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?
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
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.