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So I want to have an alien creature on a planet with a halogen atmosphere. It will have to be at a temperature where the hydrogen halide is liquid or solid.

This hydrogen halide I would have as the main useable component of the creature's blood as a substitute for water along with some halogen gas carried in the blood cells as a substitute for oxygen. So now I need to figure out which hydrogen halide, in other words which acid to have in the creature's blood.

A body temperature of 37°C completely rules out hydroflouric acid. Plus, flourine is very reactive, in fact it is the most reactive of all the halogens. So every molecule would end up flourinated and the creature would die just from the flourine not letting go, not to mention that hydrogen flouride would boil off and make the entire creature swell up and burst. The only thing that doesn't react with flourine or hydrogen flouride that I know of is teflon.

Highest survived fever for a human is 46°C, just 2 degrees below the boiling point of concentrated hydrochloric acid. Hydrochloric acid though is the most common hydrogen halide and is known to be in living organisms as stomach acid. Chlorine is the most common halogen as well so this would be the most practical. A heavy gas that absorbs UV light and forms a strong acid is a pretty high contender as far as acidic blood is concerned. And it is pretty easy to form a buffer solution to prevent tissue from being corroded by hydrochloric acid by just producing a lot of sodium bicarbonate. But the fact that the boiling point is just 2 degrees above the highest survived fever in humans makes it not ideal in that aspect.

Hydrobromic acid and hydroiodic acid both have boiling points above that of water. But bromine would be a fuming liquid at body temperature and iodine would be a sublimating solid. No chance of getting a significant amount out with each breath so the lungs(or whatever breathing mechanism the creatures use) would just get flooded with liquid bromine or iodine powder and the creature would asphyxiate and die.

This really only leaves me with the option of hydrochloric acid. And I have been told to find out if any organic molecules would be resistant or not react with hydrochloric acid and the only ones I can think of are those already saturated with chlorine or flourine. Because of the lack of hydrogens it wouldn't be acidic in the same way hydrochloric acid is acidic. But it wouldn't be alkaline either. So it wouldn't protect tissues from being corroded unless it was like in a mucus or other viscous substance. And mucus if there is too much of it, can make you susceptible to infections. Also it would probably lead to the death of the creature because the chlorine and the acid would go through it way too slowly.

So now I'm kind of stuck, mucus isn't the ideal protectant in terms of the speed the chlorine and the acid will get into the cells where they are needed, sodium bicarbonate is basic so the blood wouldn't be acidic, other hydrogen halides aren't really an option for reasons already described, and hydrochloric acid boils at 48°C.

So how can I make this hydrogen halide idea work besides decreasing the creature's body temperature?

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    $\begingroup$ Are you trying to make a Xenomorph?? Because this is how you get Xenomorphs. $\endgroup$
    – Joe Bloggs
    Oct 6, 2018 at 20:27
  • $\begingroup$ If you look it up, you may find that polypropylene and polyethylene are not easily attacked by strong acids, including hydrochloric acid. $\endgroup$
    – AlexP
    Oct 6, 2018 at 20:31
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    $\begingroup$ "So how can I make this hydrogen halide idea work besides decreasing the creature's body temperature?" Decreasing the body temperature is perfectly a perfectly valid option. Even better is... ignore the problem and just make your Xenomorph. After all, was Alien a worse movie because the biochemistry wasn't scientifically valid? $\endgroup$
    – RonJohn
    Oct 6, 2018 at 20:52
  • $\begingroup$ @RonJohn Perfectly a perfectly? $\endgroup$
    – Joe Bloggs
    Oct 6, 2018 at 20:53
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    $\begingroup$ You might have better luck with your google searches if you realize that it's spelled "fluorine", not "flourine" - which seems like something you'd use in baking :-) $\endgroup$
    – jamesqf
    Oct 7, 2018 at 4:16

2 Answers 2

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No hydrogen halide could be a likely substitute for water. They are ionic acidic compounds. Water is a covalent molecule which has its own more durable charge asymmetry, called a dipole. Water does not easily decompose, as do strong acids.

Blood has to be suitable for carrying CO2, O2, sugars, hormones, various organics,

An analog would most likely be in the same column on the periodic table as O, the best analog to O is S. H2S is a gas at STP, so that would work, if at all, only in a colder environment. S is also much larger than O, with different properties, but good luck. :)

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  • $\begingroup$ See I figured with going with halogens and thus hydrogen halides because of their acidity. I mean after all if there is anaerobic life and life that is very acid resistant, even to hydrochloric acid, no reason why there couldn't be life that absolutely needs an acidic pH to function properly and uses halogens instead of oxygen. I can more easily imagine a planet with a chlorine atmosphere and hydrochloric acid rivers than a planet with sulfur rocks, an H2S atmosphere and H2SO4 rivers. $\endgroup$
    – Caters
    Oct 7, 2018 at 18:51
  • $\begingroup$ @Caters - water is the solvent within blood. any suitable solvent must be more chemically stable relative to any of its solutes. most any solvents we would imagine to be covalent compounds upon that basis. ionic bonds as in acids or salts are brittle, in the sense that they are more reactive, and consequently less stable, relative to water. this is the issue. $\endgroup$
    – theRiley
    Oct 8, 2018 at 9:01
  • $\begingroup$ You're making an assumption that this creature's biology uses oxygen for respiration and burns sugars. If we're going to throw water out of the window, we most likely have to throw aerobic processes out as well. $\endgroup$
    – stix
    Jul 24, 2019 at 19:57
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There is a difference between hydrochloric acid and hydrogen chloride. In your question your refer to hydrochloric acid, which is a solution of hydrogen chloride (usually) in water. Concentrated hydrochloric acid still consists of ~63% water and the pure hydrogen chloride is a gas with a boiling point of -85 °C. Hydrogen bromide and hydrogen fluoride are liquid at roomtemperature, but if you want to stick with hydrogen chloride here are three ideas that came to my mind (they might be unhealthy ideas for living organisms).

  1. Stick with aqueous hydrochloric acid, you could decrease the concentration -this would increase the boiling point. However you had quite alot of water in your bloodstream.
  2. Decrease the temperature. Hydrogen Chloride is a liquid between -114 °C and -85 °C.
  3. Keep a solution of hydrogen chloride, but replace the water. Hydrogen chloride is soluble in quite a few organic solvents. However this would replace the idea of using a hydrogen halide as main component. On the bright side, hydrogen chloride is not necessary acidic in unpolar solvents.
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