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Acetylene is useful for making fire underwater. This might be used for cooking, or so you have a way to start a fire right after coming out of the water, or for some underwater welding. Or, maybe even to use a torch as a weapon, or as some sort of explosive device?

Whatever they use it for, the main question is how they make it, with pre-modern technology. That is, the tech level is mostly prior to the 18th century. I have some exceptions I try to name.

Capabilities

I'll note these mermaids have access to electricity in this case via a special domesticated breed of electric eels, are very skilled with copper and its alloys, and have very good gas science. They can electrolyse O2 and hydrogen, for example. They do a lot of work on islands, or sometimes in large submarine air-filled habitats--by which I mean basically just a big, fancy diving bell, an underwater air-tight house filled with air.

With the excellent copper and gas sciences, I am figuring the mermaids can store gasses such as acetylene at high pressures, but not as a liquid. I also presume they have high quality valves.

It has been brought to my attention that copper does not work well with acetylene. They might be able to plate the inside with another metal, due to being capable with electrolysing?

Methane

One method that's mentioned that seems easiest is to partially burn methane mixed with oxygen. Getting some organic matter to create methane sounds simple. This article even reckons you can use a mixture of hydrogen and electricity with it to make it more efficient, via modern means.

This seems the simplest, but I'm not enough of an expert to know if it's the best method. I expect some mermaid nations would use this method.

Carbide

Calcium Carbide can be mixed with water to make acetylene gas. Calcium Carbide can be made from Calcium Carbonate mixed with coke and heat. The mermaids can get the Calcium Carbonate from their coral farms. They may have lumber and woodworking industries I'm developing, that I think might allow them to make white coal--can that be made into coke? They could trade for coke, if necessary.

I have ideas and specifics about their brick and wood industries, but I have been brief to prevent the question being too long. Suffice to say I think they could feasibly make acetylene this way?

Electricity and Hydrocarbons

If mermaids have abundance of oil or other hydrocarbons, they could put them through an electrical arc to make it. Exactly how costly this is, or how it compares to the other methods, is a bit beyond me. I expect the eels are pretty efficient, at least.

Bonus Question: How do you light it underwater?

Would they be able to make a battery to electrically like the acetylene torch? Or would you need a (small) diving bell to light it inside?

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  • $\begingroup$ Can you clarify what you mean by 'without pre-modern technology'? Usually the line there is drawn at the Enlightenment or Industrial Revolution, but if mermaids have large-scale electrical infrastructure and functioning air-fulled submarines, I would say they are well into the modern era. Also, acetylene reacts with water to form acetaldeyde, which is very toxic (pubchem.ncbi.nlm.nih.gov/compound/acetaldehyde), so I doubt your mermaids would want anything to do with it outside of very controlled laboratories. $\endgroup$
    – E Tam
    Feb 19 at 4:38
  • $\begingroup$ mermaid: "alpha to romeo over, target at 2o'clock looks like he's hyperbaric welding. seriously are we going to rob this poor guy?" $\endgroup$
    – user6760
    Feb 19 at 4:43
  • $\begingroup$ They have submarine air-filled habitats and you're asking how to make acetylene? What's stopping them from doing what we do on land? Mix lime with coke in a blast furnace to get calcium carbide, then subject the calcium carbide to water to get acetylene gas.... I agree with @ETam that using acetylene gas underwater is a bad idea. Must it be acetylene? I doubt there's a solution that allows an open flame underwater, especially if you have access to electricity and can arc-weld. Note that promoting your answer and asking for more is prohibited in the help center. $\endgroup$
    – JBH
    Feb 19 at 4:44
  • $\begingroup$ Also think about how you're going to store acetylene. Woodwork isn't going to cut it, and a very quick Google suggests that copper storage vessels are a really bad idea $\endgroup$ Feb 19 at 4:48
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    $\begingroup$ "Acetylene is useful for making fire underwater.": No, it isn't. oxyacetylene torches work underwater because they provide oxygen. Acetylene has some advantages in this application, but it's the oxygen that's the critical component. $\endgroup$ Feb 19 at 19:13

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Underwater everything is more difficult. Especially as there is no access to metals. So it's bio-organic systems all the way for chemistry. Meaning, you basically create a ablative organism that can contain chemicals and filter out the inert colon like linement in the final product. Obviously such a organism has to be tailored to what is produced.

It can work for acids (stomach, colon) and it can for other chemistries as long as you are willing to accept the degradation of the final product due to reactions with containment material.

If that can not be done, the only way i can imagine chemical synthesis under water, would be hotspot blown glass. As in you melt it in volcanic cracks and blow/extrude it into beakers/pipes inside the hotspot. That could be used to synthesize even aggressive chemicals in high purity, but always at ambient pressure.

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    $\begingroup$ This was quite a good answer, in that it had some really creative and interesting concepts about mermaids breeding unusual chemistry fish or other creatures. I wonder what base creature would be most ideal? Some bottom feeder? Probably a creature tolerant of toxins like have metals? Maybe something unusual like a starfish? Have upvoted this answer, though I will note it didn't exactly follow the data I gave for the mermaids, who do work on land. Thank you for the answer, Pica! $\endgroup$ Feb 19 at 8:41
  • $\begingroup$ Eh, worms.. in particular worms adapated to black smokers.. they are already adapted to hardcore chemistry - actually need it to life. Finally organisms in oxygen poor environments, like worms in mud with loads of cyanobacteria? And lighting is easy - bioluminescence bacteria. en.wikipedia.org/wiki/Bioluminescence $\endgroup$
    – Pica
    Feb 19 at 14:15
  • $\begingroup$ Ah, worms is an interesting one. Will remember that. Maybe mermaids will perform some chemistry on land and a lot of it via unusual bacteria they collect via colony organisms or processes through specially bred worms? That's a really interesting concept. I guess any bacterial chemistry is technically possible via a larger organism....? Hmm, not sure where to even start my research. $\endgroup$ Feb 19 at 19:07

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