So, dragons are hexapodal creatures, slightly larger than most draft horses (around 190-200 cm at the withers). Compared to the Quetzalcoatlus, dragons have twice the muscles mass in the pectoral region, but the length of the fibers stays consistent. Basically, the chest is just as deep, but the length is extended.

The extra muscle is attached to the heavily modified humerus. Dragon bones are very strong, due to their microscopic structure, which mimics limpet teeth.

That's how dragons remain volant, despite their size. Now, their breath weapon is a two-component acid, aqua regia. Its constituents, nitric acid and hydrochloric acid, are stored separately for safety reasons and only combine moments before exiting the dragon's mouth. Still, having too much acid could interfere with the dragon's ability to fly.

On top of that, the dragon's saliva and mucus are only able to stall (but not completely stop) and (with sodium bicarbonate) neutralize the acids, when present in large-enough quantities.

The breath weapon is useful for dealing with humans and their equipment, but not for much else.

Dragons usually spray targets with the acids, so the two components don't really have a chance to stay combined.

Given that, what's the realistic amount of aqua regia (so, the sum of the two components in liters) a dragon should store?

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    $\begingroup$ Just a note: aqua regia is neither more nor less reactive with organic materials than the component acids. It's more reactive with gold, but that's of little concern for the dragon. $\endgroup$
    – AlexP
    Commented Feb 5, 2020 at 20:11
  • $\begingroup$ @AlexP Aqua regia is only a temporary state of the mixture of these two acids, so combining them long before it's necessary defeats the purpose of aqua regia. $\endgroup$ Commented Feb 5, 2020 at 20:16
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    $\begingroup$ My comment is that there is no "purpose of aqua regia". Aqua regia is not more reactive with knights etc. than the nitric acid on its own. Unless the knight is made of gold, of course. $\endgroup$
    – AlexP
    Commented Feb 5, 2020 at 20:18
  • $\begingroup$ @AlexP Not really, but the dragon does "drool" on objects if he wants to dissolve them and aqua regia simply just expands the list of dissolvable thing. $\endgroup$ Commented Feb 5, 2020 at 20:24
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    $\begingroup$ Dragons or lore are gold hoarders. I see a story here since the OP is obviously putting a lot of effort into a story based around aqua regia in dragons. I remember you said they are as smart as humans, but immature... maybe you have a story here about a dragon that turned a whole town's fortune into aqueous gold because presumably that's what aqua regia dragons do. Then, the story is about the discovery about where the gold went, and negotiations on how to get it back.... or something. Anyways, acid is a bad weapon compared to, for example, the horns on a bull, etc. $\endgroup$
    – doe
    Commented Feb 6, 2020 at 2:50

4 Answers 4


My acid spitting dragons use sulphuric acid, not hydrocloric acid.

The chemistry is pretty simple. When an acid is put in water, it disassociates into H+ ions. Hydrocloric acid is already in water and mostly disassociated with a ratio of 30-40% acid and the rest water. Sulphuric acid on the other hand is 98% acid and 2% water, so it is seeking water to disassociate with.

When my dragon breathes his acid breath on the human trying to kill him, it immediately dehydrates the skin it comes in contact with, the skin carbonizes resulting in 3rd degree burns.

Your aqua-regia dragon is more into processing and refining gold than dealing with humans or other beasts that might attack it. The Hydrocloric acid will burn eventually, but the affected areas don't immediately react to the acid.

In both cases, the dragon's mouth and surfaces that may have been affected by the acid would be doused in glands that create a base mucous coating that stops any major damage from the remaining acid. This mixture causes "drooling" and is a way of disposing the mixture off of the dragon.


Draft horses weigh around 900kg, modern estimates for the weight of the Quetzalcoatlus are around 200kg (lower-bound) to 400kg (upper-bound). Also, the Quetzalcoatlus is about the size of a giraffe with the wingspan of a small cessna. Assuming your dragon is about the same size of the Quetzalcoatlus then it will probably have difficulty going from stationary to flying if it weighs more than 500kg.

Having that stablished it's easy to determine how much aqua regia it carries. Just pick how bulky your dragon should be and subtract that from 500kg, the remainder is your available aqua regia storage.

If it fits within your story you can also say that the dragons have a much bigger acid storage (thus exceeding the 500kg limit) but when they're full they cannot takeoff, so before flying they have to spit enough acid to make themselves lighter.

Another possibility if you want them to be heavier is to choose their natural habitat so that they don't need to takeoff from the ground (i.e. if your dragon lives on cliff-edges then it can fly by jumping from the cliff edge and gliding).


Ok so we have a dragon the size of a horse with some special anatomy. In this case, as you used quetzalcoatlus and considering it's wings have similar size and area, I'll just assume your dragon weights as much as the heaviest quetzalcoatlus estimate to allow for flight (around 250 kg) since despite smaller it has added weight due to the extra muscle and bone for the 3rd pair of limbs. I'll also assume it's predominantly a carnivore.

Apparently you're going for aqua Regia both because it melts most metals as well as due to the fact it can heat up to over 100 degrees celsius (232 Fahrenheit) once you mix both acids (so that's why you want them separated?). However, with flight being the main tool here, your dragon is already at basically the weight limit (though I'd think it can likely get a bit heavier due to its powerful muscles, let's not rely too much on that). So we need a quantity that doesn't make it too much heavier to allow for flight. Additionally we don't want a volume too large for our dragon, so we need to watch for that.

Considering aqua Regia (luckily or unluckily) has a density of about 1,21 grams per cubic centimeter, we see that a kilogram of aqua Regia would occupy a volume of around 0.826 or approximately 0.8 liters. Since horses have a total volume of around 416 liters and your dragon shouldn't have as much guts (herbivores requires larger digestive systems to process plant matter while carnivores don't need them as long to process meat, which is why a herbivore tends to have a body cavity twice as large as a similar sized carnivore) I'd say, on a moderate estimate of someone with very little expertise, that your dragon should be able to store in the best of cases (in a moderate estimation), that it could potentially store from 10 to 20 liters total of aqua Regia. The issue with this is that every 10 liters of aqua Regia would result in an increase of 12,5 kilograms. This much weight added would almost certainly hinder flight, so we have a dragon that can't store much aqua Regia if it plans to fly, unless it acts like a vulture that just fed and begins to empty its reserves every time it wants to take to the skies. Based on this, I'd say the best outcome would be a storage space of around 1/4 of a liter, which would still result in a. 3,125 kg increase in weight, but should be an acceptable one as to not make flight a problem.

If I got something wrong in my calculations please let me know. For now, it seems like your dragon likely act like a skunk, carefully choosing when to use its Aqua Regia spray (remember you'll need compartments capable of containing said acids though, so we might have a slightly smaller amount than that available).

  • $\begingroup$ you have done some very wrong maths. 1 liter is 1000 cubic cm. 1 kilogram is 1000 grams. 1000/1.2 = 826.44. Therefore you only need 830 ml to contain one kilogram of aqua regia. Please correct your answer. $\endgroup$ Commented Jun 18, 2020 at 5:24
  • $\begingroup$ @meaninglessname thank you very much for warning me, I have no idea how I got it wrong like that. $\endgroup$ Commented Jun 18, 2020 at 11:19

Did the dragons evolve, or were they birthed with magic? If birthed with magic, then it might as well just be matched to the average amount of humans they run into in a day.

If they evolved, then actually how much acid breath they have has nothing to do with humans, because dragons evolved before them (or at least before humans had weapons).

So if not evolved for humans, what was the original purpose of the acid? This is important because sometimes evolution works so hard on one thing that it is an impediment to other things. Just try and imagine a male peacock trying to escape a predator.

If the original use of the acid was something massively essential for survival and selection, they might carry so much that it actually hurts their flying distance and limits their lifespan by slowly dissolving their teeth (an animal that can't chew starves). A fun opportunity to consider their past situation and enemies.

But if they used it for something less, like maybe occasionally burning holes in rocks to hide eggs, a liter is more than enough (one human a day worth).

Although as AlexP points out, this particular acid is worst case scenario for a dragon, since its claim to fame is it dissolves gold. That's more of a curse you would put on a poor hoarding dragon who just wants to lick his treasure!

  • $\begingroup$ Just because it's useful against humans doesn't mean it was meant for them specifically, they just happen to be a threat. It's simply a defense mechanism, and therefore, the more the dragon can use it the best. Think of how skunks are essentially defenseless for about a week after a full spray. Additionally, an animal can have more than one defense mechanism, as seen in the frilled lizard's many strategies to escape predators, with flight here clearly being the focus over the acid, like he said himself. $\endgroup$ Commented May 18, 2020 at 15:10

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