour and bitter. Thankfully discussing taste in non-food related chemistry is an abandoned practice. $\endgroup$ – Agriculturist Feb 8 '16 at 15:32
  • $\begingroup$ Interesting, thanks for the historical perspective :) $\endgroup$ – Thawn Feb 8 '16 at 18:25

One chemistry related answer could be that chlorine gas has a greenish hue after which it was named(wikipedia):

Sir Humphry Davy in 1810, who named it from Ancient Greek: χλωρóς (khlôros) "pale green

Also, chlorine gas is closely related to hydrochloric and hypochlorous acid:

Solutions of chlorine in water contain chlorine (Cl2), hydrochloric acid (which is arguably the most commonly known acid), and hypochlorous acid:

$\ce{Cl2 + H2O <=> HCl + HClO}$

Finally, in WWI, chlorine gas was used as a chemical weapon, that caused acid burns to skin, eyes and lung:

By April, German chemists had tested a method of releasing chlorine gas from pressurised cylinders and thousands of French Algerian troops were smothered in a ghostly green cloud of chlorine at the second Battle of Ypres. With no protection, many died from the agonies of suffocation

  • $\begingroup$ Maybe it could be good answer, but right now it looks like speculation. $\endgroup$ – Mithoron Jan 20 '16 at 22:51
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    $\begingroup$ @Mithoron Well, this question is unlikely to get a definitive answer because it basically asks for the intentions of comic artists. $\endgroup$ – Thawn Jan 21 '16 at 6:15

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