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An answer in this question by Gillgamesh inspired this question, mostly to see if it's at all possible.

Basically the end-goal is to have an organism that would make use of endothermic reactions to either:

  1. Cool itself off(or possibly others of its kind), or
  2. Make use of the sub-zero temperatures for some other purpose like food preservation, nest construction(ice nest perhaps?), or perhaps some odd form of hunting or defense tactic where it either covers its prey in the chemicals that'd proceed to cool down them down to lethal levels or numb a predator's extremities with the cold through a reactionary(bombardier beetle style?) mechanism and making it less likely to succeed in hunting the organism or avoid the organism altogether.

Would an earth-life organism be able to produce or otherwise synthesize the chemicals required to produce and make use of endothermic reactions?

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    $\begingroup$ Yes, obviously. For example, humans use evaporative cooling to cool their bodies. The endothermic process in this case is the transformation of water from a liquid to a gas; the process proceeds spontaneously in this direction provided that the ambient air is not at 100% humidity. This works by means of a large number of sweat glans, which, when commanded by the autonomous nervous system, extract water from the blood plasma and produce a thin film of liquid water on the skin. The water evaporates, which is an endothermic process, and thus absorbs heat from the body, cooling it down. $\endgroup$
    – AlexP
    Dec 15, 2021 at 15:02
  • $\begingroup$ as a note to everyone here. only chemical reactions can be endothermic. evaporation takes a lot of energy, yes. but can not be classified as endothermic since it is not a chemical reaction. $\endgroup$
    – Postlim Fort
    Dec 16, 2021 at 8:20
  • $\begingroup$ @PostlimFort: That is what I called it an endothermic process, which it is. $\endgroup$
    – AlexP
    Dec 16, 2021 at 9:22

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Apart from the difficulty of getting enough cooling, getting two or more reactants to mix thoroughly and remain in contact with the target long enough to have an effect seems unlikely.

Instead, consider evaporative cooling. Have them accumulate a liquid such as dimethyl ether or butane in a pressurized bladder, mixed with a gelling agent so it sticks to the target as it evaporates. You still aren't going to instantly freeze a target, but you could potentially put them into shock and quickly make small animals hypothermic. Displacement of oxygen and direct intoxication by the vapors would also have an effect.

Just...avoid ignition sources, or the effect will be the opposite of what you desire. And the critters might explode on hot days as pressure in their...cryo-snot bladders gets too high.

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  • $\begingroup$ The obvious answer to avoiding extreme pressures building in the cryo-snot bladder is through a slow and controlled, probably messy leak. ;) $\endgroup$
    – Lemming
    Dec 16, 2021 at 6:55
  • $\begingroup$ So uh... would something like, hmm I don't know, a dragon in a setting by a certain wizard king be able to produce and spray a whole heckin' load of butane or dimethyl ether to freeze things? If so they might not need to handwave that the dragon uses liquid nitrogen. $\endgroup$
    – Lemming
    Dec 16, 2021 at 7:45
  • $\begingroup$ @Lemming butane has a boiling point of -0.5°C at atmospheric pressure, but can be liquefied at "room temperature" by just 2-3 atm of pressure. Dimethyl ether boils at -24°C at atmospheric pressure and requires about 5 atm to liquefy. Nitrogen will not liquefy at all above -147°C, and even at that temperature it requires 33 atm of pressure. $\endgroup$
    – Christopher James Huff
    Dec 16, 2021 at 13:54
  • $\begingroup$ Nice to know the temps but I was mostly asking if spilling or spraying a lot of butane or dimethyl ether onto something, like an animal the dragon might want to eat or defend itself against, will lead to it freezing, partially if not completely. I know the substances are used in pressurized implements to perform cryotherapy to freeze off warts and such can lead to frostbite if not handled correctly but whole-body exposure is where my knowledge of it fades. If they can, then great, and Wizard King will just have to figure out how to make their dragon produce and keep the substances pressurized. $\endgroup$
    – Lemming
    Dec 17, 2021 at 7:01
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    $\begingroup$ @Lemming Nothing's going to instantly freeze a human or large animal. The problem isn't spraying them with something cold enough, flesh just has too much heat capacity and too little thermal conductivity for large pieces to freeze that way, especially since it becomes an even poorer thermal conductor when frozen. $\endgroup$
    – Christopher James Huff
    Dec 17, 2021 at 14:53
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One of the simpler endothermic reactions is the dissolution of various salts in water (or other solvents). Ammonium nitrate demonstrates this, as found in commercial chemical cold packs; these work (or once did, the manufacturers may have changed chemicals over time) by packaging dry ammonium nitrate prills with a capsule of water, and are activated by rupturing the water capsule and kneading the pack to spread it around. When the ammonium nitrate dissolves, the increase in entropy is sufficient to drive a reaction that lowers the temperature of the solution quite markedly from the ambient temperature of the water and dry salt.

There are a number of other simple inorganic salts that exhibit this to a lesser extent (I suspect all do, but I'm not a physical chemist and haven't checked).

Nitrates are within the possibility of living organisms to create; my partner has "denitrate rock" in an aquarium, which is misnamed because it harbors bacteria that, in turn, convert ammonium ion in solution to nitrite, then nitrite to nitrate, and finally break down the nitrate to nitrogen and oxygen. The first two steps are all that's needed here to give an organism internal production of nitrate from ammonium (produced by other bacteria from urea), followed by a process similar to the ion pumps in the kidneys or reverse osmosis in the intestinal walls to dehydrate the salt for storage (producing heat slowly that can be gotten rid of the usual way, by conduction or evaporation).

Once the dry ammonium nitrate (or other salt) is stored, when the animal needs a quick burst of cooling it need merely introduce water (such as lymph or intercellular fluid; it need not be particularly pure due to the very high solubility of ammonium nitrate) to enjoy a fairly sudden burst of cooling without having to wait for the environment to take up the heat.

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  • $\begingroup$ What would be the potency of this? Would the animal be able to freeze something? $\endgroup$
    – Wizard King
    Dec 15, 2021 at 16:58
  • $\begingroup$ From my limited experience with ammonium nitrate cold packs, no, they won't freeze anything. I don't know that there's a chemical endothermic reaction that will (but as noted above, I'm not a chemist). $\endgroup$
    – Zeiss Ikon
    Dec 15, 2021 at 17:00
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The reaction between hydrochloric acid (HCl, or stomach acid) and baking soda (NaHCO3, a dry powder) is endothermic. If a creature like a bombardier beetle can excrete the two substances at once then. They could create a plume of colder carbon dioxide filled air creating a chocking hazard and a little drop in temperature. One might be able to create sub-zero temperatures with this. But it would require a lot of chemicals. And therefore a lot of CO2 needs to be generated to reduce the temperature enough.

I had looked this up earlier for a question about animals generating a snowstorm. But the amount of chemicals involved for that kind of a thing is a little much.

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