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We perceive water as a fire extinguisher, based on observations related to the atmospheric conditions on our planet. However, Let's see if we can circumvent this limitation. Actually, water extinguishes fire because it forms a separation between the fuel (such as charcoal) and the air around it. That's why we can't extinguish gasoline fire with water, because the gasoline floats on water and remains in contact with the air.

Assumption: Carbon dioxide, dissolves readily in water. On the other hand, oxygen is less soluble. Now, let's say that the atmosphere of some planet was very dense (something like Venus's atmosphere or more) and yet very rich in oxygen both in percentage and pressure. Pressurized oxygen at sea level means there will be lots of oxygen dissolved in the water. Carbon dioxide is soluble enough to be moved away from the fire location, so in theory water will be unable to separate the fuel from its oxygen supply, and therefore even "wet" objects may catch fire.

Some points to consider: The fire should neither occur from chemicals reacting with water, nor chemical mixes capable of combusting without additional oxygen. It must be something that (For instance) a primitive underwater society should be able to maintain without complex chemistry and physics.

Clarifications: Combustion should be maintained via reaction with oxygen dissolved in water, whether naturally or not. I'm not speaking about electrolysis to obtain oxygen and hydrogen like this question here. In addition, I'm aware about the heat properties of water. i.e., water absorbs heat more readily than air, so that the fuel is cooled down and it is therefore much harder to maintain fire underwater. You can see this video, where a firework stick extinguishes in water very easily even though it does not rely on oxygen from the air. On the other hand, the same stick isolated with tape can burn underwater without any oxygen supply. You may cause combustion by heating the fuel. One way is to expose it to hydrothermal vents. One other way is to supply heat from other sources, such as a laser beam which serves to maintain high heat at a pinpoint location.

Christmas Snow
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  • Just to be clear: you don’t want any oxidising agents present in the material that’s burning (a’la underwater flares)? – Joe Bloggs Jun 01 '18 at 10:23
  • Try lithium. See https://www.youtube.com/watch?v=MQjangB1dPI – pojo-guy Jun 01 '18 at 11:19
  • A laser beam for a primitive underwater society?! – L.Dutch Jun 01 '18 at 11:27
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    There are several existing compounds that will burn just fine underwater, all you need is a material with its own oxidizer, thermite being a famous example.Why do these not fulfill your needs? Also most fireworks still rely on oxygen from the air. – John Jun 01 '18 at 11:49
  • Does thermite occur naturally? @john I would think most such compounds are flammable enough that they wouldn’t be around in nature ... we have them because we make them and then carefully store them. Are there any that aren’t so easily triggerable? – SRM Jun 01 '18 at 13:18
  • Thermite would hardly be called easy to ignite but no by definition something that will burn underwater will need a to maintain the chain reaction thus will not occur naturally in large quantities. That is why most naturally occurring flammable materials are biological in nature. – John Jun 01 '18 at 13:56
  • So.. what's the actual question here? – Aify Jun 01 '18 at 17:02
  • Magic. The lock of oxygen and the heat dissipation of water pretty much rules out anything else. – ShadoCat Jun 01 '18 at 17:46

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Flush hydrogen peroxide near the spot where you want the flame.

The peroxide will decompose and release oxygen, which will locally enrich its concentration and support the combustion. Also, peroxide decomposition releases heat, thus will help keeping the burning spot warm.

L.Dutch
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  • Hydrogen peroxide and kerosene can be used as rocket fuel. I had read that concentrated peroxide splashed on stuff would cause it to burst into flames but I could not find that on youtube. – Willk Jun 01 '18 at 12:22
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    Hydrogen peroxide is a powerful enough oxidiser that when I was a Hazmat FRO, we learned that spills of HO from tanker trucks had caused otherwise noncombustible items to become highly combustible, and at that point you're just a minor ignition source from a *bad* day. Excerpted directly from Solvay's MSDS for Hydrogen Peroxide: Decomposition of hydrogen peroxide can generate sufficient heat and oxygen to initiate combustion of ignitable materials. – GerardFalla Jun 01 '18 at 15:43
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    From USP's website section on Hydrogen Peroxide safe handling: From USP Tech's website: Hydrogen peroxide is not considered an explosive. However, when it is mixed with organic substances at significant concentrations, hazardous impact-sensitive compounds may result. Small amounts of other materials that contain catalysts (silver, lead, copper, chromium, mercury, and iron oxide rust) can cause rapid decomposition and an explosive pressure rupture of the containing vessel if it is not properly vented. – GerardFalla Jun 01 '18 at 15:48