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I did a lot of research and asking questions about dragons' breathing ability:

Here's what I found

  • using pyrophoric subtances : impossible

  • using two chemical reactions like a bombadier beetle: impossible

  • using flammable subtances and sparks by either electric or metallic

    May work, but can only be used once and having that kind of stuff may or may not be impossible

I can see how breathing fire sounds a little more far fetched so my only theory is that dragons in real life are able to spit acid

Let's say that this dragon sprays acid like a spitting cobra, and it spits for defense.

What kind of acid, or maybe enzyme, can a dragon spray that's strong enough to feel like it's fire?

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    $\begingroup$ I gave you 4 different pyrophoric liquids across your 2 questions; NaK might be tricky to integrate with biology but acetone, ethanol and oil are all real biology. But no good for your dragon? $\endgroup$ – Willk Apr 5 at 15:17
  • $\begingroup$ Any acid will do. I personally put my vote on sulphuric acid, though containment can get a little hard. $\endgroup$ – Mephistopheles Apr 5 at 15:57
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The easiest option is Hydrochloric acid. Also, maybe less like Spitting and more like vomiting

You see, acid in an organism is a hard task, after producing it, you'll require special structures capable of resisting the corrosive effects of said acid. Luckily, many animals already have it, it's called stomach. In our case for example, it's mostly hydrochloric acid that used to digest our food, as well as other chemicals and enzymes that break down the food. So the main problem is a way to convert stomach secretions into a projectile form, so I'd suggest your dragon to have a stomach like that of the Fulmar. You see, mostly seen in chicks, fulmars are capable of projectile-vomit a foul smelling, oily substance that can kill other birds trying to attack it(it's also a rich energy source they feed their young with and as energy storage, but I assume that's not the focus). Your dragon could have a similar structure to their digestive system, with the exception of producing highly concentrated hydrochloric acid to spill on it's target instead. However, we might have to remember some key issues:

1-acid that eats through others' organic tissue will eat through yours. They'll need especially reinforced mouth tissues, throat and esophagus, as all of these structures will have to deal with the acid on a common basis. Otherwise their excessive acid production might do them more harm than good.

2-they will need a diet which replenishes the elements required for such a large acid production, as we can assume a certain part of it will be lost once out of its mouth.

Presonally I'd think your dragons, having this ability, would use it to start the digestive process outside of the body, vomiting digestive enzymes along with the acid. BUT I don't think they would use this like we see in Fire breathing dragons, as acid, much like venom in snakes, is costly do produce.

So, summing up: could your dragon "spit" acid? Not that unlikely, we have birds doing the spitting-stuff-off-their-stomachs part and many animals naturally produce acid. Would they use it all the time like their fire-breathing counterparts do? No, I'm mostly sure they'd save it as a defense mechanism (when feeling cornered and being out of options) and for hunting, vomiting on prey once it's pinned down to soften their flesh and aid in digestion, similar to how crocodiles will at times submerge dead prey in water and let it start rotting before eating it.

Ps: when acids and enzymes are eating through your skin, you'll easily mistake the sensation for burning, especially since acids like hydrochloric acid cause something known as "chemical burn". As not everyone would like to see what that is, you can see a picture depicting a hydrochloric acid burn in this link

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    $\begingroup$ That's basically my thoughts, too. A duplication could cause a second unused stomach that only gets used for this purpose. It could evolve to have stronger linings and more concentrated acid. With added toxins or enzymes this stuff could burn, then slowly dissolve while poisoning through the damaged skin. Big dragon means used lightly there might be enough to spray more than once. $\endgroup$ – DWKraus Apr 6 at 3:37
  • $\begingroup$ @DWKraus exactly, it might not even need to use toxins, as the acid would naturally hinder healing of the wound and allow for infection by bacteria resistant to lower pH environments, which could even be present at the mouth or gut of the dragon. $\endgroup$ – ProjectApex Apr 6 at 3:45
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Use concentrated stomach acid ...

And to do that safely ...

The dragon's innards, for acid delivery, are lined with "ear wax"

This answer is built with an engineer's eye to the inner workings of a dragon, which we will assume is some kind of carbon based life form.

First, posit that the dragon will internally produce a kind of acid (as the human body produces acid in the stomach to aid in food digestion). We then find an overflow valve (muscle flap or opening like a sphincter) that can port excess stomach acid into another organ / reservoir. That reservoir is a hybrid between a human bladder and a cow's second stomach - it holds an amount of fluid, and is also where a chemical reaction occurs that (in this case) increases the acid's concentration (which is what makes this ejected acid dangerous/lethal). {Something like hydrochloric, hydrofluoric or sulfuric acid will do; pick one}.

  • A note regarding acids: stomach acid (primarily hydrochloric acid) and hydroflouric acid are not the same. The former can quite safely be stored in glass vessels, but wax should still be a good protective material for the dragon to use since its innards are assumed to be made out of something other than glass. (thank you @Someone_Evil)

This requires another bit of plumbing and a process to secrete wax internally into the tissues of that bladder/stomach to coat the lining with the "ear wax" analogue. (Humans create "ear wax" so our dragon will secrete/create something similar based on dragon physiology).

We now need piping that takes the fluid from this bladder via a single path out of the dragon. The "Ear Wax" is internally secreted into that tubing (wider than blood vessels, narrower than a throat, similar material) to coat the tubes.

The acid ejection itself is a result of a very hard squeeze by the bladder muscle. Dragons are big and strong, dragons have very strong muscle tissue ... acid will issue forth in bursts rather than streams, and will thus appear to be more like "spitting" than a fire hose in application.

The opening of these tubes (more or less discharge ports) are in the corner of the dragon's mouth on either side - the fleshy tubes normally hang down, but there is muscle tissue surrounding them to point 'the open ends out' and thus these flesh tubes must be connected by the nervous system to the bladder: the tubes' muscles contract when the bladder squeezes. The dragon aims by moving its head while it's mouth is open, and there's your acid spitting dragon.

Practical limitations

This rig poses some limitations on the dragon: it only has a few bursts per day, or a few bursts every few hours, before it empties its acid bladder and must refill it. (This would be similar to a limitation in old Dungeons and Dragon days when dragons were only able to use their breath weapons a few times per day). It makes sense for there to be a limited amount per unit time if you are already going for this degree of verisimilitude. The bladder is rechargeable, it just takes a bit of time and digestive process ...

What about the valves?

The valves at the top of the bladder are similiar to the Eustachian tube in the human ear. (You can clear you ears by doing "valsalva" which resembles the activation of a valve against pressure). For the dragon's protection, these one-way check valves only open when the internal bladder-muscle squeezes, and open in only one direction: the acid stream's ejection direction.

The reservoir, similar to a human bladder, stores a few to a dozen gallons of fluid (based on how big of a dragon you have). It's a big sack made out of mostly muscle tissue that can either squeeze (contract) or relax.

Why wax? ("Ear Wax" for illustrative purposes)

When I was in 8th grade, we learned that hydrofluoric acid ate most (glass) materials that might contain it, but if the vessel were wax lined the acid would not eat through it. While these days plastics are used for acid storage, wax is a compound that animals (ear "wax" from humans, wax made by bees) could produce and you thereby avoid your world building needing to include a petrochemicals industry. :-)

All of this is "similar to" rather than a cut and paste from Earth biology, since you are making up a fantastic world that isn't real in the first place.

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Sulfuric acid

I made quite a few question about this very topic. The reason for choosing this acid is becaue it actually burns (well, oxidizes) stuff and it can be synthesized by bacteria.

Now, potency. You see, pH doesn't tell us much about how concentrated is the acid. At least, it's difficult to approximate. Battery acid, for instance, has a concentration of 36% and a pH of 0.7.

An acidophilic bacteria, Picrophilus, can grow in pHs as low as -0.06 and prefers high (45 °C) temperatures. However, they can't survive in pHs above 6. They're also strictly aerobic, so they must have ways to protect themselves from oxidative damage. Also, their cell membranes have high acid stability.

Perhaps dragons could adapt to be able to handle such strong acids.

As for spitting/vomiting it, we're talking about flying creatures, so they'd most likely use their flight muscles to propel it. I'm yet to come up with the exact method, but I have an outline that's inspired by weightlifters projectile-vommiting protein shake because their posture applied extra pressure on their stomach.

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If you want something that really does feel like fire, instead of an acid, why not go for the solution that has been evolved by nature and has been weaponized and synthesized by human chemists: oleoresin capsicum (OC), the active ingredient of chilli peppers.

OC is widely used in pepper spray, pepper balls and a variety of crowd-control and suspect-disabling 'less-lethal' weapons.

In spray from it could easily be produced by a biological creature, and something like a dragon could disperse it in aerosol for perhaps tens of metres. It would be extremely effective at disabling (and basically blinding) any adversaries not taking effective protective measures so they could be easily taken out by the dragon's other weapons.

The cloud of aerosol might look like steam or smoke, adding to the impression - along with the cries of the victims - that the dragon's breath is some kind of fire.

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  • $\begingroup$ what about some creatures with furs, can it also affect them too? $\endgroup$ – user3556983 Apr 11 at 15:55
  • $\begingroup$ Yes, it mainly gets in your eyes, nose and lungs - anything which breaths will be affected, but bare skin is worse $\endgroup$ – David Hambling Apr 12 at 8:27

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