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I have alchemists who can transmute materials at a small magic cost. I have a scene where some alchemists need to improvise a light. They have a bottle of water and some coins. What would be a good material to transmute the coins into, that would burn under water to fashion a makeshift bottle lantern?

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  • $\begingroup$ Could they instead transmute the water to oil or alcohol? Water is a very stable molecule. $\endgroup$ – Charles Bamford Apr 7 at 18:56
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Your alchemist should create calcium carbide which when dropped into water releases acetylene which will burn in the air above the water's surface. This is the reaction behind the classic carbide lantern which has been used by miners for centuries.

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    $\begingroup$ Just an extra detail - the actual material Calcium Carbide is made using electricity from Lime and Coke. If your alchemists are in a cave - those are relatively common. The coins could be used as some sort of tool to help the process or add a little stylish flair. $\endgroup$ – David Shader Apr 7 at 17:28
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The alkali metals such as sodium, potassium and lithium all burn under water, but in general, they will burn so fast and hot, you end up with more of an explosion than a sustainable light source.

For a more controllable reaction aluminum releases flammable hydrogen gas when it interacts with water which can be burned. This are much easier to control by simply adjusting the PH of the water to slowly dissolve the aluminum oxide coating that normally stops the reaction which I assume your alchemist can also do. If you want don't have control over the PH, but do have control over the shape of what you are turning the coin into, turning aluminum into fine enough of a dust will create a useable supply of hydrogen fuel without needing to modify the PH.

Aluminum has about 27x the atomic mass of hydrogen, and exposing pure aluminum to water creates Al2O3 + 3(H2) meaning that 1 gram of aluminum can liberate 1/9th of a gram of hydrogen from the water. A quarter is 5.67 grams; so, it could be used to create about 0.63 grams of hydrogen gas. That may not sound like much, but hydrogen is some potent stuff. That amount of it when burned yields 79 kilojoules of energy. About the same as you get out of 2 birthday candles; so, a handful of coins should be enough for a suitable lamp.

There have also been many claims throughout history of catalysts that can be used to burn water by separating hydrogen and oxygen into it's parts, and then burning them. These have all been debunked as either hoaxes, or involving what someone thought was a catalyst really being an fuel source in a hydrogen on demand system. But if you are already talking about magic/alchemy, it is not really that big of a stretch.

For a more hard science solution which would look like an alchemist throwing coins into water to make a makeshift lamp, is if you replace the water with Hydrochloric Acid. A casual observer would see an alchemist with a glass vile of clear liquid that he drops some penies into. The zinc in the penies would cause the acid to release hydrogen which he could burn

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    $\begingroup$ Separating water into hydrogen and oxygen and then burning it back to water cannot create a positive sum of energy. It cannot work without magic or other external energy sources. $\endgroup$ – Aganju Apr 7 at 13:58
  • $\begingroup$ @Aganju I always thought that sounded wrong, and yup, all such cases I can find have turned out to actually be hydrogen on demand systems or fraud. Updated answer accordingly. $\endgroup$ – Nosajimiki Apr 7 at 19:39
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    $\begingroup$ This would produce very poor light. Hydrogen fire produces very little visible light, so little in fact that a steady burn would be easier to hear than to see. I know this because I have actually burned hydrogen produced from both aluminum reaction with water and NaOH, and zinc in water. In both instances, the only time you can actually see the flame is when it explodes, not when it burns steadily. $\endgroup$ – BlackThorn Apr 7 at 21:44
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    $\begingroup$ @Aganju This doesn't work by separating water into oxygen and hydrogen. It works by converting aluminum and water into aluminum oxide and hydrogen--at no point is there free oxygen. And that does indeed release energy, and you can recover more energy bu then burning the hydrogen with ambient free oxygen. $\endgroup$ – Logan R. Kearsley Apr 7 at 22:11
  • $\begingroup$ @Nosajimiki-ReinstateMonica yOU CAN GET hYDROGEN WITH OTHER ENERGY INPUT USING EG LIGHT INPUT + CATALYSTS, OR LIGHT + BIO PROCESSES. sPLIITING NATURAL GAS C2.N(CH2) with less energy than is liberated also works. $\endgroup$ – Russell McMahon Apr 8 at 4:12
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White phosphorus. Glows with or without water, but water adds a touch of safety.

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The answer about platinum was close, but thermodynamically impossible in terms of an ongoing reaction.

Instead, they need to perform two transmutations: convert the coins into platinum (doesn't need to be particularly pure, it'll work as well in the form of nearly any alloy with iridium, rhodium, palladium, even nickel), ideally in the form of a mesh of fine wire -- then transform the water into hydrogen. When the hydrogen is passed over the platinum alloy in air, it will ignite and heat the platinum white-hot, and the glow from the incandescent platinum will seem as if the wire is burning (because the hydrogen flame is virtually invisible).

The platinum will be unconsumed and undamaged; the hydrogen will need frequent or continuous replenishment, but there's quite a lot of hydrogen (as gas, by volume) in a fairly small amount of water.

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